[PATCH V2 0/5] ublk: NUMA-aware memory allocation

[PATCH V2 0/5] ublk: NUMA-aware memory allocation

[Ming Lei]

Hi Jens,

The 1st two patches implement ublk driver NUMA aware memory allocation.

The last two patches implement it for ublk selftest utility.

`taskset -c 0-31 ~/git/fio/t/io_uring -p0 -n16 -r 40 /dev/ublkb0` shows
5%~10% IOPS improvement on one AMD zen4 dual socket machine when creating
ublk/null with 16 queues and AUTO_BUF_REG(zero copy).


V2:
	- use a flexible array member for queues field, save one indirection
	  for retrieving ublk queue
	- rename __queues into queues 
	- remove the queue_size field from struct ublk_device
	- Move queue allocation and deallocation into ublk_init_queue() and
	ublk_deinit_queue() 
	- use flexible array for ublk_queue.ios
	- convert ublk_thread_set_sched_affinity() to use pthread_setaffinity_np()

Ming Lei (5):
  ublk: reorder tag_set initialization before queue allocation
  ublk: implement NUMA-aware memory allocation
  ublk: use flexible array for ublk_queue.ios
  selftests: ublk: set CPU affinity before thread initialization
  selftests: ublk: make ublk_thread thread-local variable

 drivers/block/ublk_drv.c             | 99 +++++++++++++++++-----------
 tools/testing/selftests/ublk/kublk.c | 70 ++++++++++++--------
 tools/testing/selftests/ublk/kublk.h |  9 +--
 3 files changed, 106 insertions(+), 72 deletions(-)

-- 
2.47.0

[PATCH V2 1/5] ublk: reorder tag_set initialization before queue allocation

[Ming Lei]

Move ublk_add_tag_set() before ublk_init_queues() in the device
initialization path. This allows us to use the blk-mq CPU-to-queue
mapping established by the tag_set to determine the appropriate
NUMA node for each queue allocation.

The error handling paths are also reordered accordingly.

Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 drivers/block/ublk_drv.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index 0c74a41a6753..2569566bf5e6 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -3178,17 +3178,17 @@ static int ublk_ctrl_add_dev(const struct ublksrv_ctrl_cmd *header)
 			ub->dev_info.nr_hw_queues, nr_cpu_ids);
 	ublk_align_max_io_size(ub);
 
-	ret = ublk_init_queues(ub);
+	ret = ublk_add_tag_set(ub);
 	if (ret)
 		goto out_free_dev_number;
 
-	ret = ublk_add_tag_set(ub);
+	ret = ublk_init_queues(ub);
 	if (ret)
-		goto out_deinit_queues;
+		goto out_free_tag_set;
 
 	ret = -EFAULT;
 	if (copy_to_user(argp, &ub->dev_info, sizeof(info)))
-		goto out_free_tag_set;
+		goto out_deinit_queues;
 
 	/*
 	 * Add the char dev so that ublksrv daemon can be setup.
@@ -3197,10 +3197,10 @@ static int ublk_ctrl_add_dev(const struct ublksrv_ctrl_cmd *header)
 	ret = ublk_add_chdev(ub);
 	goto out_unlock;
 
-out_free_tag_set:
-	blk_mq_free_tag_set(&ub->tag_set);
 out_deinit_queues:
 	ublk_deinit_queues(ub);
+out_free_tag_set:
+	blk_mq_free_tag_set(&ub->tag_set);
 out_free_dev_number:
 	ublk_free_dev_number(ub);
 out_free_ub:
-- 
2.47.0

[PATCH V2 2/5] ublk: implement NUMA-aware memory allocation

[Ming Lei]

Implement NUMA-friendly memory allocation for ublk driver to improve
performance on multi-socket systems.

This commit includes the following changes:

1. Convert struct ublk_device to use a flexible array member for the
   queues field (using DECLARE_FLEX_ARRAY) instead of a separate
   pointer array allocation. This eliminates one level of indirection
   and simplifies memory management. The queues array is now allocated
   as part of struct ublk_device using struct_size().

