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Re: [PATCH 1/9] dma-buf: add dma_fence_was_initialized function v2

by Philipp Stanner

On Mon, 2026-07-13 at 10:58 +0200, Jiri Slaby wrote: > Hi, > > On 20. 01. 26, 11:54, Christian König wrote: > > Some driver use fence->ops to test if a fence was initialized or not. > > The problem is that this utilizes internal behavior of the dma_fence > > implementation. > > > > So better abstract that into a function. > > > > v2: use a flag instead of testing fence->ops, rename the function, move > > to the beginning of the patch set. > ... > > --- a/drivers/gpu/drm/qxl/qxl_release.c > > +++ b/drivers/gpu/drm/qxl/qxl_release.c > > @@ -146,7 +146,7 @@ qxl_release_free(struct qxl_device *qdev, > > idr_remove(&qdev->release_idr, release->id); > > spin_unlock(&qdev->release_idr_lock); > > > > - if (release->base.ops) { > > + if (dma_fence_was_initialized(&release->base)) { Could you verify the cause with sth like if (release->base.ops && dma_fence_was_initialized(…)) { ? > > WARN_ON(list_empty(&release->bos)); > > qxl_release_free_list(release); > > > > This likely breaks qxl: > https://bugzilla.suse.com/show_bug.cgi?id=1271081 > > > refcount_t: underflow; use-after-free. > > WARNING: lib/refcount.c:28 at refcount_warn_saturate+0x59/0x90, > CPU#0: kworker/0:0/1534 > > Modules linked in: af_packet nft_fib_inet ... > > CPU: 0 UID: 0 PID: 1534 Comm: kworker/0:0 Not tainted 7.1.3-1-default > #1 PREEMPT(full) openSUSE Tumbleweed > b041a6527f6e58424f4cd3de0fade8d408b378fd > > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS > rel-1.16.3-2-gc13ff2cd-prebuilt.qemu.org 04/01/2014 > > Workqueue: events qxl_gc_work [qxl] > > RIP: 0010:refcount_warn_saturate+0x59/0x90 > > Code: 44 48 8d 3d a9 72 b7 01 67 48 0f b9 3a c3 cc cc cc cc 48 8d 3d > a8 72 b7 01 67 48 0f b9 3a c3 cc cc cc cc 48 8d 3d a7 72 b7 01 <67> 48 > 0f b9 3a c3 cc cc cc cc 48 8d 3d a6 72 b7 01 67 48 0f b9 3a > > RSP: 0018:ffffce09019efe00 EFLAGS: 00010246 > > RAX: 0000000000000000 RBX: ffff8b5a56f819c0 RCX: 0000000000000017 > > RDX: 0000000000011696 RSI: 0000000000000003 RDI: ffffffffbc953b70 > > RBP: ffff8b5a4a314000 R08: 0000000000000001 R09: ffffffffc07d488a > > R10: fffffa6fc4787d80 R11: ffff8b5a40044000 R12: 0000000000000004 > > R13: 0000000000000003 R14: ffffce09003f1434 R15: ffff8b5a410c4640 > > FS: 0000000000000000(0000) GS:ffff8b5afe954000(0000) > knlGS:0000000000000000 > > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > > CR2: 00007ff463d65000 CR3: 000000012b476004 CR4: 0000000000772ef0 > > PKRU: 55555554 > > Call Trace: > > <TASK> > > qxl_release_free+0xee/0xf0 [qxl > d93e9381353e619799d56790f5f8dda6cce491f6] > > qxl_garbage_collect+0xd1/0x1b0 [qxl > d93e9381353e619799d56790f5f8dda6cce491f6] > > process_one_work+0x19e/0x3a0 > > ... > > The function does now: > > > void > > qxl_release_free(struct qxl_device *qdev, > > struct qxl_release *release) > > { > > ... > > if (dma_fence_was_initialized(&release->base)) { > > WARN_ON(list_empty(&release->bos)); > > > qxl_release_free_list(release); > > > > > > dma_fence_signal(&release->base); > > > dma_fence_put(&release->base); > > > > refcount_dec_and_test() in kref_put() in this ^^ crashes. Apparently, > the reference count was not increased anywhere. QXL does not call > dma_fence_get() -- who is supposed to increase the refcount. > dma_fence_init() initializes the refcount to 1. So since there is only one place with dma_fence_put(), it probably aims at countering that. > And why it > did not fail before? My guess would be that base->ops was used as a boolean to see whether qxl_release contains a valid fence. It seems qxl_release_free() is intended as some sort of universal function; it is called at many places. So I would suppose it is sometimes being called without the fence being initialized. Should most certainly have been documented. Anyways, verifying the issue would probably be cool Cheers, P. > > > > } else { > > > qxl_release_free_list(release); > > > kfree(release); > > > } > > > atomic_dec(&qdev->release_count); > > > } > > thanks,

