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Re: [PATCH v9 4/5] rust: Add dma_fence abstractions

by Gary Guo

On Wed Aug 5, 2026 at 3:59 PM BST, Philipp Stanner wrote: > C's dma_fence's are synchronisation primitives that will be needed by all > Rust GPU drivers. > > The dma_fence framework sets a number of rules, notably: > - fences must only be signaled once > - all fences must be signaled at some point > - fence error codes must only be set before signaling > - every pointer to a fence must be backed by a reference > > All those rules are being addressed by these abstractions. > > To cleanly decouple fence issuers and consumers, two types are provided: > - DriverFence: the only fence type that can be signaled and that > carries driver-specific data. > - Fence: the fence type to be shared with other drivers and / or > userspace. The only type callbacks can be registered on. > Cannot be signaled. > > Hereby, a Fence lives in the same chunk of memory as a DriverFence. Both > share the refcount of the underlying C dma_fence. Since this > implementation does not provide a custom dma_fence_backend_ops.release() > function, the memory is freed by the dma_fence backend once the refcount > drops to 0. > > To create a DriverFence, the user must first allocate a > DriverFenceAllocation, so that the creation of the DriverFence later on > can always succeed. Otherwise, deadlocks could occur if fences need to > be created in a GPU job submission path. > > Synchronization is ensured by the dma_fence backend. > > All DriverFence's created through this abstraction must be signaled by > the creator with an error code. In case a DriverFence drops without > being signaled beforehand, it is signaled with -ECANCELLED as its > error and a warning is printed. This allows the Rust abstraction to very > cleanly decouple fence issuer and consumer by relying on the decoupling > mechanisms in the C backend, which ensures through RCU and the > 'signaled' fence-flag that dma_fence_backend_ops functions cannot > access the potentially unloaded driver code anymore. > > Signalling fences on drop thus grants many advantages. Not signaling > fences on drop would risk deadlock and does not grant real advantages: > By definition only the drivers can ensure that a fence always represents > the hardware's state correctly. > > This implementation models a DmaFenceContext object on which fences are > to be created, thereby ensuring correct sequence numbering according to > the timeline. > > dma_fence supports a variety of callbacks. The mandatory callbacks > (get_timeline_name() and get_driver_name()) are implemented in this > patch. For convenience, they store those name parameters in the fence > context, saving the driver from implementing these two callbacks. > > Support for other callbacks (like for hardware signaling) is prepared > for through the fact that both DriverFence and Fence live in the same > allocation, allowing for usage of container_of from the callback to > access the driver-specific data. > > It is expected that other callbacks, added in the future, also mostly > operate on the generic data in the FenceContext. To make this safe, the > implementation ensures through a lifetime that a DriverFence cannot > outlive its FenceContext. > > Synchronization for dma_fence_ops callbacks is ensured by only running the > Rust deconstructor delayed with call_rcu(), which prevents UAF-bugs > should a DriverFence drop while a Fence callback is currently operating > on the associated driver data. Since they can also operate on the > FenceContext's data, its drop implementation also performs the necessary > delay with rcu_barrier(). > > An additional issue discovered during the review process of this code is > that there is (currently) no mechanism in Rust to prevent someone from > circumventing the DriverFence's FenceContext-reference's lifetime by > "forgetting" the fence, e.g. with core::mem::forget(). This would enable > UAF bugs on the FenceContext. Throw a panic if this happens and document > a path towards a more robust solution. > > Add abstractions for dma_fence in Rust. > > Signed-off-by: Philipp Stanner <phasta(a)kernel.org> > Tested-by: Daniel Almeida <daniel.almeida(a)collabora.com> > --- > rust/bindings/bindings_helper.h | 1 + > rust/helpers/dma_fence.c | 48 ++ > rust/helpers/helpers.c | 1 + > rust/kernel/dma_buf/dma_fence.rs | 1002 ++++++++++++++++++++++++++++++ > rust/kernel/dma_buf/mod.rs | 14 + > rust/kernel/lib.rs | 1 + > 6 files changed, 1067 insertions(+) > create mode 100644 rust/helpers/dma_fence.c > create mode 100644 rust/kernel/dma_buf/dma_fence.rs > create mode 100644 rust/kernel/dma_buf/mod.rs > > diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h > index 1124785e210b..54b62d952e01 100644 > --- a/rust/bindings/bindings_helper.h > +++ b/rust/bindings/bindings_helper.h > @@ -53,6 +53,7 @@ > #include <linux/debugfs.h> > #include <linux/device/faux.h> > #include <linux/dma-direction.h> > +#include <linux/dma-fence.h> > #include <linux/dma-mapping.h> > #include <linux/dma-resv.h> > #include <linux/errname.h> > diff --git a/rust/helpers/dma_fence.c b/rust/helpers/dma_fence.c > new file mode 100644 > index 000000000000..0e08411098fa > --- /dev/null > +++ b/rust/helpers/dma_fence.c > @@ -0,0 +1,48 @@ > +// SPDX-License-Identifier: GPL-2.0 > + > +#include <linux/dma-fence.h> > + > +__rust_helper void rust_helper_dma_fence_get(struct dma_fence *f) > +{ > + dma_fence_get(f); > +} > + > +__rust_helper void rust_helper_dma_fence_put(struct dma_fence *f) > +{ > + dma_fence_put(f); > +} > + > +__rust_helper bool rust_helper_dma_fence_begin_signalling(void) > +{ > + return dma_fence_begin_signalling(); > +} > + > +__rust_helper void rust_helper_dma_fence_end_signalling(bool cookie) > +{ > + dma_fence_end_signalling(cookie); > +} > + > +__rust_helper bool rust_helper_dma_fence_is_signaled(struct dma_fence *f) > +{ > + return dma_fence_is_signaled(f); > +} > + > +__rust_helper bool rust_helper_dma_fence_test_signaled_flag(struct dma_fence *f) > +{ > + return dma_fence_test_signaled_flag(f); > +} > + > +__rust_helper void rust_helper_dma_fence_lock_irqsave(struct dma_fence *f, unsigned long *flags) > +{ > + dma_fence_lock_irqsave(f, *flags); > +} > + > +__rust_helper void rust_helper_dma_fence_unlock_irqrestore(struct dma_fence *f, unsigned long *flags) > +{ > + dma_fence_unlock_irqrestore(f, *flags); > +} > + > +__rust_helper void rust_helper_dma_fence_set_error(struct dma_fence *f, int error) > +{ > + dma_fence_set_error(f, error); > +} > diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c > index 998e31052e66..4ab8aa9da7e7 100644 > --- a/rust/helpers/helpers.c > +++ b/rust/helpers/helpers.c > @@ -58,6 +58,7 @@ > #include "cred.c" > #include "device.c" > #include "dma.c" > +#include "dma_fence.c" > #include "dma-resv.c" > #include "drm.c" > #include "drm_gpuvm.c" > diff --git a/rust/kernel/dma_buf/dma_fence.rs b/rust/kernel/dma_buf/dma_fence.rs > new file mode 100644 > index 000000000000..e61b4b2d8b8c > --- /dev/null > +++ b/rust/kernel/dma_buf/dma_fence.rs > @@ -0,0 +1,1002 @@ > +// SPDX-License-Identifier: GPL-2.0 > +/* > + * Copyright (C) 2025-2026 Red Hat Inc. > + * Author: Philipp Stanner <pstanner(a)redhat.com> > + */ > + > +//! DriverFence support. > +//! > +//! Reference: <https://docs.kernel.org/driver-api/dma-buf.html#c.dma_fence> > +//! > +//! header: [`include/linux/dma-fence.h`](srctree/include/linux/dma-fence.h) > + > +use crate::{ > + alloc::AllocError, > + bindings, > + container_of, > + error::to_result, > + prelude::*, > + types::ForeignOwnable, > + types::Opaque, // > +}; > + > +use core::{ > + marker::PhantomData, > + mem::ManuallyDrop, > + ops::Deref, > + ptr, > + ptr::{ > + drop_in_place, > + NonNull, // > + }, // > +}; > + > +use kernel::{ > + str::CString, > + sync::{ > + aref::{ > + ARef, > + AlwaysRefCounted, // > + }, > + atomic::{ > + Atomic, > + Relaxed, // > + }, > + rcu::rcu_barrier, // > + }, // > +}; > + > +/// VTable for dma_fence backend_ops callbacks. > +// > +// Mandatory dma_fence backend_ops are implemented implicitly through > +// [`FenceContext`]. Additional ones shall get implemented on this trait. > +pub trait FenceContextOps { > + /// The generic payload data for [`DriverFence`]s created on this fctx. > + type FenceDataType: Send + Sync; > +} > + > +/// A dma-fence context. A fence context takes care of associating related fences > +/// with each other, providing each with raising sequence numbers and a common > +/// identifier. > +#[pin_data(PinnedDrop)] > +pub struct FenceContext<T: FenceContextOps + Send + Sync> { > + /// The fence context number. > + nr: u64, > + /// The sequence number for the next fence created. > + seqno: Atomic<u64>, > + // The name parameters can be accessed by the dma_fence backend_ops. UAF > + // errors are prevented by the `call_rcu()` in `drop_driver_fence_data()`. > + /// The name of the driver this FenceContext's fences belong to. > + driver_name: CString, > + /// The name of the timeline this FenceContext's fences belong to. > + timeline_name: CString, > + /// The number of all unsignaled fences on this context. > + // Used to prevent bugs due to forgotten fences. > + // > + // The lifetime on `DriverFence`s should typically prevent this from > + // happening. > + // > + // However, we cannot fully guarantee in Rust that `DriverFence`s will not > + // be forgotten, e.g., through `core::mem::forget()`. This could circumvent > + // the lifetime which intends to enforce that all fences disappear before > + // their context. > + nr_of_unsignaled_fences: Atomic<u64>, This can be `Atomic<usize>` so it doesn't need to go through the generic 64-bit atomic mechanism on 32-bit systems. > + /// The user's data. > + #[pin] > + data: T, > +} > + > +impl<'a, T: Send + Sync + FenceContextOps> FenceContext<T> { > + // This can later be extended as a vtable in case other parties need support > + // for the more "exotic" callbacks. > + const OPS: bindings::dma_fence_ops = bindings::dma_fence_ops { > + get_driver_name: Some(Self::get_driver_name), > + get_timeline_name: Some(Self::get_timeline_name), > + enable_signaling: None, > + signaled: None, > + wait: None, > + release: None, > + set_deadline: None, > + }; > + > + /// Create a new `FenceContext`. > + pub fn new<E>( > + initial_seqno: u64, > + driver_name: CString, > + timeline_name: CString, > + data: impl PinInit<T, E>, > + ) -> impl PinInit<Self, Error> > + where > + Error: From<E>, > + { > + try_pin_init!