[Linaro-mm-sig] Re: [PATCH v6 01/13] dma-buf: introduce initial file I/O infrastructure

On Mon, Sep 21, 2026 at 02:38:45PM +0100, 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.

[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@gmail.com

drivers/dma-buf/Makefile | 2 +- drivers/dma-buf/dma-buf-io.c | 219 +++++++++++++++++++++++++++++++++++ include/linux/dma-buf-io.h | 113 ++++++++++++++++++ include/linux/fs.h | 2 + 4 files changed, 335 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..8312637a299f --- /dev/null +++ b/drivers/dma-buf/dma-buf-io.c @@ -0,0 +1,219 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +/*

• • Common infrastructure for supporing dma-buf in the I/O path.
• • Copyright (C) 2026 Pavel Begunkov asml.silence@gmail.com
• */

+#include <linux/dma-buf-io.h> +#include <linux/dma-resv.h>

+static void dma_buf_io_put_ctx(struct dma_buf_io_ctx *ctx) +{

• might_sleep();
• if (WARN_ON_ONCE(rcu_dereference_protected(ctx->map, true)))

• return;
  

• ctx->dev_ops->release(ctx);
• dma_buf_put(ctx->dmabuf);
• mutex_destroy(&ctx->map_mutex);
• mutex_destroy(&ctx->map_create_mutex);
• 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_ctx *ctx = map->ctx;
• struct dma_buf *dmabuf = ctx->dmabuf;
• dma_resv_lock(dmabuf->resv, NULL);
• ctx->dev_ops->unmap(ctx, map);
• dma_resv_unlock(dmabuf->resv);
• percpu_ref_exit(&map->refs);
• kfree(map);
• atomic_dec(&ctx->all_maps);
• wake_up(&ctx->maps_wq);

+}

+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);
• struct dma_buf_io_ctx *ctx = map->ctx;
• /* There are no more requests using the map. */
• atomic_dec(&ctx->active_maps);
• wake_up(&ctx->maps_wq);
• /* might sleep, use a worker */
• INIT_WORK(&map->release_work, dma_buf_io_map_release_work);
• queue_work(system_percpu_wq, &map->release_work);

+}

+static void dma_buf_io_wait_active_maps(struct dma_buf_io_ctx *ctx) +{

• wait_event(ctx->maps_wq, atomic_read(&ctx->active_maps) == 0);

+}

+static void dma_buf_io_wait_maps(struct dma_buf_io_ctx *ctx) +{

• wait_event(ctx->maps_wq, atomic_read(&ctx->all_maps) == 0);

+}

+int dma_buf_io_init_map(struct dma_buf_io_ctx *ctx, struct dma_buf_io_map *map,

• 	struct sg_table *sgt)
  

+{

• unsigned seg_shift = ~0U;
• struct scatterlist *sg;
• unsigned long tmp;
• int ret;
• for_each_sgtable_dma_sg(sgt, sg, tmp)

• seg_shift = min(seg_shift, __ffs(sg_dma_len(sg)));
  

• ret = percpu_ref_init(&map->refs, dma_buf_io_map_refs_release, 0,

• 	      GFP_KERNEL);
  

• if (ret)

• return ret;
  

• map->min_seg_shift = seg_shift;
• 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;
• guard(mutex)(&ctx->map_create_mutex);
• scoped_guard(mutex, &ctx->map_mutex) {

• if (ctx->maps_killed)
  

• 	return ERR_PTR(-ENOENT);
  

• /* recheck under the lock in case it has already been re-created */
  

• map = __dma_buf_io_get_map(ctx);
  

• if (map)
  

• 	return map;
  

• }
• dma_buf_io_wait_active_maps(ctx);
• ret = dma_resv_lock_interruptible(dmabuf->resv, NULL);
• if (ret)

• return ERR_PTR(ret);
  

• ret = dma_resv_wait_timeout(dmabuf->resv, DMA_RESV_USAGE_KERNEL,

• 		    true, MAX_SCHEDULE_TIMEOUT);
  

• if (ret <= 0) {

• if (!ret)
  

• 	ret = -EAGAIN;
  

• dma_resv_unlock(dmabuf->resv);
  

• return ERR_PTR(ret);
  

• }
• map = ctx->dev_ops->map(ctx);

I'm playing around this code now.

I think you need the dma_resv_wait_timeout after the 'map'?

If a device doesn't support p2p ->map() will typically trigger an async migrate to system memory and data will be moving but the map is valid - Xe 100% does this, I checked AMDGPU and fairly confident it has the same async behavior.