2 hours, 25 minutes

Re: [RFC PATCH 1/2] dma-buf/udmabuf: Introduce CONFIG_UDMABUF_SIZE_LIMIT_MBYTES

by Kasireddy, Vivek

Hi Robert, > Subject: Re: [RFC PATCH 1/2] dma-buf/udmabuf: Introduce > CONFIG_UDMABUF_SIZE_LIMIT_MBYTES > > Hi, > > On 13.07.26 11:12, Christian König wrote: > > On 7/11/26 16:48, Robert Mader wrote: > >> As udmabuf increasingly enjoys popularity - being used in projects > like > >> libcamera, Gstreamer, Mesa and KWin - users more frequently > encounter > >> cases where the current default size limit of 64MB is too low. > Examples > >> include allocating video buffers at a 8K resolution - and even 4K is > >> affected when using non-subsampled video formats or high bit > depths. > >> > >> While the limit can already be changed via the kernel command line, > >> exposing it as a kernel config makes that easier and more > discoverable > >> for distros. Thus let's do that. > > Well config options are usually only useful if the value can't be > changed on runtime through a module parameter, but that is clearly > not the case here. > > > > On the other hand I do see your problem. I would just vote to disable > the limit by default, there is nothing preventing userspace from > allocating multiple uDMA-bufs so it doesn't seem to prevent any > security issue or similar. > > I fully agree and would prefer that as well. If there is no resistance / > concerns from anybody I'll send a corresponding patch at the end of the > week. AFAIU, the main motivation behind the limits was to ensure that there are some kind of guardrails. I am not opposed to their removal but I think it would now allow users to pin large arbitrary amounts of memory and I feel like there needs to be a way to prevent users from doing that. Early RFC versions of udmabuf had mlock accounting to address this issue but not sure why it was dropped eventually. I am wondering if it makes sense to bring that back. Thanks, Vivek > > Thanks, > > Robert > > > > > Regards, > > Christian. > > > > > >> Signed-off-by: Robert Mader <robert.mader(a)collabora.com> > >> --- > >> drivers/dma-buf/Kconfig | 6 ++++++ > >> drivers/dma-buf/udmabuf.c | 4 ++++ > >> 2 files changed, 10 insertions(+) > >> > >> diff --git a/drivers/dma-buf/Kconfig b/drivers/dma-buf/Kconfig > >> index 7efc0f0d0712..35f0779cdc80 100644 > >> --- a/drivers/dma-buf/Kconfig > >> +++ b/drivers/dma-buf/Kconfig > >> @@ -40,6 +40,12 @@ config UDMABUF > >> A driver to let userspace turn memfd regions into dma-bufs. > >> Qemu can use this to create host dmabufs for guest > framebuffers. > >> > >> +config UDMABUF_SIZE_LIMIT_MBYTES > >> + int "Size limit in Mega Bytes" > >> + default 64 > >> + help > >> + Maximum size of a udmabuf, in megabytes. Default is 64. > >> + > >> config DMABUF_DEBUG > >> bool "DMA-BUF debug checks" > >> depends on DMA_SHARED_BUFFER > >> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c > >> index bced421c0d65..a83153326362 100644 > >> --- a/drivers/dma-buf/udmabuf.c > >> +++ b/drivers/dma-buf/udmabuf.c > >> @@ -20,7 +20,11 @@ static int list_limit = 1024; > >> module_param(list_limit, int, 0644); > >> MODULE_PARM_DESC(list_limit, "udmabuf_create_list->count limit. > Default is 1024."); > >> > >> +#ifdef CONFIG_UDMABUF_SIZE_LIMIT_MBYTES > >> +static int size_limit_mb = CONFIG_UDMABUF_SIZE_LIMIT_MBYTES; > >> +#else > >> static int size_limit_mb = 64; > >> +#endif > >> module_param(size_limit_mb, int, 0644); > >> MODULE_PARM_DESC(size_limit_mb, "Max size of a dmabuf, in > megabytes. Default is 64."); > >> > >> -- > >> 2.55.0 > >>

4 hours, 26 minutes

Re: [RFC PATCH 10/30] vfio/pci: Export vfio dma-buf specific info for importers

by Jason Gunthorpe

On Mon, Jul 13, 2026 at 08:08:14PM +0100, Fuad Tabba wrote: > On Sun, 12 Jul 2026 at 02:02, Ackerley Tng <ackerleytng(a)google.com> wrote: > > > > Jason Gunthorpe <jgg(a)nvidia.com> writes: > > > > > On Thu, May 29, 2025 at 01:34:53PM +0800, Xu Yilun wrote: > > >> Export vfio dma-buf specific info by attaching vfio_dma_buf_data in > > >> struct dma_buf::priv. Provide a helper vfio_dma_buf_get_data() for > > >> importers to fetch these data. Exporters identify VFIO dma-buf by > > >> successfully getting these data. > > >> > > >> VFIO dma-buf supports disabling host access to these exported MMIO > > >> regions when the device is converted to private. Exporters like KVM > > >> need to identify this type of dma-buf to decide if it is good to use. > > >> KVM only allows host unaccessible MMIO regions been mapped in private > > >> roots. > > >> > > >> Export struct kvm * handler attached to the vfio device. This > > >> allows KVM to do another sanity check. MMIO should only be assigned to > > >> a CoCo VM if its owner device is already assigned to the same VM. > > > > > > This doesn't seem right, it should be encapsulated into the standard > > > DMABUF API in some way. > > > > > > > I'd like to propose an alternative. I've been working on guest_memfd and > > new to the world of IO, please help me along! :) > > > > It seems like using dmabufs are used a little awkwardly here. IIUC > > dmabufs were originally meant to expose memory of one device to another > > device, mostly meant to share memory. Dmabufs do expose MMIO too, for > > device to device communications. Without virtualization, userspace MMIO > > would be done by mmap()-ing a VFIO fd and having the userspace program > > write to the userspace addresses. > > > > Before CoCo, device passthrough (MMIO) is mostly handled by mmap()-ing a > > VFIO fd and setting up the userspace address in a KVM memslot for the > > guest. > > > > With CoCo, is the problem we're solving that we want KVM to know what > > pfns to set up in stage 2 page tables, but not via userspace addresses? > > > > guest_memfd already does that for regular host memory, tracks the > > private/shared-ness of the memory, tracks which struct kvm the memory > > belongs to. > > > > guest_memfd functions as KVM's bridge to host memory. KVM already can > > ask guest_memfd for the pfn to map into stage 2 page tables, and already > > asks guest_memfd for the shared/private state of the memory. guest_memfd > > already also blocks the host from faulting guest private memory > > (mmap()-ing is always allowed). > > > > > > Instead of using dmabuf as the intermediary between the MMIO PFNs and > > KVM, why not use guest_memfd? > > > > What if we make guest_memfd accept a VFIO fd, or a dmabuf fd? > > This is interesting for pKVM too, provided it covers more than MMIO. > > We need guest_memfd to be backable by a dmabuf for ordinary guest memory, not > only for device MMIO. There is mobile hardware that doesn't tolerate scattered > private memory (DMA engines that can't gather, IOMMU page-table size > constraints), and a CMA-backed dmabuf heap is the practical way to get > contiguous memory at runtime. Why can't guestmemfd allocate directly from CMA? Allocating struct page memory through dmabuf just to put it back in a guestmemfd sounds very ugly to me. > HugeTLB doesn't help, it wants boot-time > reservation. Those pages are struct-page backed, so it's a different problem > from the non-struct-page MMIO case, and the shared parts still need to be > GUP-able. Isn't dmabuf pretty allergic to mmaping refcounted struct page backed memory since that wrecks its lifetime model? > More important for the API shape: conversions have to work on subsets of such a > region, at page granularity. A pKVM guest doesn't know what backs its memory, so > it will issue share/unshare hypercalls over arbitrary ranges of whatever it was > given. If a dmabuf-backed guest_memfd can only be converted as a whole, we can't > use it for memory, and the guest can't be taught to care. More reasons not to involve DMABUF since guestmemfd already does all of this... Jason

