[PATCH V5 0/6] loop: improve loop aio perf by IOCB_NOWAIT

[PATCH V5 0/6] loop: improve loop aio perf by IOCB_NOWAIT

[Ming Lei]

Hello Jens,

This patchset improves loop aio perf by using IOCB_NOWAIT for avoiding to queue aio
command to workqueue context, meantime refactor lo_rw_aio() a bit.

In my test VM, loop disk perf becomes very close to perf of the backing block
device(nvme/mq virtio-scsi).

And Mikulas verified that this way can improve 12jobs sequential readwrite io by
~5X, and basically solve the reported problem together with loop MQ change.

https://lore.kernel.org/linux-block/a8e5c76a-231f-07d1-a394-847de930f638@redhat.com/

Zhaoyang Huang also mentioned it may fix their performance issue on Android
use case.

The loop MQ change will be posted as standalone patch, because it needs
UAPI change.

V5:
	- only try nowait in case that backing file supports it (Yu Kuai)
	- fix one lockdep assert (syzbot) 
	- improve comment log (Christoph)

V4:
	- rebase
	- re-organize and make it more readable 

V3:
	- add reviewed-by tag
	- rename variable & improve commit log & comment on 5/5(Christoph)

V2:
	- patch style fix & cleanup (Christoph)
	- fix randwrite perf regression on sparse backing file
	- drop MQ change

Ming Lei (6):
  loop: add helper lo_cmd_nr_bvec()
  loop: add helper lo_rw_aio_prep()
  loop: add lo_submit_rw_aio()
  loop: move command blkcg/memcg initialization into loop_queue_work
  loop: try to handle loop aio command via NOWAIT IO first
  loop: add hint for handling aio via IOCB_NOWAIT

 drivers/block/loop.c | 233 +++++++++++++++++++++++++++++++++++--------
 1 file changed, 194 insertions(+), 39 deletions(-)

-- 
2.47.0

[PATCH V5 1/6] loop: add helper lo_cmd_nr_bvec()

[Ming Lei]

Add lo_cmd_nr_bvec() and prepare for refactoring lo_rw_aio().

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 drivers/block/loop.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 053a086d547e..af443651dff5 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -337,6 +337,19 @@ static void lo_rw_aio_complete(struct kiocb *iocb, long ret)
 	lo_rw_aio_do_completion(cmd);
 }
 
+static inline unsigned lo_cmd_nr_bvec(struct loop_cmd *cmd)
+{
+	struct request *rq = blk_mq_rq_from_pdu(cmd);
+	struct req_iterator rq_iter;
+	struct bio_vec tmp;
+	int nr_bvec = 0;
+
+	rq_for_each_bvec(tmp, rq, rq_iter)
+		nr_bvec++;
+
+	return nr_bvec;
+}
+
 static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
 		     loff_t pos, int rw)
 {
@@ -348,12 +361,9 @@ static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
 	struct file *file = lo->lo_backing_file;
 	struct bio_vec tmp;
 	unsigned int offset;
-	int nr_bvec = 0;
+	int nr_bvec = lo_cmd_nr_bvec(cmd);
 	int ret;
 
-	rq_for_each_bvec(tmp, rq, rq_iter)
-		nr_bvec++;
-
 	if (rq->bio != rq->biotail) {
 
 		bvec = kmalloc_array(nr_bvec, sizeof(struct bio_vec),
-- 
2.47.0

[PATCH V5 2/6] loop: add helper lo_rw_aio_prep()

[Ming Lei]

Add helper lo_rw_aio_prep() to separate the preparation phase(setting up bio
vectors and initializing the iocb structure) from the actual I/O execution
in the loop block driver.