Matt

• dma_resv_unlock(dmabuf->resv);
• if (IS_ERR(map))

• return map;
  

• if (WARN_ON_ONCE(!map->min_seg_shift))

• return ERR_PTR(-EFAULT);
  

• atomic_inc(&ctx->active_maps);
• atomic_inc(&ctx->all_maps);
• /* get a reference for the caller */
• percpu_ref_get(&map->refs);
• scoped_guard(mutex, &ctx->map_mutex)

• rcu_assign_pointer(ctx->map, map);
  

• return map;

+}

+static void dma_buf_io_kill_maps(struct dma_buf_io_ctx *ctx, bool final) +{

• struct dma_buf_io_map *map;
• scoped_guard(mutex, &ctx->map_mutex) {

• if (final)
  

• 	ctx->maps_killed = true;
  

• map = rcu_dereference_protected(ctx->map,
  

• 			lockdep_is_held(&ctx->map_mutex));
  

• if (!map)
  

• 	return;
  

• rcu_assign_pointer(ctx->map, NULL);
  

• percpu_ref_kill(&map->refs);
  

• }

+}

+void dma_buf_io_invalidate_mappings(struct dma_buf_io_ctx *ctx) +{

• dma_buf_io_kill_maps(ctx, false);
• dma_buf_io_wait_active_maps(ctx);

+} +EXPORT_SYMBOL_NS_GPL(dma_buf_io_invalidate_mappings, "DMA_BUF");

+void dma_buf_io_ctx_release(struct dma_buf_io_ctx *ctx) +{

• /* Remove and wait for the last map, there should be no new ones. */
• dma_buf_io_kill_maps(ctx, true);
• dma_buf_io_wait_maps(ctx);
• dma_buf_io_put_ctx(ctx);

+}

+int dma_buf_io_ctx_create(struct file *file,

• 	   struct dma_buf *dmabuf,
  

• 	   enum dma_data_direction dir,
  

• 	   struct dma_buf_io_ctx **out_ctx)
  

+{

• struct dma_buf_io_ctx *ctx;
• int ret;
• if (!file->f_op->init_dma_buf_io_ctx)

• return -EOPNOTSUPP;
  

• ctx = kmalloc_obj(*ctx);
• if (!ctx)

• return -ENOMEM;
  

• memset(ctx, 0, sizeof(*ctx));
• ctx->dir = dir;
• ctx->dmabuf = dmabuf;
• get_dma_buf(dmabuf);
• mutex_init(&ctx->map_mutex);
• mutex_init(&ctx->map_create_mutex);
• atomic_set(&ctx->active_maps, 0);
• atomic_set(&ctx->all_maps, 0);
• init_waitqueue_head(&ctx->maps_wq);
• ret = file->f_op->init_dma_buf_io_ctx(file, ctx);
• if (ret) {

• kfree(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;
  

• *out_ctx = ctx;
• return 0;

+}

+void dma_buf_io_detach(struct dma_buf_io_ctx *ctx) +{

• guard(mutex)(&ctx->map_create_mutex);
• dma_buf_io_kill_maps(ctx, true);
• dma_buf_io_wait_maps(ctx);

+} +EXPORT_SYMBOL_NS_GPL(dma_buf_io_detach, "DMA_BUF"); diff --git a/include/linux/dma-buf-io.h b/include/linux/dma-buf-io.h new file mode 100644 index 000000000000..5ea92fc78582 --- /dev/null +++ b/include/linux/dma-buf-io.h @@ -0,0 +1,113 @@ +/* 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_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;
• /*

• * Shift for the minimum segment size of the mapping.
  

• */
  

• unsigned min_seg_shift;
• struct work_struct release_work;
• 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;
• /* synchronises map reassignment */
• struct mutex map_mutex;
• struct mutex map_create_mutex;
• /* maps that may still be in use. */
• atomic_t active_maps;
• atomic_t all_maps;
• struct wait_queue_head maps_wq;
• bool maps_killed;
• void *dev_priv;
• const struct dma_buf_io_ops *dev_ops;

+};

+int dma_buf_io_ctx_create(struct file *file,

• 	   struct dma_buf *dmabuf,
  

• 	   enum dma_data_direction dir,
  

• 	   struct dma_buf_io_ctx **ctx);
  

+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 struct dma_buf_io_map * +dma_buf_io_get_map(struct dma_buf_io_ctx *ctx, bool nowait) +{

• struct dma_buf_io_map *map;
• map = __dma_buf_io_get_map(ctx);
• if (likely(map))

• return map;
  

• if (nowait)

• return ERR_PTR(-EAGAIN);
  

• return dma_buf_io_create_map(ctx);

+}

+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,

• 	struct sg_table *sgt);
  

+void dma_buf_io_detach(struct dma_buf_io_ctx *ctx);

+#endif diff --git a/include/linux/fs.h b/include/linux/fs.h index f9d1e05e8ae6..05c1ff495732 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; struct file_operations { @@ -1960,6 +1961,7 @@ 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 */ -- 2.54.0