15 hours, 51 minutes

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21 hours

[PATCH v3 00/10] Add dmabuf read/write via io_uring

by Pavel Begunkov

The patch set allows to register a dmabuf to an io_uring instance for a specified file and use it with io_uring read / write requests. The infrastructure is not tied to io_uring and there could be more users in the future. A similar idea was attempted some years ago by Keith [1], from where I borrowed a good number of changes, and later was brough up by Tushar and Vishal from Intel. It's an opt-in feature for files, and they need to implement a new file operation to use it. Only NVMe block devices are supported in this series. The user API is built on top of io_uring's "registered buffers", where a dmabuf is registered in a special way, but after it can be used as any other "registered buffer" with IORING_OP_{READ,WRITE}_FIXED requests. It's created via a new file operation and the resulted map is then passed through the I/O stack in a new iterator type. There is some additional infrastructure to bind it all, which also counts requests using a dmabuf map and managing lifetimes, which is used to implement map invalidation. It was tested for GPU <-> NVMe transfers. Also, as it maintains a long-term dma mapping, it helps with the IOMMU cost. The numbers below are for udmabuf reads previously run by Anuj for different IOMMU modes: - STRICT: before = 570 KIOPS, after = 5.01 MIOPS - LAZY: before = 1.93 MIOPS, after = 5.01 MIOPS - PASSTHROUGH: before = 5.01 MIOPS, after = 5.01 MIOPS There are some liburing tests that can serve as an example: git: https://github.com/isilence/liburing.git rw-dmabuf-tests-v3 url: https://github.com/isilence/liburing/tree/rw-dmabuf-tests-v3 [1] https://lore.kernel.org/io-uring/20220805162444.3985535-1-kbusch@fb.com/ v3: - Rework io_uring registration - Move token/map infrastructure code out of blk-mq - Simplify callbacks: remove a separate blk-mq table, which was mostly just forwarding calls (to nvme). - Don't skip dma sync depending on request direction - Fix a couple of hangs - Rename s/dma/dmabuf/ - Other small changes v2: - Don't pass raw dma addresses, wrap it into a driver specific object - Split into two objects: token and map - Implement move_notify Pavel Begunkov (10): file: add callback for creating long-term dmabuf maps iov_iter: add iterator type for dmabuf maps block: move bvec init into __bio_clone block: introduce dma map backed bio type lib: add dmabuf token infrastructure block: forward create_dmabuf_token to drivers nvme-pci: implement dma_token backed requests io_uring/rsrc: introduce buf registration structure io_uring/rsrc: extend buffer update io_uring/rsrc: add dmabuf backed registered buffers block/bio.c | 28 +++- block/blk-merge.c | 14 ++ block/blk.h | 3 +- block/fops.c | 16 ++ drivers/nvme/host/pci.c | 282 ++++++++++++++++++++++++++++++++ include/linux/bio.h | 19 ++- include/linux/blk-mq.h | 9 + include/linux/blk_types.h | 8 +- include/linux/fs.h | 2 + include/linux/io_dmabuf_token.h | 92 +++++++++++ include/linux/io_uring_types.h | 5 + include/linux/uio.h | 11 ++ include/uapi/linux/io_uring.h | 31 +++- io_uring/io_uring.c | 3 +- io_uring/rsrc.c | 266 +++++++++++++++++++++++++----- io_uring/rsrc.h | 30 +++- io_uring/rw.c | 4 +- lib/Kconfig | 4 + lib/Makefile | 2 + lib/io_dmabuf_token.c | 272 ++++++++++++++++++++++++++++++ lib/iov_iter.c | 29 +++- 21 files changed, 1071 insertions(+), 59 deletions(-) create mode 100644 include/linux/io_dmabuf_token.h create mode 100644 lib/io_dmabuf_token.c -- 2.53.0