(Self { > + // SAFETY: `dma_fence_context_alloc()` merely works on a global > + // atomic. Parameter `1` is the number of contexts we want to > + // allocate. > + nr: unsafe { bindings::dma_fence_context_alloc(1) }, > + seqno: Atomic::new(initial_seqno), > + driver_name, > + timeline_name, > + nr_of_unsignaled_fences: Atomic::new(0), > + data <- data, > + }) > + } > + > + fn next_seqno(&self) -> u64 { > + self.seqno.fetch_add(1, Relaxed) > + } > + > + /// Allocate the memory for a [`DriverFence`] and already store `data` inside. > + /// > + /// This is needed because many times, creation of a [`DriverFence`] must not > + /// fail, and allocating might deadlock in some situations. > + /// > + /// The `data` you pass here must not perform any operations that are illegal > + /// in atomic context in its [`Drop`] implementation. > + pub fn new_fence_allocation( > + &self, > + data: T::FenceDataType, > + ) -> Result<DriverFenceAllocation<'_, T>> { > + let fence_data = DriverFenceData { > + rcu_head: Default::default(), > + // `inner` remains uninitialized until a `DriverFence` takes over. > + inner: Fence { > + inner: Opaque::uninit(), > + }, > + fctx: self, > + data, > + }; > + > + // In order to support the C dma_fence callbacks, it is necessary for > + // a `Fence` and a `DriverFence` to live in the same allocation, > + // because the C backend passes a dma_fence, from which the driver most > + // likely wants to be able to access its `data` in `DriverFence`. > + // > + // Hence, we need the manage the memory manually. It will be freed by the > + // C backend automatically once the refcount within `Fence` drops to 0. > + let data = KBox::new(fence_data, GFP_KERNEL | __GFP_ZERO)?; > + > + Ok(DriverFenceAllocation { > + data, > + ops: &Self::OPS, > + }) > + } > + > + extern "C" fn get_driver_name(ptr: *mut bindings::dma_fence) -> *const c_char { > + // SAFETY: The C backend only invokes this callback with `ptr` pointing > + // to a valid, unsignaled `bindings::dma_fence`. All fences created in > + // this module always reside within `Fence` which always resides in a > + // `DriverFenceData`, thus satisfying the function's safety > + // requirements. > + let fctx = unsafe { Self::from_raw_fence(ptr) }; > + > + fctx.driver_name.as_char_ptr() > + } > + > + extern "C" fn get_timeline_name(ptr: *mut bindings::dma_fence) -> *const c_char { > + // SAFETY: The C backend only invokes this callback with `ptr` pointing > + // to a valid, unsignaled `bindings::dma_fence`. All fences created in > + // this module always reside within `Fence` which always resides in a > + // `DriverFenceData`, thus satisfying the function's safety > + // requirements. > + let fctx = unsafe { Self::from_raw_fence(ptr) }; > + > + fctx.timeline_name.as_char_ptr() > + } > + > + /// Create a [`FenceContext`] from an associated [`bindings::dma_fence`]. > + /// > + /// # Safety > + /// > + /// `ptr` must be a valid pointer to a [`bindings::dma_fence`] which resides > + /// within a [`Fence`], which in turn resides in a [`DriverFenceData`]. > + unsafe fn from_raw_fence(ptr: *mut bindings::dma_fence) -> &'a Self { > + let opaque_fence = Opaque::cast_from(ptr); > + > + // SAFETY: Safe due to the function's overall safety requirements. > + let fence_ptr = unsafe { container_of!(opaque_fence, Fence, inner) }; > + > + // CAST: `DriverFenceData` is repr(C) and a `Fence` is its first member. `repr(C)` > + let fence_data_ptr = fence_ptr as *mut DriverFenceData<'a, T>; `*const` should work here too? > + > + // SAFETY: Safe because of the comments directly above. > + let fence_data = unsafe { &*fence_data_ptr }; > + > + fence_data.fctx > + } > +} > + > +// FenceContext's drop() ensures that the driver cannot unload while there are > +// still dma_fence callbacks running. This also prevents UAF problems with > +// `fctx.driver_name` and `fctx.timeline_name`. > +// > +// DriverFence data gets dropped through `call_rcu()` in `DriverFence::drop`. > +// This `rcu_barrier()` also serves to wait for their completion. These should be comment on the drop code itself, not as comment of `PinnedDrop` impl. This can be more detailed about why a UAF problem exists (and this should be commented on the panic part, as something like: // Fence ops callbacks may be called on unsignaled fences, so we may not // leak any driver fences, otherwise `fctx.driver_name` and // `fctx.timeline_name` can be accessed after drop (UAF). if self.nr_of_unsignaled_fences.load(Relaxed) != 0 { // Fence ops callbacks use RCU to sychronize callbacks and thus we use `call_rcu` // to destroy `DriverFence` data. `rcu_barrier` here synchronize with driver // fence's destruction. > +#[pinned_drop] > +impl<T: FenceContextOps + Send + Sync> PinnedDrop for FenceContext<T> { > + fn drop(self: Pin<&mut Self>) { > + // TODO: > + // It would be better if the fence context signals all forgotten fences > + // itself. To do so, it would keep a list of unsignaled fences. That > + // list members would have to be pre-allocated (see > + // `FenceCallback::new_fence_allocation()`). > + if self.nr_of_unsignaled_fences.load(Relaxed) != 0 { > + panic!("Forgotten fences in FenceContext."); > + } > + > + rcu_barrier(); > + } > +} > + > +/// Error type for fence callback registration. > +/// > +/// Generic over `T` so that `AlreadySignaled` can return the callback to the > +/// caller, allowing it to reclaim any resources owned by the callback (e.g., > +/// a fence handle that needs to be signaled). > +#[derive(Debug)] > +pub enum CallbackError<T = ()> { What is this `= ()` used for? > + /// The fence was already signaled. The callback is returned so the caller > + /// can extract owned resources without losing them. > + AlreadySignaled(T), > + /// Some other error occurred during registration. > + Other(Error), > +} > + > +impl<T> From<CallbackError<T>> for Error { #[inline] > + fn from(err: CallbackError<T>) -> Self { > + match err { > + CallbackError::AlreadySignaled(_) => ENOENT, > + CallbackError::Other(e) => e, > + } > + } > +} > + > +impl<T> From<AllocError> for CallbackError<T> { #[inline] > + fn from(e: AllocError) -> Self { > + CallbackError::Other(Error::from(e)) > + } > +} > + > +/// Trait for callbacks that can be registered on fences. > +/// > +/// When the fence signals, the callback will be invoked. > +/// > +/// # Example > +/// > +/// ```rust > +/// use kernel::dma_buf::FenceCallback; > +/// > +/// struct MyCallback { > +/// // Your callback state here > +/// } > +/// > +/// impl FenceCallback for MyCallback { > +/// fn called(&mut self) { > +/// pr_info!("Fence signaled!"); > +/// // Handle fence completion > +/// } > +/// } > +/// ``` > +pub trait FenceCallback: Send + 'static { > + /// Called when the fence is signaled. > + /// > + /// This is called from the fence signaling path, which may be in interrupt > + /// context or with locks held, which is why `self` is only borrowed, so that > + /// it cannot drop. Implementations must not sleep or perform > + /// long-running operations. > + /// > + /// An implementation likely wants to inform itself (e.g., through a work item) > + /// within this callback that the associated [`FenceCallbackRegistration`] > + /// can now be dropped. > + fn called(&mut self); The name feels a bit awkward to me. I think this should either look like an action on the callback, in which case "call" or describe an event on the fence, i.e. "on_signal" or "signaled". Naming it "called" is very weird because it's not a event that is triggered when something is "called". > +} > + > +/// A callback registration on a fence. > +/// > +/// When this object is dropped, the callback is automatically removed if it > +/// hasn't been called yet. > +#[pin_data(PinnedDrop)] > +pub struct FenceCallbackRegistration<T: FenceCallback + 'static> { > + #[pin] > + callback_foreign: Opaque<bindings::dma_fence_cb>, > + callback: ManuallyDrop<T>, > + fence: ARef<Fence>, > +} > + > +impl<T: FenceCallback> FenceCallbackRegistration<T> { > + /// Create a [`PinInit`] closure for registering a callback on a fence. > + /// > + /// The actual attempt at registering the callback will take place once you > + /// call an allocator's `pin_init()` function. > + /// > + /// On success the callback is pinned in place and will fire when the fence > + /// signals. On `AlreadySignaled` the callback is returned to the caller so > + /// that owned resources can be reclaimed. > + pub fn new<'a>(fence: &'a Fence, callback: T) -> impl PinInit<Self, CallbackError<T>> + 'a > + where > + T: 'a, > + { > + try_pin_init!(Self { > + // We need to fully initialize the fence because after > + // `dma_fence_add_callback()` ran, the callback might immediately > + // get invoked. > + callback: ManuallyDrop::new(callback), > + fence: ARef::from(fence), > + callback_foreign <- Opaque::try_ffi_init(|ptr| { > + // SAFETY: `fence.inner.get()` is a valid, initialized `struct > + // dma_fence`. `ptr` points to the `struct dma_fence_cb` field > + // within the pinned allocation, so it remains valid until > + // `dma_fence_remove_callback()` in `PinnedDrop` or until the > + // callback fires. > + let ret = unsafe { > + to_result(bindings::dma_fence_add_callback( > + fence.inner.get(), > + ptr, > + Some(Self::dma_fence_callback), > + )) > + }; > + match ret { > + Ok(()) => Ok(()), > + Err(e) => { > + // SAFETY: We could not register the callback. Thus, > + // C will not use it. So we can just take it back > + // and pass it to the user again. > + let cb_back = unsafe { ManuallyDrop::take(callback) }; > + if e == ENOENT { > + Err(CallbackError::AlreadySignaled(cb_back)) > + } else { > + Err(CallbackError::Other(e)) > + } > + }, > + } > + }), > + }? CallbackError<T>) > + } > + > + /// Raw dma fence callback that is called by the C code. > + /// > + /// # Safety > + /// > + /// This is only called by the dma_fence subsystem with valid pointers. > + unsafe extern "C" fn dma_fence_callback( > + _fence: *mut bindings::dma_fence, > + callback_foreign: *mut bindings::dma_fence_cb, > + ) { > + let ptr = Opaque::cast_from(callback_foreign).cast_mut(); > + > + // SAFETY: All `cb` we can receive here have been created in such a way > + // that they are embedded into a `FenceCallbackRegistration`. The > + // backend ensures synchronisation so whoever holds the registration > + // object cannot drop it while this code is running. See > + // `FenceCallbackRegistration::drop`. > + unsafe { > + let reg: *mut Self = container_of!(ptr, Self, callback_foreign); > + > + (*reg).callback.called(); > + } > + } > + > + /// Returns a reference to the fence this callback is registered on. > + pub fn fence(self: Pin<&Self>) -> &Fence { > + &self.get_ref().fence > + } This can just be `fence(&self) -> &Fence`. > +} > + > +#[pinned_drop] > +impl<T: FenceCallback> PinnedDrop for FenceCallbackRegistration<T> { > + fn drop(self: Pin<&mut Self>) { > + // Always call dma_fence_remove_callback, even if `callback` has already > + // been taken by `dma_fence_callback`. This is necessary for Is this still up-to-date? You're not taking callback anymore in `dma_fence_callback`. > + // synchronization: `dma_fence_remove_callback` acquires `fence->lock`, > + // which ensures that any in-flight `dma_fence_signal` (which calls our > + // callback while holding the same lock) has completed before we free > + // the struct. > + // > + // Without this, Drop can race with a concurrent signal: > + // CPU0 (signal, lock held): take() -> signaled(fence_ref) (in progress) > + // CPU1 (drop): sees is_some()==false -> skips lock -> frees struct > + // CPU0: accesses fence_ref -> use-after-free > + // > + // When the callback has already fired, the signal path detached the > + // list node via INIT_LIST_HEAD, so dma_fence_remove_callback just sees > + // an empty node and returns false — the lock acquisition is the only > + // thing that matters. > + // > + // SAFETY: The fence pointer is valid and the cb was initialized by > + // dma_fence_add_callback during construction. > + unsafe { > + bindings::dma_fence_remove_callback(self.fence.as_raw(), self.callback_foreign.get()); > + } > + > + // SAFETY: This is literally the drop implementation, so no one has > + // dropped this so far; so we can do it now. > + unsafe { ManuallyDrop::<T>::drop(self.project().callback) }; > + } > +} > + > +// SAFETY: FenceCallbackRegistration can be sent between threads. > +unsafe impl<T: FenceCallback> Send for FenceCallbackRegistration<T> {} > + > +// SAFETY: &FenceCallbackRegistration can be shared between threads if &T can. > +unsafe impl<T: FenceCallback> Sync for FenceCallbackRegistration<T> where T: Sync {} > + > +/// The receiving counterpart of a [`DriverFence`]. > +/// > +/// The Rust DMA fence implementation has a dualistic design: [`DriverFence`]s > +/// are the producer-side, intended to be always owned by only one party. That > +/// party has the monopoly on signalling the fence. > +/// > +/// A [`Fence`] is the counterpart for consumers. Thus, [`Fence`]s are always > +/// refcounted and can shared with an arbitrary number of parties, including > +/// userspace. A [`Fence`] can only be used for actions such as checking the > +/// fence's status or for registering callbacks on it. > +/// > +/// Once the associated [`DriverFence`] signals, all > +/// [`FenceCallbackRegistration`]s registered on the [`Fence`] will be executed. > +/// > +/// A [`Fence`] can arbitrarily outlive its [`DriverFence`] and the > +/// [`FenceContext`]. Signalling a [`DriverFence`] decouples it from its > +/// [`Fence`]s. > +#[repr(transparent)] > +pub struct Fence { > + /// The actual dma_fence passed to C. > + inner: Opaque<bindings::dma_fence>, > +} > + > +/// Guard helper for locking within this module. > +/// > +/// Its only purpose for now is to avoid a number of unsafe lock-unlock cycles. > +/// It is never used outside of this module. > +// TODO: This should be made more canonical, probably by basing it on a > +// SpinLockIrqGuard once available. > +struct FenceGuard { > + inner: *mut bindings::dma_fence, > + flags: usize, > +} > + > +impl Deref for FenceGuard { > + type Target = *mut bindings::dma_fence; Why not store and return `&Fence`? > + > + fn deref(&self) -> &Self::Target { > + &self.inner > + } > +} > + > +impl Drop for FenceGuard { > + fn drop(&mut self) { > + // SAFETY: `fence` is valid because `self` is valid. `flag_ptr` is > + // merely a pointer to an integer, which lives as long as this function. > + // When a `FenceGuard` exists, the lock has been taken by definition. > + unsafe { bindings::dma_fence_unlock_irqrestore(self.inner, &raw mut self.flags) }; > + } > +} > + > +// SAFETY: Fences are literally designed to be shared between threads. > +unsafe impl Send for Fence {} > +// SAFETY: Fences are literally designed to be shared between threads. > +unsafe impl Sync for Fence {} > + > +impl Fence { > + /// Check whether the fence was signaled at the moment of the function call. > + /// > + /// Note that this can return `true` for a [`Fence`] whose [`DriverFence`] > + /// has not yet been dropped. The reason is that the fence ops callbacks can > + /// cause the fence to get signaled by the C backend. > + pub fn is_signaled(&self) -> bool { > + // We should not use `dma_fence_is_signaled_locked()` here, because > + // according to the C backend's recommendations, that function is > + // problematic and we should avoid calling that function with a lock > + // held. > + > + // SAFETY: Inner `fence` is valid because `self` is valid. > + let ret = unsafe { bindings::dma_fence_is_signaled(self.as_raw()) }; > + > + // To be as robust as possible for the future we guarantee that an API > + // caller can 100% rely on the signalling being completed (i.e., all > + // fence callbacks ran), so we have to take the lock. > + // > + // The reason is that the C dma_fence backend currently does not > + // carefully synchronize the `dma_fence_is_signaled()` function with the > + // proper spinlock. This can lead to the function returning `true` while > + // fence callbacks are still being executed. This can be mitigated by > + // guarding the entire function with the spinlock. > + // > + // The fundamental reason is that the C backend currently does guard > + // setting of the fence's signaled-bit with the fence's spinlock, but > + // reading is done locklessly. > + // > + // See commit c8a5d5ea3ba6a. > + Extra newline here. > + let _ = self.lock(); > + > + ret > + } > + #[inline] here and many more below. > + fn lock(&self) -> FenceGuard { > + let mut guard = FenceGuard { > + inner: self.as_raw(), > + flags: 0, > + }; > + > + // SAFETY: `fence` is valid because `self` is valid. `flag_ptr` is > + // merely a pointer to an integer, whose lifetime is tied to the guard > + // object. > + unsafe { bindings::dma_fence_lock_irqsave(guard.inner, &raw mut guard.flags) }; > + > + guard > + } > + > + /// Get the fence's sequence number. > + pub fn seqno(&self) -> u64 { > + // SAFETY: Valid because `self` is valid. > + unsafe { (*self.as_raw()).seqno } > + } > + > + fn as_raw(&self) -> *mut bindings::dma_fence { > + self.inner.get() > + } > + > + /// Create a [`Fence`] from a raw C [`bindings::dma_fence`]. > + /// > + /// # Safety > + /// > + /// `ptr` must point to an initialized fence that is embedded into a [`Fence`]. > + pub unsafe fn from_raw<'a>(ptr: *mut bindings::dma_fence) -> &'a Self { > + // SAFETY: Safe as per the function's overall safety requirements. > + unsafe { &*ptr.cast() } > + } > +} > + > +// SAFETY: These implement the C backends refcounting methods which are proven > +// to work correctly. > +unsafe impl AlwaysRefCounted for Fence { > + fn inc_ref(&self) { > + // SAFETY: `self.as_raw()` is a pointer to a valid `struct dma_fence`. > + unsafe { bindings::dma_fence_get(self.as_raw()) } > + } > + > + /// # Safety > + /// > + /// `ptr`must be a valid pointer to a [`DriverFence`]. > + unsafe fn dec_ref(ptr: NonNull<Self>) { > + // SAFETY: `ptr` is never a NULL pointer; and when `dec_ref()` is called > + // the fence is by definition still valid. > + let fence = unsafe { (*ptr.as_ptr()).inner.get() }; > + > + // SAFETY: `fence` was created validly above. When `dec_ref()` is called, > + // there is by definition still a reference alive that can be put. > + unsafe { bindings::dma_fence_put(fence) } > + } > +} > + > +// Necessary to guarantee that `inner` always comes first and can be freed by C. > +// Also useful for using casts instead of container_of(). > +#[repr(C)] > +#[pin_data] > +struct DriverFenceData<'a, T: Send + Sync + FenceContextOps> { > + #[pin] > + /// The inner fence. > + // Must always be the first member so that unsafe casting works; but also > + // necessary so that the C backend can free the allocation (coming from our > + // Rust code) with kfree_rcu(). > + inner: Fence, > + /// Callback head for dropping this in a deferred manner through RCU. > + rcu_head: bindings::callback_head, > + /// Reference to access the FenceContext. Useful for obtaining name parameters. > + fctx: &'a FenceContext<T>, > + /// The API user's data. It is essential that the data only performs > + /// operations legal in atomic context in its [`Drop`] implementation. > + #[pin] > + data: T::FenceDataType, > +} > + > +/// A synchronization primitive mainly for GPU drivers. > +/// > +/// The Rust DMA fence implementation has a dualistic design: [`DriverFence`]s > +/// are the producer-side, intended to be always owned by only one party. That > +/// party has the monopoly on signalling the fence. > +/// > +/// A [`Fence`] is the counterpart for consumers. Thus, [`Fence`]s are always > +/// refcounted and can shared with an arbitrary number of parties, including > +/// userspace. A [`Fence`] can only be used for actions such as checking the > +/// fence's status or for registering callbacks on it. > +/// > +/// Once the associated [`DriverFence`] signals, all > +/// [`FenceCallbackRegistration`]s registered on a [`Fence`] will be executed. > +/// > +/// A [`Fence`] can arbitrarily outlive its [`DriverFence`] and the > +/// [`FenceContext`]. Signalling a [`DriverFence`] decouples it from its > +/// [`Fence`]s. > +/// > +/// It is crucial that a [`DriverFence`] always correctly represents the state > +/// of the associated job on the hardware. Especially, it is strictly necessary > +/// that the owner ensures that all [`DriverFence`]s eventually get signaled. > +/// As a last resort, a [`DriverFence`] will signal itself if it drops > +/// unsignaled and print a warning. > +/// > +/// This design intends to implement the [`bindings::dma_fence_ops`] in such a > +/// way that the driver-data necessary to implement the callback's functionality > +/// resides in the [`FenceContext`]. Thus, a [`DriverFence`] contains a > +/// reference to the context, which can be accessed in the callbacks. The > +/// implementation, therefore, ensures that a [`DriverFence`] cannot outlive its > +/// [`FenceContext`]. Unfortunately, this can be circumvented under certain > +/// circumstances in Rust (e.g., usage of [`core::mem::forget`]). > +/// > +/// In the unlikely case of such violations, error warnings are printed. > +/// > +/// # Examples > +/// > +/// ``` > +/// use kernel::dma_buf::{ > +/// DriverFence, > +/// FenceContext, > +/// FenceContextOps, > +/// FenceCallback, > +/// FenceCallbackRegistration, // > +/// }; > +/// use kernel::str::CString; > +/// use kernel::sync::aref::ARef; > +/// use core::fmt::Display; > +/// > +/// struct CallbackData { } > +/// > +/// impl FenceCallback for CallbackData { > +/// fn called(&mut self) { > +/// pr_info!("DmaFence callback executed.\n"); > +/// } > +/// } > +/// > +/// #[pin_data] > +/// struct FenceContextData {} > +/// > +/// impl FenceContextData { > +/// fn new() -> impl PinInit<Self> { > +/// pin_init!(Self {}) > +/// } > +/// } > +/// > +/// impl FenceContextOps for FenceContextData { > +/// type FenceDataType = FenceData; > +/// } > +/// > +/// let fctx_data = FenceContextData::new(); > +/// > +/// let driver_name = CString::try_from_fmt(fmt!("dummy_driver"))?; > +/// let timeline_name = CString::try_from_fmt(fmt!("dummy_timeline"))?; > +/// > +/// let mut fctx = KBox::pin_init( > +/// FenceContext::new(0, driver_name, timeline_name, fctx_data), GFP_KERNEL)?; > +/// > +/// struct FenceData { > +/// data: CString, > +/// } > +/// > +/// let data = CString::try_from_fmt(fmt!("dummy_data"))?; > +/// let fence_data = FenceData { data }; > +/// > +/// let fence_alloc = fctx.new_fence_allocation(fence_data)?; > +/// let mut fence = fence_alloc.new_fence(); > +/// > +/// let cb_data = CallbackData { }; > +/// let waiting_fence = ARef::from(fence.as_fence()); > +/// let cb_reg = FenceCallbackRegistration::new(&waiting_fence, cb_data); > +/// let cb_reg = KBox::pin_init(cb_reg, GFP_KERNEL)?; > +/// > +/// // TODO signalling guards > +/// fence.signal(Ok(())); > +/// assert_eq!(waiting_fence.is_signaled(), true); > +/// > +/// Ok::<(), Error>(()) > +/// ``` > +pub struct DriverFence<'a, T: Send + Sync + FenceContextOps> { > + /// The actual content of the fence. Lives in a [`NonNull`] so that its > + /// memory can be managed independently. Valid until both the [`DriverFence`] > + /// and all associated [`Fence`]s have disappeared. > + data: NonNull<DriverFenceData<'a, T>>, > +} > + > +/// A pre-prepared DMA fence, carrying the user's data and the memory it and the > +/// fence reside in. Only useful for creating a [`DriverFence`]. Splitting > +/// allocation and full initialization is necessary because fences cannot be > +/// allocated dynamically in some circumstances (deadlock). > +pub struct DriverFenceAllocation<'a, T: Send + Sync + FenceContextOps> { > + /// The memory for the actual content of the fence. > + /// Handed over to a [`DriverFence`], or deallocated once the > + /// [`DriverFenceAllocation`] drops. > + data: KBox<DriverFenceData<'a, T>>, > + /// Pointer for the ops for the associated [`FenceContext`] > + ops: *const bindings::dma_fence_ops, > +} > + > +impl<'a, T: Send + Sync + FenceContextOps> DriverFenceAllocation<'a, T> { > + /// Create a new fence, consuming `data`. There's no `data`. > + /// > + /// This increments the sequence number in the associated [`FenceContext`]. > + pub fn new_fence(self) -> DriverFence<'a, T> { > + // We feed the C dma_fence backend a NULL for the spinlock so that it > + // uses per-fence locks automatically. > + let null_ptr: *mut bindings::spinlock = ptr::null_mut(); > + let seqno = self.data.fctx.next_seqno(); > + let fence_ptr = self.as_raw(); > + // SAFETY: `fence_ptr` has been created directly above. It will live > + // at least as long as `Self`. The same applies to `&Self::OPS`. > + unsafe { > + bindings::dma_fence_init(fence_ptr, self.ops, null_ptr, self.data.fctx.nr, seqno) > + }; > + > + self.data.fctx.nr_of_unsignaled_fences.fetch_add(1, Relaxed); > + > + // A `DriverFenceAllocation`'s purpose is to carry allocated memory, so that > + // `DriverFence`s can always be created without allocating. In this > + // method, ownership over that memory is transferred to the new > + // `DriverFence` and managed through refcounting. The C dma_fence > + // backend will ultimately free the memory once the refcount reaches 0. > + let ptr = KBox::into_raw(self.data); > + // SAFETY: `ptr` was just created validly directly above. > + let ptr = unsafe { NonNull::new_unchecked(ptr) }; > + > + DriverFence { data: ptr } > + } > + > + fn as_raw(&self) -> *mut bindings::dma_fence { > + self.data.inner.inner.get() > + } > +} > + > +impl<'a, T: Send + Sync + FenceContextOps> DriverFence<'a, T> { > + fn as_raw(&self) -> *mut bindings::dma_fence { > + // SAFETY: Valid because `self` is valid. > + let fence_data = unsafe { &*self.data.as_ptr() }; > + > + fence_data.inner.inner.get() > + } > + > + /// Create a [`DriverFence`] from a raw pointer to a [`bindings::dma_fence`]. > + /// > + /// # Safety > + /// > + /// `ptr` must be a valid pointer to a `dma_fence` that was obtained through > + /// a [`DriverFence`] with matching generic data for both fence and associated > + /// [`FenceContext`]. > + unsafe fn from_raw(ptr: *mut bindings::dma_fence) -> Self { > + let opaque_fence = Opaque::cast_from(ptr); > + > + // SAFETY: Safe due to the function's overall safety requirements. > + let fence_ptr = unsafe { container_of!