2. Rename __queues to queues, dropping the __ prefix since the field is
   now accessed directly throughout the codebase rather than only through
   the ublk_get_queue() helper.

3. Remove the queue_size field from struct ublk_device as it is no longer
   needed.

4. Move queue allocation and deallocation into ublk_init_queue() and
   ublk_deinit_queue() respectively, improving encapsulation. This
   simplifies ublk_init_queues() and ublk_deinit_queues() to just
   iterate and call the per-queue functions.

5. Add ublk_get_queue_numa_node() helper function to determine the
   appropriate NUMA node for a queue by finding the first CPU mapped
   to that queue via tag_set.map[HCTX_TYPE_DEFAULT].mq_map[] and
   converting it to a NUMA node using cpu_to_node(). This function is
   called internally by ublk_init_queue() to determine the allocation
   node.

6. Allocate each queue structure on its local NUMA node using
   kvzalloc_node() in ublk_init_queue().

7. Allocate the I/O command buffer on the same NUMA node using
   alloc_pages_node().

This reduces memory access latency on multi-socket NUMA systems by
ensuring each queue's data structures are local to the CPUs that
access them.

Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 drivers/block/ublk_drv.c | 84 +++++++++++++++++++++++++---------------
 1 file changed, 53 insertions(+), 31 deletions(-)

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index 2569566bf5e6..394e9b5f512f 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -209,9 +209,6 @@ struct ublk_queue {
 struct ublk_device {
 	struct gendisk		*ub_disk;
 
-	char	*__queues;
-
-	unsigned int	queue_size;
 	struct ublksrv_ctrl_dev_info	dev_info;
 
 	struct blk_mq_tag_set	tag_set;
@@ -239,6 +236,8 @@ struct ublk_device {
 	bool canceling;
 	pid_t 	ublksrv_tgid;
 	struct delayed_work	exit_work;
+
+	DECLARE_FLEX_ARRAY(struct ublk_queue *, queues);
 };
 
 /* header of ublk_params */
@@ -781,7 +780,7 @@ static noinline void ublk_put_device(struct ublk_device *ub)
 static inline struct ublk_queue *ublk_get_queue(struct ublk_device *dev,
 		int qid)
 {
-       return (struct ublk_queue *)&(dev->__queues[qid * dev->queue_size]);
+	return dev->queues[qid];
 }
 
 static inline bool ublk_rq_has_data(const struct request *rq)
@@ -2662,9 +2661,13 @@ static const struct file_operations ublk_ch_fops = {
 
 static void ublk_deinit_queue(struct ublk_device *ub, int q_id)
 {
-	int size = ublk_queue_cmd_buf_size(ub);
-	struct ublk_queue *ubq = ublk_get_queue(ub, q_id);
-	int i;
+	struct ublk_queue *ubq = ub->queues[q_id];
+	int size, i;
+
+	if (!ubq)
+		return;
+
+	size = ublk_queue_cmd_buf_size(ub);
 
 	for (i = 0; i < ubq->q_depth; i++) {
 		struct ublk_io *io = &ubq->ios[i];
@@ -2676,57 +2679,76 @@ static void ublk_deinit_queue(struct ublk_device *ub, int q_id)
 
 	if (ubq->io_cmd_buf)
 		free_pages((unsigned long)ubq->io_cmd_buf, get_order(size));
+
+	kvfree(ubq);
+	ub->queues[q_id] = NULL;
+}
+
+static int ublk_get_queue_numa_node(struct ublk_device *ub, int q_id)
+{
+	unsigned int cpu;
+
+	/* Find first CPU mapped to this queue */
+	for_each_possible_cpu(cpu) {
+		if (ub->tag_set.map[HCTX_TYPE_DEFAULT].mq_map[cpu] == q_id)
+			return cpu_to_node(cpu);
+	}
+
+	return NUMA_NO_NODE;
 }
 
 static int ublk_init_queue(struct ublk_device *ub, int q_id)
 {
-	struct ublk_queue *ubq = ublk_get_queue(ub, q_id);
+	int depth = ub->dev_info.queue_depth;
+	int ubq_size = sizeof(struct ublk_queue) + depth * sizeof(struct ublk_io);
 	gfp_t gfp_flags = GFP_KERNEL | __GFP_ZERO;
-	void *ptr;
+	struct ublk_queue *ubq;
+	struct page *page;
+	int numa_node;
 	int size;
 