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Re: [RFC PATCH 1/2] dma-buf/udmabuf: Introduce CONFIG_UDMABUF_SIZE_LIMIT_MBYTES

by Christian König

On 7/11/26 16:48, Robert Mader wrote: > As udmabuf increasingly enjoys popularity - being used in projects like > libcamera, Gstreamer, Mesa and KWin - users more frequently encounter > cases where the current default size limit of 64MB is too low. Examples > include allocating video buffers at a 8K resolution - and even 4K is > affected when using non-subsampled video formats or high bit depths. > > While the limit can already be changed via the kernel command line, > exposing it as a kernel config makes that easier and more discoverable > for distros. Thus let's do that. Well config options are usually only useful if the value can't be changed on runtime through a module parameter, but that is clearly not the case here. On the other hand I do see your problem. I would just vote to disable the limit by default, there is nothing preventing userspace from allocating multiple uDMA-bufs so it doesn't seem to prevent any security issue or similar. Regards, Christian. > > Signed-off-by: Robert Mader <robert.mader(a)collabora.com> > --- > drivers/dma-buf/Kconfig | 6 ++++++ > drivers/dma-buf/udmabuf.c | 4 ++++ > 2 files changed, 10 insertions(+) > > diff --git a/drivers/dma-buf/Kconfig b/drivers/dma-buf/Kconfig > index 7efc0f0d0712..35f0779cdc80 100644 > --- a/drivers/dma-buf/Kconfig > +++ b/drivers/dma-buf/Kconfig > @@ -40,6 +40,12 @@ config UDMABUF > A driver to let userspace turn memfd regions into dma-bufs. > Qemu can use this to create host dmabufs for guest framebuffers. > > +config UDMABUF_SIZE_LIMIT_MBYTES > + int "Size limit in Mega Bytes" > + default 64 > + help > + Maximum size of a udmabuf, in megabytes. Default is 64. > + > config DMABUF_DEBUG > bool "DMA-BUF debug checks" > depends on DMA_SHARED_BUFFER > diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c > index bced421c0d65..a83153326362 100644 > --- a/drivers/dma-buf/udmabuf.c > +++ b/drivers/dma-buf/udmabuf.c > @@ -20,7 +20,11 @@ static int list_limit = 1024; > module_param(list_limit, int, 0644); > MODULE_PARM_DESC(list_limit, "udmabuf_create_list->count limit. Default is 1024."); > > +#ifdef CONFIG_UDMABUF_SIZE_LIMIT_MBYTES > +static int size_limit_mb = CONFIG_UDMABUF_SIZE_LIMIT_MBYTES; > +#else > static int size_limit_mb = 64; > +#endif > module_param(size_limit_mb, int, 0644); > MODULE_PARM_DESC(size_limit_mb, "Max size of a dmabuf, in megabytes. Default is 64."); > > -- > 2.55.0 >

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[PATCH v19 00/40] DEPT(DEPendency Tracker)