(opaque_fence, Fence, inner) }; > + > + // DriverFenceData is repr(C) and a Fence is its first member. > + let fence_data_ptr = fence_ptr as *mut DriverFenceData<'a, T>; > + > + // SAFETY: `fence_data_ptr` was created validly above. > + let data = unsafe { NonNull::new_unchecked(fence_data_ptr) }; > + > + Self { data } > + } > + > + /// Return the underlying [`Fence`]. > + pub fn as_fence(&self) -> &Fence { > + // SAFETY: `self` is by definition still valid, and it cannot drop until > + // this new reference is gone. > + unsafe { Fence::from_raw(self.as_raw()) } > + } > + > + /// Signal the fence. This will invoke all registered callbacks. > + pub fn signal(self, res: Result) { > + let fence = self.as_fence().lock(); > + > + // SAFETY: `fence` is valid because `self` is valid. The lock must be > + // held, which we acquired directly above. > + if !unsafe { bindings::dma_fence_test_signaled_flag(*fence.deref()) } { These `*fence.deref()` are quite weird as consequence of `FenceGuard` design. If `FenceGuard` just derefs to `&Fence` then this can be `fence.as_raw()`. > + if let Err(err) = res { > + // SAFETY: `fence` is valid because `self` is valid. The fence > + // must not have been signaled yet, which we check directly above. > + unsafe { bindings::dma_fence_set_error(*fence.deref(), err.to_errno()) }; > + } > + // SAFETY: `fence` is valid because `self` is valid. The lock must > + // be held, which we acquired above. > + unsafe { bindings::dma_fence_signal_locked(*fence.deref()) }; > + } > + > + // SAFETY: `self.data` is valid because `self` is valid. > + let fctx = unsafe { self.data.as_ref().fctx }; > + let _ = fctx.nr_of_unsignaled_fences.fetch_sub(1, Relaxed); Drop impl of `self` here will neededlessly take lock again before checking it's signaled already and unlock. > + } > +} > + > +// SAFETY: Fences are literally designed to be shared between threads. > +unsafe impl<'a, T: Send + Sync + FenceContextOps> Send for DriverFence<'a, T> {} > +// SAFETY: Fences are literally designed to be shared between threads. > +unsafe impl<'a, T: Send + Sync + FenceContextOps> Sync for DriverFence<'a, T> {} > + > +impl<'a, T: Send + Sync + FenceContextOps> Deref for DriverFence<'a, T> { > + type Target = T::FenceDataType; > + > + fn deref(&self) -> &Self::Target { > + // SAFETY: Thanks to refcounting, `data` is always valid as long as `self` is. > + let data = unsafe { &*self.data.as_ptr() }; > + > + &data.data > + } > +} > + > +/// A borrow wrapper for [`DriverFence`]. Implements [`Deref`]. > +pub struct DriverFenceBorrow<'a, T: Send + Sync + FenceContextOps> { > + driver_fence: ManuallyDrop<DriverFence<'a, T>>, > + _lifetime: PhantomData<&'a T>, > +} > + > +impl<'a, T: Send + Sync + FenceContextOps> Deref for DriverFenceBorrow<'a, T> { > + type Target = DriverFence<'a, T>; > + > + fn deref(&self) -> &Self::Target { > + self.driver_fence.deref() > + } > +} > + > +// SAFETY: The Rust dma_fence abstractions are already designed around the inner > +// C `dma_fence`, which can serve safely as the identification point when being > +// owned by C. Moreover, safety is ensured by not dropping `DriverFence` and by > +// only allowing operations without side effects on the Borrowed type. > +unsafe impl<T: Send + Sync + FenceContextOps + 'static> ForeignOwnable for DriverFence<'_, T> { The `'static` shouldn't be needed here. > + type Borrowed<'a> > + = DriverFenceBorrow<'a, T> > + where > + Self: 'a; > + type BorrowedMut<'a> > + = DriverFenceBorrow<'a, T> > + where > + Self: 'a; > + > + const FOREIGN_ALIGN: usize = core::mem::align_of::<bindings::dma_fence>(); > + > + fn into_foreign(self) -> *mut c_void { > + let fence = self; > + > + let ptr = fence.as_raw(); > + > + // DriverFence must not drop. > + let _ = ManuallyDrop::new(fence); > + > + ptr.cast() > + } > + > + unsafe fn from_foreign(ptr: *mut c_void) -> Self { > + // SAFETY: Safe because the trait implementation only invokes this with > + // a valid `ptr`, associated to a `DriverFence` with matching generic data. > + unsafe { Self::from_raw(ptr.cast()) } > + } > + > + unsafe fn borrow<'a>(ptr: *mut c_void) -> Self::Borrowed<'a> > + where > + Self: 'a, > + { > + // SAFETY: The trait implementation ensures that `ptr` always resides > + // within a [`Fence`] within a [`DriverFenceData`]. > + let driver_fence = unsafe { Self::from_raw(ptr.cast()) }; > + > + let driver_fence = ManuallyDrop::new(driver_fence); > + > + DriverFenceBorrow { > + driver_fence, > + _lifetime: PhantomData, > + } > + } > + > + unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> Self::BorrowedMut<'a> > + // FIXME: The bound below and the one above in `borrow` should actually be > + // unnecessary since the compiler should be able to completely derive all > + // necessary information automatically. There is currently a compiler bug > + // preventing that, though: > + // > + // https://github.com/rust-lang/rust/issues/155430. > + // > + // (Help to) fix the compiler bug and remove the bounds afterwards. > + where > + Self: 'a, > + { > + // SAFETY: The trait implementation ensures that `ptr` always resides > + // within a [`Fence`] within a [`DriverFenceData`]. > + let driver_fence = unsafe { Self::from_raw(ptr.cast()) }; > + > + let driver_fence = ManuallyDrop::new(driver_fence); > + > + DriverFenceBorrow { > + driver_fence, > + _lifetime: PhantomData, > + } > + } > +} > + > +impl<'a, T: Send + Sync + FenceContextOps> Drop for DriverFence<'a, T> { > + fn drop(&mut self) { > + let guard = self.as_fence().lock(); > + > + // Use dma_fence_test_signaled_flag() instead of > + // dma_fence_is_signaled_locked() because the C backend wants to get rid > + // of the latter. > + > + // SAFETY: `guard` is valid until the `call_rcu()` below. > + let signaled: bool = unsafe { bindings::dma_fence_test_signaled_flag(*guard.deref()) }; > + if !signaled { > + pr_err!("DriverFence drops unsignaled. Danger of memory corruption!\n"); > + // SAFETY: `guard` is valid until the `call_rcu()` below. The fence > + // must not have been signaled yet, which we check directly above. > + unsafe { bindings::dma_fence_set_error(*guard.deref(), ECANCELED.to_errno()) }; > + // SAFETY: `guard` is valid until the `call_rcu()` below. The lock > + // must be held, which we acquired above. > + unsafe { bindings::dma_fence_signal_locked(*guard.deref()) }; > + > + // SAFETY: `self.data` is valid because `self` is valid. > + let fctx = unsafe { self.data.as_ref().fctx }; > + let _ = fctx.nr_of_unsignaled_fences.fetch_sub(1, Relaxed); > + } > + drop(guard); > + > + // SAFETY: Valid because `self` is valid. > + let rcu_head_ptr = unsafe { &raw mut (*self.data.as_ptr()).rcu_head }; > + > + // `DriverFenceData` but could be accessed through some dma_fence > + // callbacks right now. Access is being revoked in principle above by > + // signalling the fence, but since the C backend does not guarantee > + // perfect full synchronization, we have to wait for one grace period to > + // ensure that all accessors of `DriverFenceData` (through the > + // dma_fence_ops accessible through a `Fence`) are gone. > + > + // SAFETY: `call_rcu()` is always safe to be called. `rcu_head_ptr` was > + // created validly above. The module must perform a `synchronize_rcu()` > + // or `rcu_barrier()` call to guard against module unload. > + unsafe { bindings::call_rcu(rcu_head_ptr, Some(drop_driver_fence_data::<T>)) }; I thought at some point it was mentioned that we want a fast path if !mem::needs_drop::<...>() { } ? > + } > +} > + > +// TODO: > +// The entire call_rcu() mechanism in the drop above and the code below would be > +// unnecessary if C's dma_fence_signal() could be reworked in a way that after it > +// ran, the caller knows that no fence_ops callbacks can be running anymore. > +// In other words, if the dma_fence backend would use its spinlock for full > +// synchronization. > +// > +// Then we could move the drop_in_place() and dma_fence_put() upwards into the > +// drop() implementation and call it a day. > + > +/// Finally really drop this `DriverFence<T>` > +/// > +/// # Safety > +/// > +/// `head` references the `rcu_head` field of an `DriverFenceData<T>`. All > +/// accessors to that `DriverFenceData<T>` must be gone by now. This must be > +/// ensured by signalling the associated `DriverFence<T>` and then waiting > +/// for a grace period until calling this function here. > +unsafe extern "C" fn drop_driver_fence_data<T: Send + Sync + FenceContextOps>( > + head: *mut bindings::callback_head, > +) { > + // SAFETY: Caller provides a pointer to the `rcu_head` field of a `DriverFenceData<C>`. > + let fence_data = unsafe { container_of!(head, DriverFenceData<'_, T>, rcu_head) }; > + > + // SAFETY: `fence_data` was created validly above. All the fence's data will > + // only drop below, but the raw pointer to the raw C `dma_fence` remains > + // valid because the reference count is only decremented at the end of the > + // function. > + let fence = unsafe { (*fence_data).inner.inner.get() }; > + > + // SAFETY: `fence_data` was created validly above. A grace period has passed. > + // All callbacks which might have had access to the `fctx` are gone now. > + unsafe { drop_in_place(&raw mut (*fence_data).fctx) }; fctx is just a reference, so this is a no-op. Best, Gary > + > + // SAFETY: `fence_data` was created validly above. The user has already > + // dropped the only conventional accessor to the user data, the `DriverFence`, > + // one grace period ago. All accessors are gone now. > + unsafe { drop_in_place(&raw mut (*fence_data).data) }; > + > + // The inner `Fence` explicitly does not get dropped because there may be > + // many more users / consumers, each holding their own reference. > + > + // SAFETY: Once a `DriverFence` is initialized, the inner `fence` is > + // valid and initialized. It is valid until the refcount drops > + // to 0, which can earliest happen once we drop the `DriverFence`'s reference > + // here. > + unsafe { bindings::dma_fence_put(fence) }; > + > + // The actual memory the data associated with a `DriverFence` lives in > + // gets freed by the C dma_fence backend once the fence's refcount reaches 0. > +} > diff --git a/rust/kernel/dma_buf/mod.rs b/rust/kernel/dma_buf/mod.rs > new file mode 100644 > index 000000000000..4764a828642e > --- /dev/null > +++ b/rust/kernel/dma_buf/mod.rs > @@ -0,0 +1,14 @@ > +// SPDX-License-Identifier: GPL-2.0 OR MIT > + > +//! DMA-buf subsystem abstractions. > + > +pub mod dma_fence; > + > +pub use self::dma_fence::{ > + DriverFence, > + Fence, > + FenceCallback, > + FenceCallbackRegistration, > + FenceContext, > + FenceContextOps, // > +}; > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs > index 68f4d9a3425d..6221ebfe71df 100644 > --- a/rust/kernel/lib.rs > +++ b/rust/kernel/lib.rs > @@ -67,6 +67,7 @@ > pub mod device_id; > pub mod devres; > pub mod dma; > +pub mod dma_buf; > pub mod driver; > #[cfg(CONFIG_DRM = "y")] > pub mod drm;