+	/* Determine NUMA node based on queue's CPU affinity */
+	numa_node = ublk_get_queue_numa_node(ub, q_id);
+
+	/* Allocate queue structure on local NUMA node */
+	ubq = kvzalloc_node(ubq_size, GFP_KERNEL, numa_node);
+	if (!ubq)
+		return -ENOMEM;
+
 	spin_lock_init(&ubq->cancel_lock);
 	ubq->flags = ub->dev_info.flags;
 	ubq->q_id = q_id;
-	ubq->q_depth = ub->dev_info.queue_depth;
+	ubq->q_depth = depth;
 	size = ublk_queue_cmd_buf_size(ub);
 
-	ptr = (void *) __get_free_pages(gfp_flags, get_order(size));
-	if (!ptr)
+	/* Allocate I/O command buffer on local NUMA node */
+	page = alloc_pages_node(numa_node, gfp_flags, get_order(size));
+	if (!page) {
+		kvfree(ubq);
 		return -ENOMEM;
+	}
+	ubq->io_cmd_buf = page_address(page);
 
-	ubq->io_cmd_buf = ptr;
+	ub->queues[q_id] = ubq;
 	ubq->dev = ub;
 	return 0;
 }
 
 static void ublk_deinit_queues(struct ublk_device *ub)
 {
-	int nr_queues = ub->dev_info.nr_hw_queues;
 	int i;
 
-	if (!ub->__queues)
-		return;
-
-	for (i = 0; i < nr_queues; i++)
+	for (i = 0; i < ub->dev_info.nr_hw_queues; i++)
 		ublk_deinit_queue(ub, i);
-	kvfree(ub->__queues);
 }
 
 static int ublk_init_queues(struct ublk_device *ub)
 {
-	int nr_queues = ub->dev_info.nr_hw_queues;
-	int depth = ub->dev_info.queue_depth;
-	int ubq_size = sizeof(struct ublk_queue) + depth * sizeof(struct ublk_io);
-	int i, ret = -ENOMEM;
+	int i, ret;
 
-	ub->queue_size = ubq_size;
-	ub->__queues = kvcalloc(nr_queues, ubq_size, GFP_KERNEL);
-	if (!ub->__queues)
-		return ret;
-
-	for (i = 0; i < nr_queues; i++) {
-		if (ublk_init_queue(ub, i))
+	for (i = 0; i < ub->dev_info.nr_hw_queues; i++) {
+		ret = ublk_init_queue(ub, i);
+		if (ret)
 			goto fail;
 	}
 
@@ -3128,7 +3150,7 @@ static int ublk_ctrl_add_dev(const struct ublksrv_ctrl_cmd *header)
 		goto out_unlock;
 
 	ret = -ENOMEM;
-	ub = kzalloc(sizeof(*ub), GFP_KERNEL);
+	ub = kzalloc(struct_size(ub, queues, info.nr_hw_queues), GFP_KERNEL);
 	if (!ub)
 		goto out_unlock;
 	mutex_init(&ub->mutex);
-- 
2.47.0

[PATCH V2 3/5] ublk: use flexible array for ublk_queue.ios

[Ming Lei]

Convert ublk_queue to use DECLARE_FLEX_ARRAY for the ios field and
use struct_size() for allocation, following kernel best practices.

Changes in this commit:

1. Convert ios field from "struct ublk_io ios[]" to use
   DECLARE_FLEX_ARRAY(struct ublk_io, ios) for consistency with
   modern kernel style.

2. Update ublk_init_queue() to use struct_size(ubq, ios, depth)
   instead of manual size calculation (sizeof(struct ublk_queue) +
   depth * sizeof(struct ublk_io)).