by Byungchul Park

Hi Linus and folks, DEPT(DEPendency Tracker) is a runtime deadlock detection framework that sees what lockdep cannot. I'm thrilled to share that DEPT has moved beyond theory and is now catching real deadlocks in the wild: https://lore.kernel.org/lkml/6383cde5-cf4b-facf-6e07-1378a485657d@I-love.SA… https://lore.kernel.org/lkml/1674268856-31807-1-git-send-email-byungchul.pa… https://lore.kernel.org/all/b6e00e77-4a8c-4e05-ab79-266bf05fcc2d@igalia.com/ I've added comprehensive documentation explaining DEPT's design and usage. Getting started is as simple as enabling CONFIG_DEPT and watching dmesg. THE PROBLEM LOCKDEP CANNOT SOLVE -------------------------------- Lockdep has been our trusted deadlock detector for two decades, but it has a fundamental blind spot: it tracks lock acquisition order, not the actual waits and events that cause deadlocks. This means lockdep misses: * Deadlocks involving folio locks (not released within the context) * Cross-context synchronization like wait_for_completion()/complete() * DMA fence waits, RCU waits, and general waitqueue patterns * Any synchronization primitive outside the classic lock/unlock model Consider this real deadlock pattern that lockdep cannot detect: context X context Y context Z mutex_lock A folio_lock B folio_lock B <- DEADLOCK mutex_lock A <- DEADLOCK folio_unlock B folio_unlock B mutex_unlock A mutex_unlock A Lockdep sees lock acquisitions. DEPT sees the actual dependency: "mutex_unlock A in context Y cannot happen until folio_lock B is awakened by the owner's folio_unlock B, and vice versa in context Z." It's a circular dependency that means deadlock. THE DEPT APPROACH ----------------- DEPT asks a simpler question: "What is this context waiting for, and what event will wake it up?" Every deadlock is fundamentally about unreachable events. DEPT tracks: [S] Where an event context begins (the code path that will trigger an event) [W] Where a wait for another event apears between [S] and [E] [E] Where the event for [S] occurs By building a dependency graph of "[E] cannot occur until the event that [W] waits for occurs", DEPT detects circular dependencies regardless of the underlying synchronization primitives involved. WHAT DEPT BRINGS TO THE TABLE ----------------------------- * Universal coverage: Works with any wait/event-based synchronization, not just locks * Correct read-lock handling: No more blind spots for read-side dependencies * Continuous operation: Unlike lockdep, DEPT keeps running after reports, catching multiple deadlocks in a single session * Clean annotation API: Simple, intuitive interfaces for subsystem maintainers to refine detection * Battle-tested: Already catching real deadlocks as the links above demonstrate FALSE POSITIVES: THE HONEST CONVERSATION ---------------------------------------- Like any powerful detection tool, DEPT faces the false positive challenge. This is not unique to DEPT — lockdep spent years building its annotation infrastructure (lock classes, subclasses, lockdep_map) to separate real bugs from intentional patterns. DEPT is on the same journey. We have: * Event site recovery: Declare when an event has fallback paths * Subclass-based classification: Distinguish per-CPU, per-device, and modality-specific waits * Page usage tracking: Separate block device mappings from regular file mappings to avoid spurious reports (currently being worked on) But comprehensive annotation requires subsystem maintainer expertise. This is where I need your help. THE PATH TO MAINLINE -------------------- DEPT is marked EXPERIMENTAL in Kconfig for a reason. Like lockdep, it will mature through collaboration: 1. Core framework: Stabilized and ready for review 2. Subsystem pilots: Working with maintainers to add annotations where they matter most (mm, block, drm, networking, ...) 3. Gradual enablement: DEPT and lockdep coexist; DEPT takes over dependency checking when ready I am not proposing to replace lockdep. Lockdep's lock usage validation remains invaluable. The vision is: LOCKDEP: Validates correct lock usage | v DEPT: Performs dependency checking with full wait/event coverage WHY MERGE NOW? -------------- Some might suggest: "Fix all false positives out-of-tree first." But the affected subsystems span the entire kernel. Like lockdep's two-decade annotation journey, DEPT needs mainline visibility for: * Proper annotation placement (maintainers know their code best) * Real-world testing across configurations and workloads * Incremental improvement through community feedback CONFIG_DEPT is opt-in. It won't affect your default kernel build. But for those debugging complex synchronization issues, DEPT is ready to help today. ACKNOWLEDGMENTS --------------- This work would not be possible without: Harry Yoo <harry.yoo(a)oracle.com> Gwan-gyeong Mun <gwan-gyeong.mun(a)intel.com> Yunseong Kim <ysk(a)kzalloc.com> Yeoreum Yun <yeoreum.yun(a)arm.com> And the countless kernel developers whose lockdep annotations over two decades showed us the path forward. FAQ --- Q. Isn't this the cross-release feature that got reverted? A. Cross-release (commit b09be676e0ff2) attempted to extend lockdep with wait/event tracking. It found real bugs but introduced false positives that masked further issues. DEPT learns from that experience with a cleaner design and flexible reporting that makes false positives less disruptive. Q. Why not build DEPT into lockdep? A. Lockdep is stable, battle-tested code. I chose separation because while DEPT borrows BFS and hashing ideas, the wait/event model requires rebuilding from scratch. Lockdep was designed for lock acquisition order — retrofitting it would risk its stability. Q. Will DEPT replace lockdep? A. No. Lockdep validates correct lock usage — that's not going away. DEPT supersedes only the dependency-checking logic when mature. Q. Should we merge DEPT now or wait for more annotations out-of-tree? A. Now. The annotation journey requires mainline collaboration. Lockdep didn't become useful overnight — it grew through maintainer contributions. DEPT needs the same path. Q. What if I enable DEPT and get false positives? A. That's the point — report them. Work with us to add annotations that distinguish your intentional patterns from real deadlocks. This is how lockdep became indispensable, and it's how DEPT will too. GETTING STARTED --------------- 1. Enable CONFIG_DEPT (EXPERIMENTAL) 2. Boot your kernel 3. Check dmesg for DEPT reports 4. Read Documentation/dev-tools/dept.rst for interpretation DEPT is a tool for understanding your code's synchronization behavior. Even if you never see a deadlock report, the visibility it provides is invaluable. I look forward to your feedback, patches, and collaboration. Let's make DEPT as indispensable to kernel developers as lockdep has been. --- Changes from v18: 1. Rebase on v7.0. 