1 hour, 45 minutes

Re: [PATCH v3 06/11] mm/cma: Allow dynamically creating CMA areas

by T.J. Mercier

On Thu, Aug 6, 2026 at 9:21 AM Thierry Reding <thierry.reding(a)kernel.org> wrote: > > On Wed, Jul 08, 2026 at 04:49:31PM -0700, T.J. Mercier wrote: > > On Wed, Jul 1, 2026 at 9:09 AM Thierry Reding <thierry.reding(a)kernel.org> wrote: > > > > > > From: Thierry Reding <treding(a)nvidia.com> > > > > > > There is no technical reason why there should be a limited number of CMA > > > regions, so extract some code into helpers and use them to create extra > > > functions (cma_create() and cma_free()) that allow creating and freeing, > > > respectively, CMA regions dynamically at runtime. > > > > > > The static array of CMA areas cannot be replaced by dynamically created > > > areas because for many of them, allocation must not fail and some cases > > > may need to initialize them before the slab allocator is even available. > > > To account for this, keep these "early" areas in a separate list and > > > track the dynamic areas in a separate list. > > Huh... going over this thread again I could've sworn that I had replied > to this before, but I can't find any evidence of any reply. > > > Hi, It looks like you'll also need to update the CMA dma-buf heap's > > add_cma_heaps init function so that it adds all the CMA areas, not > > just the early ones. > > I would actually rather not do that. The case that we need this for is > creating a special purpose dma-buf heap that's composed of multiple CMA > areas. These areas must not be used by anyone else. Exposing these to > the CMA dma-buf heap would allow anyone to allocate from individual > chunks and throw off the accounting that we need to do in order to keep > the protected memory from working correctly. > > Besides, given the dynamic nature of this means that the CMA dma-buf > heap might already have probed when these dynamic CMA areas are added. > So we would either need to have some way of notifying the CMA dma-buf > heap of newly created areas or we might end up with an incomplete set > of registered heaps. > > If you really insist, I could look into this, but I think it's actually > a nice feature that the CMA dma-buf heap only registers these "early" > areas and exposes them. > > Thierry No problem, thanks for responding. I see there is now discussion about subdividing a single CMA area for this, and that seems like it could end up simpler in terms of the CMA code for this series. So I'm supportive of that idea.

11 hours, 37 minutes

Re: [PATCH 3/5] drm/amdgpu/userq: bound the eviction fence rearm retry loop

by Christian König

On 8/10/26 18:13, Junrui Luo via B4 Relay wrote: > From: Junrui Luo <moonafterrain(a)outlook.com> > > amdgpu_userq_ensure_ev_fence() loops until the eviction fence is both > present and unsignaled. The only producer of such a fence is > amdgpu_evf_mgr_rearm(), which runs as the very last step of > amdgpu_userq_vm_validate(). Every failure point ahead of it - the > kzalloc() in the rearm itself, amdgpu_hmm_range_alloc(), the > ttm_bo_validate() calls, the GART binding of the wptr BOs - makes > amdgpu_userq_restore_worker() give up with only a drm_file_err(). > Nothing propagates that back, so the waiting thread reschedules the > worker and flushes it again, forever. > > Both flush_delayed_work() and mutex_lock() sleep in > TASK_UNINTERRUPTIBLE, so the looping task cannot be killed and the OOM > killer cannot reclaim it. An unprivileged render node client > reaches this from both AMDGPU_USERQ and AMDGPU_USERQ_SIGNAL. > > The eviction fence sequence number is already bumped by every > successful rearm, so use it as the loop's progress condition: if a > completed flush of the restore worker did not move it then no rearm > happened and retrying cannot help. Return -ENOMEM in that case and > let both callers report it to userspace. > > Fixes: a242a3e4b5be ("drm/amdgpu: simplify eviction fence suspend/resume") > Reported-by: Yuhao Jiang <danisjiang(a)gmail.com> > Assisted-by: Claude:claude-opus-5 > Cc: stable(a)vger.kernel.org > Signed-off-by: Junrui Luo <moonafterrain(a)outlook.com> Absolutely clear NAK! This functions needs to loop forever should the rearm worker fails to re-arm the fence. The only thing which could potentially get out of that is to kill the process or maybe that the eviction fence is signaled with an error. Regards, Christian. > --- > drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c | 21 +++++++++++++++++++-- > drivers/gpu/drm/amd/amdgpu/amdgpu_userq.h | 4 ++-- > drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c | 10 +++++++++- > 3 files changed, 30 insertions(+), 5 deletions(-) > > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c > index bec107216811..208b53ae5bd1 100644 > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c > @@ -448,12 +448,16 @@ static void amdgpu_userq_cleanup(struct amdgpu_usermode_queue *queue) > * Ensures that a valid and not yet signaled eviction fence is attached to the > * usermode queue before any queue operations proceed. If it is signalled, then > * rearm a new eviction fence. > + * > + * Returns 0 with @uq_mgr->userq_mutex held, or -ENOMEM with the mutex released > + * when the restore worker could not rearm the fence. > */ > -void > +int > amdgpu_userq_ensure_ev_fence(struct amdgpu_userq_mgr *uq_mgr, > struct amdgpu_eviction_fence_mgr *evf_mgr) > { > struct dma_fence *ev_fence; > + int seq, prev_seq = -1; > > retry: > /* Flush any pending resume work to create ev_fence */ > @@ -463,7 +467,16 @@ amdgpu_userq_ensure_ev_fence(struct amdgpu_userq_mgr *uq_mgr, > ev_fence = amdgpu_evf_mgr_get_fence(evf_mgr); > if (dma_fence_is_signaled(ev_fence)) { > dma_fence_put(ev_fence); > + seq = atomic_read(&evf_mgr->ev_fence_seq); > mutex_unlock(&uq_mgr->userq_mutex); > + /* > + * The sequence number is only bumped by a successful rearm, so > + * if the flush above ran the worker without moving it then the > + * restore failed and looping again would never terminate. > + */ > + if (seq == prev_seq) > + return -ENOMEM; > + prev_seq = seq; > /* > * Looks like there was no pending resume work, > * add one now to create a valid eviction fence > @@ -472,6 +485,8 @@ amdgpu_userq_ensure_ev_fence(struct amdgpu_userq_mgr *uq_mgr, > goto retry; > } > dma_fence_put(ev_fence); > + > + return 0; > } > > > @@ -747,7 +762,9 @@ amdgpu_userq_create(struct drm_file *filp, union drm_amdgpu_userq *args) > if (r) > goto clean_mqd; > > - amdgpu_userq_ensure_ev_fence(&fpriv->userq_mgr, &fpriv->evf_mgr); > + r = amdgpu_userq_ensure_ev_fence(&fpriv->userq_mgr, &fpriv->evf_mgr); > + if (r) > + goto erase_doorbell; > > /* don't map the queue if scheduling is halted */ > if (!adev->userq_halt_for_enforce_isolation || > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.h b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.h > index 6412a7f7b6ef..c35909bf7ceb 100644 > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.h > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.h > @@ -164,8 +164,8 @@ void amdgpu_userq_mgr_fini(struct amdgpu_userq_mgr *userq_mgr); > > void amdgpu_userq_evict(struct amdgpu_userq_mgr *uq_mgr); > > -void amdgpu_userq_ensure_ev_fence(struct amdgpu_userq_mgr *userq_mgr, > - struct amdgpu_eviction_fence_mgr *evf_mgr); > +int amdgpu_userq_ensure_ev_fence(struct amdgpu_userq_mgr *userq_mgr, > + struct amdgpu_eviction_fence_mgr *evf_mgr); > > u32 amdgpu_userq_get_supported_ip_mask(struct amdgpu_device *adev); > bool amdgpu_userq_enabled(struct drm_device *dev); > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c > index 7e80442ec3e5..1c287ce59736 100644 > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c > @@ -523,7 +523,15 @@ int amdgpu_userq_signal_ioctl(struct drm_device *dev, void *data, > goto put_queue; > > /* We are here means UQ is active, make sure the eviction fence is valid */ > - amdgpu_userq_ensure_ev_fence(&fpriv->userq_mgr, &fpriv->evf_mgr); > + r = amdgpu_userq_ensure_ev_fence(&fpriv->userq_mgr, &fpriv->evf_mgr); > + if (r) { > + /* The fence is not initialized yet, so unwind it by hand */ > + amdgpu_userq_fence_put_fence_drv_array(fence); > + amdgpu_userq_fence_driver_put(fence->fence_drv); > + kvfree(fence->fence_drv_array); > + kfree(fence); > + goto put_queue; > + } > > /* Create the new fence */ > amdgpu_userq_fence_init(queue, fence, wptr); >