This provides better type safety and makes the code more maintainable
by using standard kernel macros for flexible array handling.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 drivers/block/ublk_drv.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index 394e9b5f512f..cef9cfa94feb 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -203,7 +203,8 @@ struct ublk_queue {
 	bool fail_io; /* copy of dev->state == UBLK_S_DEV_FAIL_IO */
 	spinlock_t		cancel_lock;
 	struct ublk_device *dev;
-	struct ublk_io ios[];
+
+	DECLARE_FLEX_ARRAY(struct ublk_io, ios);
 };
 
 struct ublk_device {
@@ -2700,7 +2701,6 @@ static int ublk_get_queue_numa_node(struct ublk_device *ub, int q_id)
 static int ublk_init_queue(struct ublk_device *ub, int q_id)
 {
 	int depth = ub->dev_info.queue_depth;
-	int ubq_size = sizeof(struct ublk_queue) + depth * sizeof(struct ublk_io);
 	gfp_t gfp_flags = GFP_KERNEL | __GFP_ZERO;
 	struct ublk_queue *ubq;
 	struct page *page;
@@ -2711,7 +2711,8 @@ static int ublk_init_queue(struct ublk_device *ub, int q_id)
 	numa_node = ublk_get_queue_numa_node(ub, q_id);
 
 	/* Allocate queue structure on local NUMA node */
-	ubq = kvzalloc_node(ubq_size, GFP_KERNEL, numa_node);
+	ubq = kvzalloc_node(struct_size(ubq, ios, depth), GFP_KERNEL,
+			    numa_node);
 	if (!ubq)
 		return -ENOMEM;
 
-- 
2.47.0

[PATCH V2 4/5] selftests: ublk: set CPU affinity before thread initialization

[Ming Lei]

Move ublk_thread_set_sched_affinity() call before ublk_thread_init()
to ensure memory allocations during thread initialization occur on
the correct NUMA node. This leverages Linux's first-touch memory
policy for better NUMA locality.

Also convert ublk_thread_set_sched_affinity() to use
pthread_setaffinity_np() instead of sched_setaffinity(), as the
pthread API is the proper interface for setting thread affinity in
multithreaded programs.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 tools/testing/selftests/ublk/kublk.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/ublk/kublk.c b/tools/testing/selftests/ublk/kublk.c
index 6b8123c12a7a..062537ab8976 100644
--- a/tools/testing/selftests/ublk/kublk.c
+++ b/tools/testing/selftests/ublk/kublk.c
@@ -839,7 +839,7 @@ static int ublk_process_io(struct ublk_thread *t)
 static void ublk_thread_set_sched_affinity(const struct ublk_thread *t,
 		cpu_set_t *cpuset)
 {
-        if (sched_setaffinity(0, sizeof(*cpuset), cpuset) < 0)
+	if (pthread_setaffinity_np(pthread_self(), sizeof(*cpuset), cpuset) < 0)
 		ublk_err("ublk dev %u thread %u set affinity failed",
 				t->dev->dev_info.dev_id, t->idx);
 }
@@ -862,15 +862,21 @@ static void *ublk_io_handler_fn(void *data)
 	t->dev = info->dev;
 	t->idx = info->idx;
 
+	/*
+	 * IO perf is sensitive with queue pthread affinity on NUMA machine
+	 *
+	 * Set sched_affinity at beginning, so following allocated memory/pages
+	 * could be CPU/NUMA aware.
+	 */
+	if (info->affinity)
+		ublk_thread_set_sched_affinity(t, info->affinity);
+
 	ret = ublk_thread_init(t, info->extra_flags);
 	if (ret) {
 		ublk_err("ublk dev %d thread %u init failed\n",
 				dev_id, t->idx);
 		return NULL;
 	}
-	/* IO perf is sensitive with queue pthread affinity on NUMA machine*/
-	if (info->affinity)
-		ublk_thread_set_sched_affinity(t, info->affinity);
 	sem_post(info->ready);
 
 	ublk_dbg(UBLK_DBG_THREAD, "tid %d: ublk dev %d thread %u started\n",
-- 
2.47.0

[PATCH V2 5/5] selftests: ublk: make ublk_thread thread-local variable

[Ming Lei]