2. Add 'Reviewed-by: Jeff Layton <jlayton(a)kernel.org>' on 37th patch, 'SUNRPC: relocate struct rcu_head to the first field of struct rpc_xprt'. (thanks to Jeff Layton) 3. Refine and supplement dept documents and comments, and fix typos. (feedbacked by Bagas Sanjaya and Yunseong Kim) 4. Add __rust_helper to rust_helper_wait_for_completion(). (feedbacked by Dirk Behme) 5. Remove the part supporting recover events tracking - I will keep maintaining it out of tree tho - as it unnecessarily complicates the initial DEPT patchset and significantly increases the review burden. 6. Get rid of 'extern' keyword with function declarations. (feedbacked by Petr Pavlu) Changes from v17: 1. Rebase on the mainline as of 2025 Dec 5. 2. Convert the documents' format from txt to rst. (feedbacked by Jonathan Corbet and Bagas Sanjaya) 3. Move the documents from 'Documentation/dependency' to 'Documentation/dev-tools'. (feedbakced by Jonathan Corbet) 4. Improve the documentation. (feedbacked by NeilBrown) 5. Use a common function, enter_from_user_mode(), instead of arch specific code, to notice context switch from user mode. (feedbacked by Dave Hansen, Mark Rutland, and Mark Brown) 6. Resolve the header dependency issue by using dept's internal header, instead of relocating 'struct llist_{head,node}' to another header. (feedbacked by Greg KH) 7. Improve page(or folio) usage type APIs. 8. Add rust helper for wait_for_completion(). (feedbacked by Guangbo Cui, Boqun Feng, and Danilo Krummrich) 9. Refine some commit messages. Changes from v16: 1. Rebase on v6.17. 2. Fix a false positive from rcu (by Yunseong Kim) 3. Introduce APIs to set page's usage, dept_set_page_usage() and dept_reset_page_usage() to avoid false positives. 4. Consider lock_page() as a potential wait unconditionally. 5. Consider folio_lock_killable() as a potential wait unconditionally. 6. Add support for tracking PG_writeback waits and events. 7. Fix two build errors due to the additional debug information added by dept. (by Yunseong Kim) Changes from v15: 1. Fix typo and improve comments and commit messages (feedbacked by ALOK TIWARI, Waiman Long, and kernel test robot). 2. Do not stop dept on detection of cicular dependency of recover event, allowing to keep reporting. 3. Add SK hynix to copyright. 4. Consider folio_lock() as a potential wait unconditionally. 5. Fix Kconfig dependency bug (feedbacked by kernel test rebot). 6. Do not suppress reports that involve classes even that have already involved in other reports, allowing to keep reporting. Changes from v14: 1. Rebase on the current latest, v6.15-rc6. 2. Refactor dept code. 3. With multi event sites for a single wait, even if an event forms a circular dependency, the event can be recovered by other event(or wake up) paths. Even though informing the circular dependency is worthy but it should be suppressed once informing it, if it doesn't lead an actual deadlock. So introduce APIs to annotate the relationship between event site and recover site, that are, event_site() and dept_recover_event(). 4. wait_for_completion() worked with dept map embedded in struct completion. However, it generates a few false positves since all the waits using the instance of struct completion, share the map and key. To avoid the false positves, make it not to share the map and key but each wait_for_completion() caller have its own key by default. Of course, external maps also can be used if needed. 5. Fix a bug about hardirq on/off tracing. 6. Implement basic unit test for dept. 7. Add more supports for dma fence synchronization. 8. Add emergency stop of dept e.g. on panic(). 9. Fix false positives by mmu_notifier_invalidate_*(). 10. Fix recursive call bug by DEPT_WARN_*() and DEPT_STOP(). 11. Fix trivial bugs in DEPT_WARN_*() and DEPT_STOP(). 12. Fix a bug that a spin lock, dept_pool_spin, is used in both contexts of irq disabled and enabled without irq disabled. 13. Suppress reports with classes, any of that already have been reported, even though they have different chains but being barely meaningful. 14. Print stacktrace of the wait that an event is now waking up, not only stacktrace of the event. 15. Make dept aware of lockdep_cmp_fn() that is used to avoid false positives in lockdep so that dept can also avoid them. 16. Do do_event() only if there are no ecxts have been delimited. 17. Fix a bug that was not synchronized for stage_m in struct dept_task, using a spin lock, dept_task()->stage_lock. 18. Fix a bug that dept didn't handle the case that multiple ttwus for a single waiter can be called at the same time e.i. a race issue. 19. Distinguish each kernel context from others, not only by system call but also by user oriented fault so that dept can work with more accuracy information about kernel context. That helps to avoid a few false positives. 20. Limit dept's working to x86_64 and arm64. Changes from v13: 1. Rebase on the current latest version, v6.9-rc7. 2. Add 'dept' documentation describing dept APIs. Changes from v12: 1. Refine the whole document for dept. 2. Add 'Interpret dept report' section in the document, using a deadlock report obtained in practice. Hope this version of document helps guys understand dept better. https://lore.kernel.org/lkml/6383cde5-cf4b-facf-6e07-1378a485657d@I-love.SA… https://lore.kernel.org/lkml/1674268856-31807-1-git-send-email-byungchul.pa… Changes from v11: 1. Add 'dept' documentation describing the concept of dept. 2. Rewrite the commit messages of the following commits for using weaker lockdep annotation, for better description. fs/jbd2: Use a weaker annotation in journal handling cpu/hotplug: Use a weaker annotation in AP thread (feedbacked by Thomas Gleixner) Changes from v10: 1. Fix noinstr warning when building kernel source. 