18 hours, 3 minutes

Re: [PATCH v4 06/10] dma-buf: heaps: Add debugfs support

by Christian König

On 8/7/26 17:54, Thierry Reding wrote: > From: Thierry Reding <treding(a)nvidia.com> > > Add a callback to struct dma_heap_ops that heap providers can implement > to show information about the state of the heap in debugfs. A top-level > directory named "dma_heap" is created in debugfs and individual files > will be named after the heaps. > > Reviewed-by: Maxime Ripard <mripard(a)kernel.org> > Signed-off-by: Thierry Reding <treding(a)nvidia.com> Reviewed-by: Christian König <christian.koenig(a)amd.com> > --- > drivers/dma-buf/dma-heap.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++ > include/linux/dma-heap.h | 2 ++ > 2 files changed, 58 insertions(+) > > diff --git a/drivers/dma-buf/dma-heap.c b/drivers/dma-buf/dma-heap.c > index 3937dd41bb0f..389cddd07c8d 100644 > --- a/drivers/dma-buf/dma-heap.c > +++ b/drivers/dma-buf/dma-heap.c > @@ -7,6 +7,7 @@ > */ > > #include <linux/cdev.h> > +#include <linux/debugfs.h> > #include <linux/device.h> > #include <linux/dma-buf.h> > #include <linux/dma-heap.h> > @@ -225,6 +226,46 @@ const char *dma_heap_get_name(struct dma_heap *heap) > } > EXPORT_SYMBOL_NS_GPL(dma_heap_get_name, "DMA_BUF_HEAP"); > > +#ifdef CONFIG_DEBUG_FS > +static int dma_heap_debug_show(struct seq_file *s, void *unused) > +{ > + struct dma_heap *heap = s->private; > + int err = 0; > + > + if (heap->ops && heap->ops->show) > + err = heap->ops->show(s, heap); > + > + return err; > +} > +DEFINE_SHOW_ATTRIBUTE(dma_heap_debug); > + > +static struct dentry *dma_heap_debugfs_dir; > + > +static void dma_heap_init_debugfs(void) > +{ > + struct dentry *dir; > + > + dir = debugfs_create_dir("dma_heap", NULL); > + if (IS_ERR(dir)) > + return; > + > + dma_heap_debugfs_dir = dir; > +} > + > +static void dma_heap_exit_debugfs(void) > +{ > + debugfs_remove_recursive(dma_heap_debugfs_dir); > +} > +#else > +static void dma_heap_init_debugfs(void) > +{ > +} > + > +static void dma_heap_exit_debugfs(void) > +{ > +} > +#endif > + > /** > * dma_heap_add - adds a heap to dmabuf heaps > * @exp_info: information needed to register this heap > @@ -299,6 +340,13 @@ struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info) > > /* Add heap to the list */ > list_add(&heap->list, &heap_list); > + > +#ifdef CONFIG_DEBUG_FS > + if (heap->ops && heap->ops->show) > + debugfs_create_file(heap->name, 0444, dma_heap_debugfs_dir, > + heap, &dma_heap_debug_fops); > +#endif > + > mutex_unlock(&heap_list_lock); > > return heap; > @@ -335,6 +383,14 @@ static int dma_heap_init(void) > } > dma_heap_class->devnode = dma_heap_devnode; > > + dma_heap_init_debugfs(); > + > return 0; > } > subsys_initcall(dma_heap_init); > + > +static void __exit dma_heap_exit(void) > +{ > + dma_heap_exit_debugfs(); > +} > +__exitcall(dma_heap_exit); > diff --git a/include/linux/dma-heap.h b/include/linux/dma-heap.h > index 648328a64b27..1c9bed1f4dde 100644 > --- a/include/linux/dma-heap.h > +++ b/include/linux/dma-heap.h > @@ -12,6 +12,7 @@ > #include <linux/types.h> > > struct dma_heap; > +struct seq_file; > > /** > * struct dma_heap_ops - ops to operate on a given heap > @@ -24,6 +25,7 @@ struct dma_heap_ops { > unsigned long len, > u32 fd_flags, > u64 heap_flags); > + int (*show)(struct seq_file *s, struct dma_heap *heap); > }; > > /** >

23 hours, 6 minutes

Re: [PATCH v9 0/5] rust / dma_buf: Add abstractions for dma_fence

by Miguel Ojeda

On Wed, Aug 5, 2026 at 5:01 PM Philipp Stanner <phasta(a)kernel.org> wrote: > > rust: types: implement ForeignOwnable for ARef<T> > rust: sync: Add abstraction for rcu_barrier() > rust: error: add remaining error codes Applied (these three) to `rust-next` -- thanks everyone! Please note that your Signed-off-by is required when sending a patch from someone else: https://docs.kernel.org/process/submitting-patches.html#sign-your-work-the-… I added it here on apply for you -- please let me know if that is not what you intended! [ Formatted comments. Added the submitter's Signed-off-by tag. - Miguel ] [ Relaxed `'static` bound and added `#[inline]` as discussed. Added the submitter's Signed-off-by tag. - Miguel ] [ Formatted documentation. Sorted tags. - Miguel ] Thanks! Cheers, Miguel

1 day, 3 hours

Re: [PATCH net-next v8 0/3] net: devmem: allow rx-buf-size > PAGE_SIZE per binding

by patchwork-bot+netdevbpf＠kernel.org

Hello: This series was applied to netdev/net-next.git (main) by Jakub Kicinski <kuba(a)kernel.org>: On Wed, 05 Aug 2026 13:42:42 -0700 you wrote: > Every devmem dmabuf binding hands the page_pool PAGE_SIZE niovs today. > On NICs that consume one descriptor per netmem, this caps a single RX > descriptor at PAGE_SIZE and burns CPU on buffer churn. > > In this series, we add a bind-time netlink attribute, > NETDEV_A_DMABUF_RX_BUF_SIZE, that lets userspace request a larger niov > size (power of two >= PAGE_SIZE). Drivers must opt in via > queue_mgmt_ops.QCFG_RX_PAGE_SIZE. > > [...] Here is the summary with links: - [net-next,v8,1/3] net: devmem: allow rx-page-size > PAGE_SIZE per dmabuf binding https://git.kernel.org/netdev/net-next/c/b27a8560eec9 - [net-next,v8,2/3] selftests/net: ncdevmem: add -b option to set rx-page-size on bind https://git.kernel.org/netdev/net-next/c/3e8c9ec4eb75 - [net-next,v8,3/3] selftests/net: devmem.py: add check_rx_large_niov https://git.kernel.org/netdev/net-next/c/8ac4255c1e0c You are awesome, thank you! -- Deet-doot-dot, I am a bot. https://korg.docs.kernel.org/patchwork/pwbot.html

3 days, 9 hours

Re: [PATCH v6 4/4] selftests: dmabuf-heaps: add fd-leak-on-EFAULT regression test

by T.J. Mercier

On Fri, Aug 7, 2026 at 3:12 AM Baineng Shou <shoubaineng(a)gmail.com> wrote: > > Add a test case that verifies no file descriptor is leaked when > DMA_HEAP_IOCTL_ALLOC succeeds internally but copy_to_user() fails > to deliver the fd number back to userspace. > > The failure is triggered by placing the ioctl argument in a private > anonymous page and flipping it to PROT_READ (via mprotect) between > the kernel's copy_from_user() and copy_to_user() calls. With the > buggy kernel the ioctl returns -EFAULT but leaves an extra open fd > in the process's fd table; with the fixed kernel the fd count is > unchanged. > > This serves as a regression test for: > "dma-buf: dma-heap: don't publish fd before copy_to_user() succeeds" > > Suggested-by: Sumit Semwal <sumit.semwal(a)linaro.org> > Signed-off-by: Baineng Shou <shoubaineng(a)gmail.com> Reviewed-by: T.J. Mercier <tjmercier(a)google.com>