Refactor ublk_thread to be a thread-local variable instead of storing
it in ublk_dev:

- Remove pthread_t thread field from struct ublk_thread and move it to
  struct ublk_thread_info

- Remove struct ublk_thread array from struct ublk_dev, reducing memory
  footprint

- Define struct ublk_thread as local variable in __ublk_io_handler_fn()
  instead of accessing it from dev->threads[]

- Extract main IO handling logic into __ublk_io_handler_fn() which is
  marked as noinline

- Move CPU affinity setup to ublk_io_handler_fn() before calling
  __ublk_io_handler_fn()

- Update ublk_thread_set_sched_affinity() to take struct ublk_thread_info *
  instead of struct ublk_thread *, and use pthread_setaffinity_np()
  instead of sched_setaffinity()

- Reorder struct ublk_thread fields to group related state together

This change makes each thread's ublk_thread structure truly local to
the thread, improving cache locality and reducing memory usage.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 tools/testing/selftests/ublk/kublk.c | 76 +++++++++++++++-------------
 tools/testing/selftests/ublk/kublk.h |  9 ++--
 2 files changed, 45 insertions(+), 40 deletions(-)

diff --git a/tools/testing/selftests/ublk/kublk.c b/tools/testing/selftests/ublk/kublk.c
index 062537ab8976..f8fa102a627f 100644
--- a/tools/testing/selftests/ublk/kublk.c
+++ b/tools/testing/selftests/ublk/kublk.c
@@ -836,62 +836,70 @@ static int ublk_process_io(struct ublk_thread *t)
 	return reapped;
 }
 
-static void ublk_thread_set_sched_affinity(const struct ublk_thread *t,
-		cpu_set_t *cpuset)
-{
-	if (pthread_setaffinity_np(pthread_self(), sizeof(*cpuset), cpuset) < 0)
-		ublk_err("ublk dev %u thread %u set affinity failed",
-				t->dev->dev_info.dev_id, t->idx);
-}
-
 struct ublk_thread_info {
 	struct ublk_dev 	*dev;
+	pthread_t		thread;
 	unsigned		idx;
 	sem_t 			*ready;
 	cpu_set_t 		*affinity;
 	unsigned long long	extra_flags;
 };
 
-static void *ublk_io_handler_fn(void *data)
+static void ublk_thread_set_sched_affinity(const struct ublk_thread_info *info)
 {
-	struct ublk_thread_info *info = data;
-	struct ublk_thread *t = &info->dev->threads[info->idx];
+	if (pthread_setaffinity_np(pthread_self(), sizeof(*info->affinity), info->affinity) < 0)
+		ublk_err("ublk dev %u thread %u set affinity failed",
+				info->dev->dev_info.dev_id, info->idx);
+}
+
+static __attribute__((noinline)) int __ublk_io_handler_fn(struct ublk_thread_info *info)
+{
+	struct ublk_thread t = {
+		.dev = info->dev,
+		.idx = info->idx,
+	};
 	int dev_id = info->dev->dev_info.dev_id;
 	int ret;
 
-	t->dev = info->dev;
-	t->idx = info->idx;
-
-	/*
-	 * IO perf is sensitive with queue pthread affinity on NUMA machine
-	 *
-	 * Set sched_affinity at beginning, so following allocated memory/pages
-	 * could be CPU/NUMA aware.
-	 */
-	if (info->affinity)
-		ublk_thread_set_sched_affinity(t, info->affinity);
-
-	ret = ublk_thread_init(t, info->extra_flags);
+	ret = ublk_thread_init(&t, info->extra_flags);
 	if (ret) {
 		ublk_err("ublk dev %d thread %u init failed\n",
-				dev_id, t->idx);
-		return NULL;
+				dev_id, t.idx);
+		return ret;
 	}
 	sem_post(info->ready);
 
 	ublk_dbg(UBLK_DBG_THREAD, "tid %d: ublk dev %d thread %u started\n",
-			gettid(), dev_id, t->idx);
+			gettid(), dev_id, t.idx);
 