2. dept has been reporting some false positives due to the folio lock's unfairness. Reflect it and make dept work based on dept annotaions instead of just wait and wake up primitives. 3. Remove the support for PG_writeback while working on 2. I will add the support later if needed. 4. dept didn't print stacktrace for [S] if the participant of a deadlock is not lock mechanism but general wait and event. However, it made hard to interpret the report in that case. So add support to print stacktrace of the requestor who asked the event context to run - usually a waiter of the event does it just before going to wait state. 5. Give up tracking raw_local_irq_{disable,enable}() since it totally messed up dept's irq tracking. So make it work in the same way as lockdep does. I will consider it once any false positives by those are observed again. 6. Change the manual rwsem_acquire_read(->j_trans_commit_map) annotation in fs/jbd2/transaction.c to the try version so that it works as much as it exactly needs. 7. Remove unnecessary 'inline' keyword in dept.c and add '__maybe_unused' to a needed place. Changes from v9: 1. Fix a bug. SDT tracking didn't work well because of my big mistake that I should've used waiter's map to indentify its class but it had been working with waker's one. FYI, PG_locked and PG_writeback weren't affected. They still worked well. (reported by YoungJun) Changes from v8: 1. Fix build error by adding EXPORT_SYMBOL(PG_locked_map) and EXPORT_SYMBOL(PG_writeback_map) for kernel module build - appologize for that. (reported by kernel test robot) 2. Fix build error by removing header file's circular dependency that was caused by "atomic.h", "kernel.h" and "irqflags.h", which I introduced - appolgize for that. (reported by kernel test robot) Changes from v7: 1. Fix a bug that cannot track rwlock dependency properly, introduced in v7. (reported by Boqun and lockdep selftest) 2. Track wait/event of PG_{locked,writeback} more aggressively assuming that when a bit of PG_{locked,writeback} is cleared there might be waits on the bit. (reported by Linus, Hillf and syzbot) 3. Fix and clean bad style code e.i. unnecessarily introduced a randome pattern and so on. (pointed out by Linux) 4. Clean code for applying dept to wait_for_completion(). Changes from v6: 1. Tie to task scheduler code to track sleep and try_to_wake_up() assuming sleeps cause waits, try_to_wake_up()s would be the events that those are waiting for, of course with proper dept annotations, sdt_might_sleep_weak(), sdt_might_sleep_strong() and so on. For these cases, class is classified at sleep entrance rather than the synchronization initialization code. Which would extremely reduce false alarms. 2. Remove the dept associated instance in each page struct for tracking dependencies by PG_locked and PG_writeback thanks to the 1. work above. 3. Introduce CONFIG_dept_AGGRESIVE_TIMEOUT_WAIT to suppress reports that waits with timeout set are involved, for those who don't like verbose reporting. 4. Add a mechanism to refill the internal memory pools on running out so that dept could keep working as long as free memory is available in the system. 5. Re-enable tracking hashed-waitqueue wait. That's going to no longer generate false positives because class is classified at sleep entrance rather than the waitqueue initailization. 6. Refactor to make it easier to port onto each new version of the kernel. 7. Apply dept to dma fence. 8. Do trivial optimizaitions. Changes from v5: 1. Use just pr_warn_once() rather than WARN_ONCE() on the lack of internal resources because WARN_*() printing stacktrace is too much for informing the lack. (feedback from Ted, Hyeonggon) 2. Fix trivial bugs like missing initializing a struct before using it. 3. Assign a different class per task when handling onstack variables for waitqueue or the like. Which makes dept distinguish between onstack variables of different tasks so as to prevent false positives. (reported by Hyeonggon) 4. Make dept aware of even raw_local_irq_*() to prevent false positives. (reported by Hyeonggon) 5. Don't consider dependencies between the events that might be triggered within __schedule() and the waits that requires __schedule(), real ones. (reported by Hyeonggon) 6. Unstage the staged wait that has prepare_to_wait_event()'ed *and* yet to get to __schedule(), if we encounter __schedule() in-between for another sleep, which is possible if e.g. a mutex_lock() exists in 'condition' of ___wait_event(). 7. Turn on CONFIG_PROVE_LOCKING when CONFIG_DEPT is on, to rely on the hardirq and softirq entrance tracing to make dept more portable for now. Changes from v4: 1. Fix some bugs that produce false alarms. 2. Distinguish each syscall context from another *for arm64*. 3. Make it not warn it but just print it in case dept ring buffer gets exhausted. (feedback from Hyeonggon) 4. Explicitely describe "EXPERIMENTAL" and "dept might produce false positive reports" in Kconfig. (feedback from Ted) Changes from v3: 1. dept shouldn't create dependencies between different depths of a class that were indicated by *_lock_nested(). dept normally doesn't but it does once another lock class comes in. So fixed it. (feedback from Hyeonggon) 2. dept considered a wait as a real wait once getting to __schedule() even if it has been set to TASK_RUNNING by wake up sources in advance. Fixed it so that dept doesn't consider the case as a real wait. (feedback from Jan Kara) 3. Stop tracking dependencies with a map once the event associated with the map has been handled. dept will start to work with the map again, on the next sleep. Changes from v2: 1. Disable dept on bit_wait_table[] in sched/wait_bit.c reporting a lot of false positives, which is my fault. Wait/event for bit_wait_table[] should've been tagged in a higher layer for better work, which is a future work. (feedback from Jan Kara) 2. Disable dept on crypto_larval's completion to prevent a false positive. Changes from v1: 1. Fix coding style and typo. (feedback from Steven) 2. Distinguish each work context from another in workqueue. 