3 days, 13 hours

Re: [PATCH v9 2/5] rust: types: implement ForeignOwnable for ARef<T>

by Gary Guo

On Wed Aug 5, 2026 at 3:59 PM BST, Philipp Stanner wrote: > From: Danilo Krummrich <dakr(a)kernel.org> > > Implement ForeignOwnable for ARef<T>, making it possible for C code to > own an ARef<T>. > > Since ARef represents shared ownership, BorrowedMut is &T rather than > &mut T, matching the semantics of the underlying reference-counted type. > > Signed-off-by: Danilo Krummrich <dakr(a)kernel.org> > Reviewed-by: Alice Ryhl <aliceryhl(a)google.com> > Tested-by: Daniel Almeida <daniel.almeida(a)collabora.com> > --- > rust/kernel/sync/aref.rs | 40 ++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 40 insertions(+) > > diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs > index b721b2e00b98..540766613659 100644 > --- a/rust/kernel/sync/aref.rs > +++ b/rust/kernel/sync/aref.rs > @@ -24,6 +24,11 @@ > ptr::NonNull, // > }; > > +use crate::{ > + prelude::*, > + types::ForeignOwnable, // > +}; > + > /// Types that are _always_ reference counted. > /// > /// It allows such types to define their own custom ref increment and decrement functions. > @@ -188,6 +193,41 @@ fn eq(&self, other: &ARef<U>) -> bool { > } > impl<T: AlwaysRefCounted + Eq> Eq for ARef<T> {} > > +// SAFETY: `into_foreign` returns a pointer from `NonNull::as_ptr`, so it's non-null. The > +// `ARef` invariant guarantees that `ptr` points to a valid `T`, so it's aligned to `T`. > +unsafe impl<T: AlwaysRefCounted + 'static> ForeignOwnable for ARef<T> { > + const FOREIGN_ALIGN: usize = core::mem::align_of::<T>(); > + > + type Borrowed<'a> = &'a T; > + type BorrowedMut<'a> = &'a T; > + `#[inline]` here and all others. Best, Gary > + fn into_foreign(self) -> *mut c_void { > + ARef::into_raw(self).as_ptr().cast() > + } > + > + unsafe fn from_foreign(ptr: *mut c_void) -> Self { > + // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous > + // call to `Self::into_foreign`. > + let ptr = unsafe { NonNull::new_unchecked(ptr.cast()) }; > + > + // SAFETY: `ptr` came from `into_foreign`, which consumed an `ARef` without decrementing > + // the refcount, so we can transfer the ownership to the new `ARef`. > + unsafe { ARef::from_raw(ptr) } > + } > + > + unsafe fn borrow<'a>(ptr: *mut c_void) -> &'a T { > + // SAFETY: The safety requirements of this method ensure that the object remains alive and > + // immutable for the duration of 'a. > + unsafe { &*ptr.cast() } > + } > + > + unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> &'a T { > + // SAFETY: The safety requirements for `borrow_mut` are a superset of the safety > + // requirements for `borrow`. > + unsafe { <Self as ForeignOwnable>::borrow(ptr) } > + } > +} > + > impl<T, U> PartialEq<&'_ U> for ARef<T> > where > T: AlwaysRefCounted + PartialEq<U>,

4 days, 21 hours

Re: [PATCH v4 01/14] dma-buf: introduce initial file I/O infrastructure

by Christian König

On 8/5/26 12:59, Pavel Begunkov wrote: > On 8/5/26 09:27, Christian König wrote: >>> + >>> + dma_resv_lock(dmabuf->resv, NULL); >>> + ctx->dev_ops->unmap(ctx, map); >>> + dma_resv_unlock(dmabuf->resv); >>> + >>> + dma_fence_put(&fence->base); >> >> You should probably set map->fence to NULL after that. > > The map is freed two lines below, but I can add it as > a defensive measure. In that case it's ok, I've just haven't seen the kfree(map) below. > ... >>> + ret = dma_resv_reserve_fences(dmabuf->resv, 1); >>> + if (WARN_ON_ONCE(ret)) { >>> + struct dma_fence *fence = &map->fence->base; >>> + >>> + dma_fence_get(fence); >>> + percpu_ref_kill(&map->refs); >>> + dma_fence_wait(fence, false); >>> + dma_fence_put(fence); >>> + return; >>> + } >>> + >>> + dma_resv_add_fence(dmabuf->resv, &map->fence->base, >>> + DMA_RESV_USAGE_KERNEL); >> >> That sequence is clearly incorrect! >> >> The fence must be created after dma_resv_reserve_fences(), otherwise you definately have an illegal memory operation here. > > I'm not sure what you mean, can you elaborate? I only cared about > pre-allocating it to avoid allocations here. We add / signal the fence > only once, no reuse. The map is going to be killed here, and if we > create a new map, it'll have its own fence. > > I can move the dma_fence_init() call here if that makes a difference? Yeah that is a good start, but you might need a bit more. Here is a summary of the usual procedure you need to follow when implementing a dma_fence backend: 1. Allocate your operation object, in this case here it's your mapping I think. 2. Prepare your operation, including all memory allocations. 3. Call dma_resv_reserve_fences() to reserve a fence slot. 4. Allocate and init your dma_fence object. After this step no memory allocation is allowed any more until your dma_fence object signals. The only exception is optional logging or crash dumping using GFP_NOWAIT (can fail trivially!) or minimal allocations using GFP_ATOMIC if you absolutely have to. 5. dma_resv_add_fence() to publish the fence. 6. dma_resv_unlock(). Having a dma_fence is certainly nice to have, but the tricky part is that memory allocations using GFP_KERNEL (or GFP_IO, GFP_FS etc...) can cycle back and wait for your dma_fence to signal which essentially can cause a deadlock very deeply inside memory management. Since those deadlocks happen only on memory contention situations they are usually just hard to reproduce but still totally break your neck if you manage to mess this up. So that needs to be super carefully implemented. Regards, Christian.