 	/* submit all io commands to ublk driver */
-	ublk_submit_fetch_commands(t);
+	ublk_submit_fetch_commands(&t);
 	do {
-		if (ublk_process_io(t) < 0)
+		if (ublk_process_io(&t) < 0)
 			break;
 	} while (1);
 
 	ublk_dbg(UBLK_DBG_THREAD, "tid %d: ublk dev %d thread %d exiting\n",
-		 gettid(), dev_id, t->idx);
-	ublk_thread_deinit(t);
+		 gettid(), dev_id, t.idx);
+	ublk_thread_deinit(&t);
+	return 0;
+}
+
+static void *ublk_io_handler_fn(void *data)
+{
+	struct ublk_thread_info *info = data;
+
+	/*
+	 * IO perf is sensitive with queue pthread affinity on NUMA machine
+	 *
+	 * Set sched_affinity at beginning, so following allocated memory/pages
+	 * could be CPU/NUMA aware.
+	 */
+	if (info->affinity)
+		ublk_thread_set_sched_affinity(info);
+
+	__ublk_io_handler_fn(info);
+
 	return NULL;
 }
 
@@ -989,14 +997,13 @@ static int ublk_start_daemon(const struct dev_ctx *ctx, struct ublk_dev *dev)
 		 */
 		if (dev->nthreads == dinfo->nr_hw_queues)
 			tinfo[i].affinity = &affinity_buf[i];
-		pthread_create(&dev->threads[i].thread, NULL,
+		pthread_create(&tinfo[i].thread, NULL,
 				ublk_io_handler_fn,
 				&tinfo[i]);
 	}
 
 	for (i = 0; i < dev->nthreads; i++)
 		sem_wait(&ready);
-	free(tinfo);
 	free(affinity_buf);
 
 	/* everything is fine now, start us */
@@ -1019,7 +1026,8 @@ static int ublk_start_daemon(const struct dev_ctx *ctx, struct ublk_dev *dev)
 
 	/* wait until we are terminated */
 	for (i = 0; i < dev->nthreads; i++)
-		pthread_join(dev->threads[i].thread, &thread_ret);
+		pthread_join(tinfo[i].thread, &thread_ret);
+	free(tinfo);
  fail:
 	for (i = 0; i < dinfo->nr_hw_queues; i++)
 		ublk_queue_deinit(&dev->q[i]);
diff --git a/tools/testing/selftests/ublk/kublk.h b/tools/testing/selftests/ublk/kublk.h
index 5e55484fb0aa..fe42705c6d42 100644
--- a/tools/testing/selftests/ublk/kublk.h
+++ b/tools/testing/selftests/ublk/kublk.h
@@ -175,23 +175,20 @@ struct ublk_queue {
 
 struct ublk_thread {
 	struct ublk_dev *dev;
-	struct io_uring ring;
-	unsigned int cmd_inflight;
-	unsigned int io_inflight;
-
-	pthread_t thread;
 	unsigned idx;
 
 #define UBLKS_T_STOPPING	(1U << 0)
 #define UBLKS_T_IDLE	(1U << 1)
 	unsigned state;
+	unsigned int cmd_inflight;
+	unsigned int io_inflight;
+	struct io_uring ring;
 };
 
 struct ublk_dev {
 	struct ublk_tgt tgt;
 	struct ublksrv_ctrl_dev_info  dev_info;
 	struct ublk_queue q[UBLK_MAX_QUEUES];
-	struct ublk_thread threads[UBLK_MAX_THREADS];
 	unsigned nthreads;
 	unsigned per_io_tasks;
 
-- 
2.47.0

Re: [PATCH V2 3/5] ublk: use flexible array for ublk_queue.ios

[Caleb Sander Mateos]

On Tue, Oct 28, 2025 at 1:57 AM Ming Lei <ming.lei@redhat.com> wrote:
>
> Convert ublk_queue to use DECLARE_FLEX_ARRAY for the ios field and
> use struct_size() for allocation, following kernel best practices.
>
> Changes in this commit:
>
> 1. Convert ios field from "struct ublk_io ios[]" to use
>    DECLARE_FLEX_ARRAY(struct ublk_io, ios) for consistency with
>    modern kernel style.