Prepare for using NOWAIT to improve loop performance.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 drivers/block/loop.c | 63 ++++++++++++++++++++++++++++----------------
 1 file changed, 40 insertions(+), 23 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index af443651dff5..b065892106a6 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -350,21 +350,15 @@ static inline unsigned lo_cmd_nr_bvec(struct loop_cmd *cmd)
 	return nr_bvec;
 }
 
-static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
-		     loff_t pos, int rw)
+static int lo_rw_aio_prep(struct loop_device *lo, struct loop_cmd *cmd,
+			  unsigned nr_bvec, loff_t pos)
 {
-	struct iov_iter iter;
-	struct req_iterator rq_iter;
-	struct bio_vec *bvec;
 	struct request *rq = blk_mq_rq_from_pdu(cmd);
-	struct bio *bio = rq->bio;
-	struct file *file = lo->lo_backing_file;
-	struct bio_vec tmp;
-	unsigned int offset;
-	int nr_bvec = lo_cmd_nr_bvec(cmd);
-	int ret;
 
 	if (rq->bio != rq->biotail) {
+		struct req_iterator rq_iter;
+		struct bio_vec *bvec;
+		struct bio_vec tmp;
 
 		bvec = kmalloc_array(nr_bvec, sizeof(struct bio_vec),
 				     GFP_NOIO);
@@ -382,8 +376,42 @@ static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
 			*bvec = tmp;
 			bvec++;
 		}
-		bvec = cmd->bvec;
+	} else {
+		cmd->bvec = NULL;
+	}
+
+	cmd->iocb.ki_pos = pos;
+	cmd->iocb.ki_filp = lo->lo_backing_file;
+	cmd->iocb.ki_ioprio = req_get_ioprio(rq);
+	if (cmd->use_aio) {
+		cmd->iocb.ki_complete = lo_rw_aio_complete;
+		cmd->iocb.ki_flags = IOCB_DIRECT;
+	} else {
+		cmd->iocb.ki_complete = NULL;
+		cmd->iocb.ki_flags = 0;
+	}
+	return 0;
+}
+
+static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
+		     loff_t pos, int rw)
+{
+	struct iov_iter iter;
+	struct bio_vec *bvec;
+	struct request *rq = blk_mq_rq_from_pdu(cmd);
+	struct bio *bio = rq->bio;
+	struct file *file = lo->lo_backing_file;
+	unsigned int offset;
+	int nr_bvec = lo_cmd_nr_bvec(cmd);
+	int ret;
+
+	ret = lo_rw_aio_prep(lo, cmd, nr_bvec, pos);
+	if (unlikely(ret))
+		return ret;
+
+	if (cmd->bvec) {
 		offset = 0;
+		bvec = cmd->bvec;
 	} else {
 		/*
 		 * Same here, this bio may be started from the middle of the
@@ -398,17 +426,6 @@ static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
 	iov_iter_bvec(&iter, rw, bvec, nr_bvec, blk_rq_bytes(rq));
 	iter.iov_offset = offset;
 
-	cmd->iocb.ki_pos = pos;
-	cmd->iocb.ki_filp = file;
-	cmd->iocb.ki_ioprio = req_get_ioprio(rq);
-	if (cmd->use_aio) {
-		cmd->iocb.ki_complete = lo_rw_aio_complete;
-		cmd->iocb.ki_flags = IOCB_DIRECT;
-	} else {
-		cmd->iocb.ki_complete = NULL;
-		cmd->iocb.ki_flags = 0;
-	}
-
 	if (rw == ITER_SOURCE) {
 		kiocb_start_write(&cmd->iocb);
 		ret = file->f_op->write_iter(&cmd->iocb, &iter);
-- 
2.47.0

[PATCH V5 3/6] loop: add lo_submit_rw_aio()

[Ming Lei]

Refactor lo_rw_aio() by extracting the I/O submission logic into a new
helper function lo_submit_rw_aio(). This further improves code organization
by separating the I/O preparation, submission, and completion handling into
distinct phases.