5 days, 4 hours

Re: [PATCH v4 01/14] dma-buf: introduce initial file I/O infrastructure

by Christian König

On 7/28/26 23:29, Pavel Begunkov wrote: > The goal is to be able to natively use dma-buf in the read-write / IO > path. This patch adds basic building blocks serving as a glue and API > between drivers and upper layer subsystems providing the uAPI. Later > patches implement it for NVMe raw block devices and expose it to the > user space via io_uring. > > There are two main objects. struct dma_buf_io_ctx and struct > dma_buf_io_map. The ctx is used during initial registration and serves > as an interface between the upper layer user like io_uring and to the > importer subsystem / driver. The map represents the actual dma map > established for the target device[s] with dma_buf_map_attachment() and > stored in a device specific format. The context is created via a new > file operation ->init_dma_buf_io_ctx. > > The ctx-map separation exists to support map invalidation (see > dma_buf_io_invalidate_mappings()). A ctx can create > multiple maps during its lifetime, but there can only be no more than > one (active) map attached to it. Invalidation drops the active map > if present, and the next map will only be attempted to be created > once there is a new request that wants to use the dma-buf IO ctx. > > The primary task of the dma_buf_io_map object is to count requests > using it and to wait for their completion when we want to destroy the > DMA map. Invalidation is delayed by inserting a dma fence, which is > signaled when all flight requests are completed. > > [un]mapping and any work with dma addresses is delegated to the > importer driver via an ops table stored in the ctx, see struct > dma_buf_io_ops. Only the target driver / subsystem knows about devices > it wants to use the dma-buf with, especially in case of multi-device > filesystems or stacking in the future. > > Signed-off-by: Pavel Begunkov <asml.silence(a)gmail.com> > --- > drivers/dma-buf/Makefile | 2 +- > drivers/dma-buf/dma-buf-io.c | 275 +++++++++++++++++++++++++++++++++++ > include/linux/dma-buf-io.h | 91 ++++++++++++ > include/linux/fs.h | 2 + > 4 files changed, 369 insertions(+), 1 deletion(-) > create mode 100644 drivers/dma-buf/dma-buf-io.c > create mode 100644 include/linux/dma-buf-io.h > > diff --git a/drivers/dma-buf/Makefile b/drivers/dma-buf/Makefile > index b25d7550bacf..523731b0f83e 100644 > --- a/drivers/dma-buf/Makefile > +++ b/drivers/dma-buf/Makefile > @@ -1,6 +1,6 @@ > # SPDX-License-Identifier: GPL-2.0-only > obj-y := dma-buf.o dma-fence.o dma-fence-array.o dma-fence-chain.o \ > - dma-fence-unwrap.o dma-resv.o dma-buf-mapping.o > + dma-fence-unwrap.o dma-resv.o dma-buf-mapping.o dma-buf-io.o > obj-$(CONFIG_DMABUF_HEAPS) += dma-heap.o > obj-$(CONFIG_DMABUF_HEAPS) += heaps/ > obj-$(CONFIG_SYNC_FILE) += sync_file.o > diff --git a/drivers/dma-buf/dma-buf-io.c b/drivers/dma-buf/dma-buf-io.c > new file mode 100644 > index 000000000000..5f545a1bcf32 > --- /dev/null > +++ b/drivers/dma-buf/dma-buf-io.c > @@ -0,0 +1,275 @@ > +/* SPDX-License-Identifier: GPL-2.0 */ > +/* > + * Common infrastructure for supporing dma-buf in the I/O path. > + * > + * Copyright (C) 2026 Pavel Begunkov <asml.silence(a)gmail.com> > + */ > +#include <linux/dma-buf-io.h> > +#include <linux/dma-resv.h> > + > +struct dma_buf_io_fence { > + struct dma_fence base; > + spinlock_t lock; > +}; Upstream has change to allow embedding the spinlock into the dma_fence, so this structure here is most likely not necessary any more. > + > +static const char *dma_buf_io_fence_drv_name(struct dma_fence *fence) > +{ > + /* default fence release kfree's the base pointer */ > + BUILD_BUG_ON(offsetof(struct dma_buf_io_fence, base)); > + > + return "dma-buf-io-ctx"; > +} > + > +static const char *dma_buf_io_fence_timeline_name(struct dma_fence *fence) > +{ > + return "dma-buf-io-ctx"; > +} > + > +const struct dma_fence_ops dma_buf_io_fence_ops = { > + .get_driver_name = dma_buf_io_fence_drv_name, > + .get_timeline_name = dma_buf_io_fence_timeline_name, > +}; > + > +static void dma_buf_io_ctx_destroy_work(struct work_struct *work) > +{ > + struct dma_buf_io_ctx *ctx = container_of(work, struct dma_buf_io_ctx, > + release_work); > + > + if (WARN_ON_ONCE(refcount_read(&ctx->refs))) > + return; > + > + ctx->dev_ops->release(ctx); > + dma_buf_put(ctx->dmabuf); > + kfree(ctx); > +} > + > +static void dma_buf_io_map_release_work(struct work_struct *work) > +{ > + struct dma_buf_io_map *map = container_of(work, struct dma_buf_io_map, > + release_work); > + struct dma_buf_io_fence *fence = map->fence; > + struct dma_buf_io_ctx *ctx = map->ctx; > + struct dma_buf *dmabuf = ctx->dmabuf; > + > + /* the release path must wait for fences */ > + if (WARN_ON_ONCE(refcount_read(&ctx->refs) == 0)) > + return; Stuff like that is usually illegal. And why are you using refcount directly instead of kref? > + > + /* Prevent from destoying the ctx while unmapping */ > + refcount_inc(&ctx->refs); > + > + /* > + * There are no more requests using the map, we can signal the fence. > + * It should be done before taking the resv lock as someone could be > + * waiting for the fence while holding the lock. > + */ > + dma_fence_signal(&fence->base); Signaling fences has a whole bunch of very strict rules associated with it. E.g. you can't alocate memory for example. Are you sure you actually need and want a dma_fence here? > + > + dma_resv_lock(dmabuf->resv, NULL); > + ctx->dev_ops->unmap(ctx, map); > + dma_resv_unlock(dmabuf->resv); > + > + dma_fence_put(&fence->base); You should probably set map->fence to NULL after that. > + percpu_ref_exit(&map->refs); > + kfree(map); > + > + if (refcount_dec_and_test(&ctx->refs)) { > + /* > + * Destruction needs to wait for I/O and dma fences. Defer it to > + * simplify locking. > + */ > + INIT_WORK(&ctx->release_work, dma_buf_io_ctx_destroy_work); > + queue_work(system_wq, &ctx->release_work); > + } > +} > + > +static void dma_buf_io_map_refs_release(struct percpu_ref *ref) > +{ > + struct dma_buf_io_map *map = container_of(ref, struct dma_buf_io_map, refs); > + > + /* might sleep, use a worker */ > + INIT_WORK(&map->release_work, dma_buf_io_map_release_work); > + queue_work(system_wq, &map->release_work); > +} > + > +int dma_buf_io_init_map(struct dma_buf_io_ctx *ctx, struct dma_buf_io_map *map) > +{ > + struct dma_buf_io_fence *fence = NULL; > + int ret; > + > + fence = kzalloc(sizeof(*fence), GFP_KERNEL); > + if (!fence) > + return -ENOMEM; > + > + ret = percpu_ref_init(&map->refs, dma_buf_io_map_refs_release, 0, GFP_KERNEL); > + if (ret) { > + kfree(fence); > + return ret; > + } > + > + spin_lock_init(&fence->lock); > + dma_fence_init(&fence->base, &dma_buf_io_fence_ops, &fence->lock, > + ctx->fence_ctx, atomic_inc_return(&ctx->fence_seq)); > + map->fence = fence; > + map->ctx = ctx; > + return 0; > +} > +EXPORT_SYMBOL_NS_GPL(dma_buf_io_init_map, "DMA_BUF"); > + > +struct dma_buf_io_map *dma_buf_io_create_map(struct dma_buf_io_ctx *ctx) > +{ > + struct dma_buf *dmabuf = ctx->dmabuf; > + struct dma_buf_io_map *map; > + long ret; > + > +retry: > + /* > + * ->dmabuf_map() will be calling dma_buf_map_attachment(), for which > + * we'll need to wait for fences. Do a bit nicer and try to wait > + * without the resv lock first. > + */ Clear NAK to that. Always wait while holding the resv lock if you can! It is absolutely not beneficial to do this outside of the lock and usually just hides problems instead and prevent fixing them. > + ret = dma_resv_wait_timeout(dmabuf->resv, DMA_RESV_USAGE_KERNEL, > + true, MAX_SCHEDULE_TIMEOUT); > + if (!ret) > + ret = -EAGAIN; > + if (ret < 0) > + return ERR_PTR(ret); > + > + ret = dma_resv_lock_interruptible(dmabuf->resv, NULL); > + if (ret) > + return ERR_PTR(ret); > + > + map = dma_buf_io_get_map(ctx); > + if (map) { > + ret = 0; > + goto out; > + } > + > + if (dma_resv_wait_timeout(dmabuf->resv, DMA_RESV_USAGE_KERNEL, > + true, 0) < 0) { > + dma_resv_unlock(dmabuf->resv); > + goto retry; > + } > + > + map = ctx->dev_ops->map(ctx); > + if (IS_ERR(map)) { > + ret = PTR_ERR(map); > + goto out; > + } > + > + percpu_ref_get(&map->refs); > + rcu_assign_pointer(ctx->map, map); > +out: > + dma_resv_unlock(dmabuf->resv); > + if (ret < 0) > + return ERR_PTR(ret); > + return map; > +} > + > +static void dma_buf_io_drop_map(struct dma_buf_io_ctx *ctx) > +{ > + struct dma_buf *dmabuf = ctx->dmabuf; > + struct dma_buf_io_map *map; > + int ret; > + > + dma_resv_assert_held(dmabuf->resv); > + > + map = rcu_dereference_protected(ctx->map, > + dma_resv_held(dmabuf->resv)); > + if (!map) > + return; > + rcu_assign_pointer(ctx->map, NULL); > + > + ret = dma_resv_reserve_fences(dmabuf->resv, 1); > + if (WARN_ON_ONCE(ret)) { > + struct dma_fence *fence = &map->fence->base; > + > + dma_fence_get(fence); > + percpu_ref_kill(&map->refs); > + dma_fence_wait(fence, false); > + dma_fence_put(fence); > + return; > + } > + > + dma_resv_add_fence(dmabuf->resv, &map->fence->base, > + DMA_RESV_USAGE_KERNEL); That sequence is clearly incorrect! The fence must be created after dma_resv_reserve_fences(), otherwise you definately have an illegal memory operation here. Regards, Christian. > + /* > + * Delay destruction until all inflight requests using the map arefence-> > + * gone. It'll also signal the fence then. > + */ > + percpu_ref_kill(&map->refs); > +} > + > +void dma_buf_io_invalidate_mappings(struct dma_buf_io_ctx *ctx) > +{ > + dma_buf_io_drop_map(ctx); > +} > +EXPORT_SYMBOL_NS_GPL(dma_buf_io_invalidate_mappings, "DMA_BUF"); > + > +static void dma_buf_io_ctx_release_work(struct work_struct *work) > +{ > + struct dma_buf_io_ctx *ctx = container_of(work, struct dma_buf_io_ctx, > + release_work); > + struct dma_buf *dmabuf = ctx->dmabuf; > + long ret; > + > + dma_resv_lock(dmabuf->resv, NULL); > + /* Remove the last map, there should be no new ones going forward. */ > + dma_buf_io_drop_map(ctx); > + dma_resv_unlock(dmabuf->resv); > + > + /* Wait until all maps are destroyed. */ > + ret = dma_resv_wait_timeout(dmabuf->resv, DMA_RESV_USAGE_KERNEL, > + false, MAX_SCHEDULE_TIMEOUT); > + > + if (WARN_ON_ONCE(ret <= 0)) > + return; > + if (WARN_ON_ONCE(rcu_dereference_protected(ctx->map, true))) > + return; > + > + if (refcount_dec_and_test(&ctx->refs)) > + dma_buf_io_ctx_destroy_work(&ctx->release_work); > +} > + > +void dma_buf_io_ctx_release(struct dma_buf_io_ctx *ctx) > +{ > + /* > + * Destruction needs to wait for I/O and dma fences. Defer it to > + * simplify locking. > + */ > + INIT_WORK(&ctx->release_work, dma_buf_io_ctx_release_work); > + queue_work(system_wq, &ctx->release_work); > +} > + > +int dma_buf_io_ctx_create(struct file *file, > + struct dma_buf_io_ctx *ctx, > + struct dma_buf *dmabuf, > + enum dma_data_direction dir) > +{ > + int ret; > + > + if (!file->f_op->init_dma_buf_io_ctx) > + return -EOPNOTSUPP; > + > + memset(ctx, 0, sizeof(*ctx)); > + ctx->fence_ctx = dma_fence_context_alloc(1); > + ctx->dir = dir; > + ctx->dmabuf = dmabuf; > + refcount_set(&ctx->refs, 1); > + get_dma_buf(dmabuf); > + > + ret = file->f_op->init_dma_buf_io_ctx(file, ctx); > + if (ret) { > + memset(ctx, 0, sizeof(*ctx)); > + dma_buf_put(dmabuf); > + return ret; > + } > + > + if (WARN_ON_ONCE(!ctx->dev_ops || > + !ctx->dev_ops->map || > + !ctx->dev_ops->unmap || > + !ctx->dev_ops->release)) > + return -EINVAL; > + > + return ret; > +} > diff --git a/include/linux/dma-buf-io.h b/include/linux/dma-buf-io.h > new file mode 100644 > index 000000000000..bcffc6202556 > --- /dev/null > +++ b/include/linux/dma-buf-io.h > @@ -0,0 +1,91 @@ > +/* SPDX-License-Identifier: GPL-2.0 */ > +#ifndef __DMA_BUF_IO_H__ > +#define __DMA_BUF_IO_H__ > + > +#include <linux/dma-buf.h> > + > +struct dma_buf_io_fence; > +struct dma_buf_io_ctx; > +struct dma_buf_io_map; > + > +struct dma_buf_io_ops { > + /* > + * Create a new map for the given ctx. Called with the reservation > + * lock held. > + */ > + struct dma_buf_io_map *(*map)(struct dma_buf_io_ctx *ctx); > + > + /* > + * Clean up device specific parts of the @map. Called with the > + * reservation lock held. > + */ > + void (*unmap)(struct dma_buf_io_ctx *ctx, struct dma_buf_io_map *map); > + > + /* > + * The user tries to destroy the ctx. Release all device specific > + * parts of the token. > + */ > + void (*release)(struct dma_buf_io_ctx *); > +}; > + > +struct dma_buf_io_map { > + /* > + * Counts attached requests and other users. Device specific unmapping > + * is deferred until all refs are dropped. > + */ > + struct percpu_ref refs; > + > + struct work_struct release_work; > + struct dma_buf_io_fence *fence; > + struct dma_buf_io_ctx *ctx; > +}; > + > +struct dma_buf_io_ctx { > + struct dma_buf_io_map __rcu *map; > + struct dma_buf *dmabuf; > + enum dma_data_direction dir; > + > + atomic_t fence_seq; > + u64 fence_ctx; > + struct work_struct release_work; > + refcount_t refs; > + > + void *dev_priv; > + const struct dma_buf_io_ops *dev_ops; > +}; > + > +int dma_buf_io_ctx_create(struct file *file, > + struct dma_buf_io_ctx *ctx, > + struct dma_buf *dmabuf, > + enum dma_data_direction dir); > +void dma_buf_io_ctx_release(struct dma_buf_io_ctx *ctx); > + > +struct dma_buf_io_map *dma_buf_io_create_map(struct dma_buf_io_ctx *ctx); > + > +static inline struct dma_buf_io_map * > +dma_buf_io_get_map(struct dma_buf_io_ctx *ctx) > +{ > + struct dma_buf_io_map *map; > + > + guard(rcu)(); > + > + map = rcu_dereference(ctx->map); > + if (unlikely(!map || !percpu_ref_tryget_live_rcu(&map->refs))) > + return NULL; > + > + return map; > +} > + > +static inline void dma_buf_io_map_drop(struct dma_buf_io_map *map) > +{ > + percpu_ref_put(&map->refs); > +} > + > +/* > + * Device API > + */ > + > +void dma_buf_io_invalidate_mappings(struct dma_buf_io_ctx *ctx); > +int dma_buf_io_init_map(struct dma_buf_io_ctx *ctx, struct dma_buf_io_map *map); > + > +#endif > diff --git a/include/linux/fs.h b/include/linux/fs.h > index d10897b3a1e3..9fe349a279c7 100644 > --- a/include/linux/fs.h > +++ b/include/linux/fs.h > @@ -1914,6 +1914,7 @@ struct dir_context { > #define COPY_FILE_SPLICE (1 << 0) > > struct io_uring_cmd; > +struct dma_buf_io_ctx; > struct offset_ctx; > > typedef unsigned int __bitwise fop_flags_t; > @@ -1962,6 +1963,7 @@ struct file_operations { > int (*uring_cmd_iopoll)(struct io_uring_cmd *, struct io_comp_batch *, > unsigned int poll_flags); > int (*mmap_prepare)(struct vm_area_desc *); > + int (*init_dma_buf_io_ctx)(struct file *, struct dma_buf_io_ctx *); > } __randomize_layout; > > /* Supports async buffered reads */

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