Documentation/process/deprecated.rst suggests that
DECLARE_FLEX_ARRAY() is discouraged except in the niche cases when
it's necessary (which don't apply here). Or am I misunderstanding
something? However, struct ublk_io ios[] does seem like a good use
case for __counted_by().

>
> 2. Update ublk_init_queue() to use struct_size(ubq, ios, depth)
>    instead of manual size calculation (sizeof(struct ublk_queue) +
>    depth * sizeof(struct ublk_io)).

Sure, this looks like an improvement.

Best,
Caleb

>
> This provides better type safety and makes the code more maintainable
> by using standard kernel macros for flexible array handling.
>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  drivers/block/ublk_drv.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> index 394e9b5f512f..cef9cfa94feb 100644
> --- a/drivers/block/ublk_drv.c
> +++ b/drivers/block/ublk_drv.c
> @@ -203,7 +203,8 @@ struct ublk_queue {
>         bool fail_io; /* copy of dev->state == UBLK_S_DEV_FAIL_IO */
>         spinlock_t              cancel_lock;
>         struct ublk_device *dev;
> -       struct ublk_io ios[];
> +
> +       DECLARE_FLEX_ARRAY(struct ublk_io, ios);
>  };
>
>  struct ublk_device {
> @@ -2700,7 +2701,6 @@ static int ublk_get_queue_numa_node(struct ublk_device *ub, int q_id)
>  static int ublk_init_queue(struct ublk_device *ub, int q_id)
>  {
>         int depth = ub->dev_info.queue_depth;
> -       int ubq_size = sizeof(struct ublk_queue) + depth * sizeof(struct ublk_io);
>         gfp_t gfp_flags = GFP_KERNEL | __GFP_ZERO;
>         struct ublk_queue *ubq;
>         struct page *page;
> @@ -2711,7 +2711,8 @@ static int ublk_init_queue(struct ublk_device *ub, int q_id)
>         numa_node = ublk_get_queue_numa_node(ub, q_id);
>
>         /* Allocate queue structure on local NUMA node */
> -       ubq = kvzalloc_node(ubq_size, GFP_KERNEL, numa_node);
> +       ubq = kvzalloc_node(struct_size(ubq, ios, depth), GFP_KERNEL,
> +                           numa_node);
>         if (!ubq)
>                 return -ENOMEM;
>
> --
> 2.47.0
>

Re: [PATCH V2 3/5] ublk: use flexible array for ublk_queue.ios

[Ming Lei]

On Tue, Oct 28, 2025 at 02:52:25PM -0700, Caleb Sander Mateos wrote:
> On Tue, Oct 28, 2025 at 1:57 AM Ming Lei <ming.lei@redhat.com> wrote:
> >
> > Convert ublk_queue to use DECLARE_FLEX_ARRAY for the ios field and
> > use struct_size() for allocation, following kernel best practices.
> >
> > Changes in this commit:
> >
> > 1. Convert ios field from "struct ublk_io ios[]" to use
> >    DECLARE_FLEX_ARRAY(struct ublk_io, ios) for consistency with
> >    modern kernel style.
> 
> Documentation/process/deprecated.rst suggests that
> DECLARE_FLEX_ARRAY() is discouraged except in the niche cases when
> it's necessary (which don't apply here). Or am I misunderstanding
> something?

You are right, DECLARE_FLEX_ARRAY is only needed:

```
when the flexible array is either alone in a struct or is part of a union.
```

> However, struct ublk_io ios[] does seem like a good use
> case for __counted_by().

Good point!

Thanks, 
Ming