3. Skip checking lock acquisition with nest_lock, which is about correct lock usage that should be checked by lockdep. Changes from RFC(v0): 1. Prevent adding a wait tag at prepare_to_wait() but __schedule(). (feedback from Linus and Matthew) 2. Use try version at lockdep_acquire_cpus_lock() annotation. 3. Distinguish each syscall context from another. Byungchul Park (39): dept: implement DEPT(DEPendency Tracker) dept: add single event dependency tracker APIs dept: add lock dependency tracker APIs dept: tie to lockdep and IRQ tracing dept: add proc knobs to show stats and dependency graph dept: distinguish each kernel context from another dept: distinguish each work from another dept: add a mechanism to refill the internal memory pools on running out dept: record the latest one out of consecutive waits of the same class dept: apply sdt_might_sleep_{start,end}() to wait_for_completion()/complete() dept: apply sdt_might_sleep_{start,end}() to swait dept: apply sdt_might_sleep_{start,end}() to waitqueue wait dept: apply sdt_might_sleep_{start,end}() to hashed-waitqueue wait dept: apply sdt_might_sleep_{start,end}() to dma fence dept: track timeout waits separately with a new Kconfig dept: apply timeout consideration to wait_for_completion()/complete() dept: apply timeout consideration to swait dept: apply timeout consideration to waitqueue wait dept: apply timeout consideration to hashed-waitqueue wait dept: apply timeout consideration to dma fence wait dept: make dept able to work with an external wgen dept: track PG_locked with dept dept: print staged wait's stacktrace on report locking/lockdep: prevent various lockdep assertions when lockdep_off()'ed dept: add documents for dept cpu/hotplug: use a weaker annotation in AP thread dept: assign dept map to mmu notifier invalidation synchronization dept: assign unique dept_key to each distinct dma fence caller dept: make dept aware of lockdep_set_lock_cmp_fn() annotation dept: make dept stop from working on debug_locks_off() dept: assign unique dept_key to each distinct wait_for_completion() caller completion, dept: introduce init_completion_dmap() API dept: call dept_hardirqs_off() in local_irq_*() regardless of irq state dept: introduce APIs to set page usage and use subclasses_evt for the usage dept: track PG_writeback with dept SUNRPC: relocate struct rcu_head to the first field of struct rpc_xprt mm: percpu: increase PERCPU_DYNAMIC_SIZE_SHIFT on DEPT and large PAGE_SIZE rust: completion: Add __rust_helper to rust_helper_wait_for_completion() dept: implement a basic unit test for dept Yunseong Kim (1): rcu/update: fix same dept key collision between various types of RCU Documentation/dev-tools/dept.rst | 905 ++++++++ Documentation/dev-tools/dept_api.rst | 124 + Documentation/dev-tools/index.rst | 2 + drivers/dma-buf/dma-fence.c | 23 +- include/linux/completion.h | 124 +- include/linux/dept.h | 267 +++ include/linux/dept_ldt.h | 78 + include/linux/dept_sdt.h | 68 + include/linux/dept_unit_test.h | 61 + include/linux/dma-fence.h | 74 +- include/linux/hardirq.h | 3 + include/linux/irq-entry-common.h | 4 + include/linux/irqflags.h | 21 +- include/linux/local_lock_internal.h | 1 + include/linux/lockdep.h | 105 +- include/linux/lockdep_types.h | 3 + include/linux/mm_types.h | 4 + include/linux/mmu_notifier.h | 26 + include/linux/mutex.h | 1 + include/linux/page-flags.h | 217 +- include/linux/pagemap.h | 37 +- include/linux/percpu-rwsem.h | 2 +- include/linux/percpu.h | 4 + include/linux/rcupdate_wait.h | 13 +- include/linux/rtmutex.h | 1 + include/linux/rwlock_types.h | 1 + include/linux/rwsem.h | 1 + include/linux/sched.h | 111 + include/linux/seqlock.h | 2 +- include/linux/spinlock_types_raw.h | 3 + include/linux/srcu.h | 2 +- include/linux/sunrpc/xprt.h | 9 +- include/linux/swait.h | 3 + include/linux/wait.h | 3 + include/linux/wait_bit.h | 3 + init/init_task.c | 2 + init/main.c | 2 + kernel/Makefile | 1 + kernel/cpu.c | 2 +- kernel/dependency/Makefile | 5 + kernel/dependency/dept.c | 3222 ++++++++++++++++++++++++++ kernel/dependency/dept_hash.h | 10 + kernel/dependency/dept_internal.h | 314 +++ kernel/dependency/dept_object.h | 13 + kernel/dependency/dept_proc.c | 94 + kernel/dependency/dept_unit_test.c | 149 ++ kernel/exit.c | 1 + kernel/fork.c | 2 + kernel/locking/lockdep.c | 33 + kernel/module/main.c | 2 + kernel/rcu/rcu.h | 1 + kernel/rcu/update.c | 5 +- kernel/sched/completion.c | 62 +- kernel/sched/core.c | 9 + kernel/workqueue.c | 3 + lib/Kconfig.debug | 48 + lib/debug_locks.c | 2 + lib/locking-selftest.c | 2 + mm/filemap.c | 38 + mm/mm_init.c | 3 + mm/mmu_notifier.c | 31 +- rust/helpers/completion.c | 5 + 62 files changed, 6247 insertions(+), 120 deletions(-) create mode 100644 Documentation/dev-tools/dept.rst create mode 100644 Documentation/dev-tools/dept_api.rst create mode 100644 include/linux/dept.h create mode 100644 include/linux/dept_ldt.h create mode 100644 include/linux/dept_sdt.h create mode 100644 include/linux/dept_unit_test.h create mode 100644 kernel/dependency/Makefile create mode 100644 kernel/dependency/dept.c create mode 100644 kernel/dependency/dept_hash.h create mode 100644 kernel/dependency/dept_internal.h create mode 100644 kernel/dependency/dept_object.h create mode 100644 kernel/dependency/dept_proc.c create mode 100644 kernel/dependency/dept_unit_test.c base-commit: 028ef9c96e96197026887c0f092424679298aae8 -- 2.17.1

1 day, 7 hours

Re: [RFC PATCH 10/30] vfio/pci: Export vfio dma-buf specific info for importers

by Jason Gunthorpe

On Sat, Jul 11, 2026 at 06:01:31PM -0700, Ackerley Tng wrote: > In the course of a CoCo guest's operation, will the guest need to > convert between private/shared MMIO? Will the guest need some pages > shared and others private? If these are required operations, guest_memfd > already provides the tracking and is going to have a conversion ioctl > very soon. Instead of further extending dmabuf to track more things, how > about letting guest_memfd track it? Use another FD type was sort of my fallback if we couldn't get DMABUF into something workable. I'm kind of surprised to see guestmemfd proposed as the other FD, but I don't know much about its insides. If VFIO can create one and fill it with MMIO physical addresses then maybe it is OK? Jason

1 day, 12 hours

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