Prepare for using NOWAIT to improve loop performance.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 drivers/block/loop.c | 41 ++++++++++++++++++++++++-----------------
 1 file changed, 24 insertions(+), 17 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index b065892106a6..3ab910572bd9 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -393,38 +393,32 @@ static int lo_rw_aio_prep(struct loop_device *lo, struct loop_cmd *cmd,
 	return 0;
 }
 
-static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
-		     loff_t pos, int rw)
+static int lo_submit_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
+			    int nr_bvec, int rw)
 {
-	struct iov_iter iter;
-	struct bio_vec *bvec;
 	struct request *rq = blk_mq_rq_from_pdu(cmd);
-	struct bio *bio = rq->bio;
 	struct file *file = lo->lo_backing_file;
-	unsigned int offset;
-	int nr_bvec = lo_cmd_nr_bvec(cmd);
+	struct iov_iter iter;
 	int ret;
 
-	ret = lo_rw_aio_prep(lo, cmd, nr_bvec, pos);
-	if (unlikely(ret))
-		return ret;
-
 	if (cmd->bvec) {
-		offset = 0;
-		bvec = cmd->bvec;
+		iov_iter_bvec(&iter, rw, cmd->bvec, nr_bvec, blk_rq_bytes(rq));
+		iter.iov_offset = 0;
 	} else {
+		struct bio *bio = rq->bio;
+		struct bio_vec *bvec = __bvec_iter_bvec(bio->bi_io_vec,
+				bio->bi_iter);
+
 		/*
 		 * Same here, this bio may be started from the middle of the
 		 * 'bvec' because of bio splitting, so offset from the bvec
 		 * must be passed to iov iterator
 		 */
-		offset = bio->bi_iter.bi_bvec_done;
-		bvec = __bvec_iter_bvec(bio->bi_io_vec, bio->bi_iter);
+		iov_iter_bvec(&iter, rw, bvec, nr_bvec, blk_rq_bytes(rq));
+		iter.iov_offset = bio->bi_iter.bi_bvec_done;
 	}
 	atomic_set(&cmd->ref, 2);
 
-	iov_iter_bvec(&iter, rw, bvec, nr_bvec, blk_rq_bytes(rq));
-	iter.iov_offset = offset;
 
 	if (rw == ITER_SOURCE) {
 		kiocb_start_write(&cmd->iocb);
@@ -433,7 +427,20 @@ static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
 		ret = file->f_op->read_iter(&cmd->iocb, &iter);
 
 	lo_rw_aio_do_completion(cmd);
+	return ret;
+}
+
+static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
+		     loff_t pos, int rw)
+{
+	int nr_bvec = lo_cmd_nr_bvec(cmd);
+	int ret;
+
+	ret = lo_rw_aio_prep(lo, cmd, nr_bvec, pos);
+	if (unlikely(ret))
+		return ret;
 
+	ret = lo_submit_rw_aio(lo, cmd, nr_bvec, rw);
 	if (ret != -EIOCBQUEUED)
 		lo_rw_aio_complete(&cmd->iocb, ret);
 	return -EIOCBQUEUED;
-- 
2.47.0

[PATCH V5 4/6] loop: move command blkcg/memcg initialization into loop_queue_work

[Ming Lei]

Move loop command blkcg/memcg initialization into loop_queue_work,
and prepare for supporting to handle loop io command by IOCB_NOWAIT.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 drivers/block/loop.c | 32 +++++++++++++++++---------------
 1 file changed, 17 insertions(+), 15 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 3ab910572bd9..99eec0a25dbc 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -829,11 +829,28 @@ static inline int queue_on_root_worker(struct cgroup_subsys_state *css)
 
 static void loop_queue_work(struct loop_device *lo, struct loop_cmd *cmd)
 {
+	struct request __maybe_unused *rq = blk_mq_rq_from_pdu(cmd);
 	struct rb_node **node, *parent = NULL;
 	struct loop_worker *cur_worker, *worker = NULL;
 	struct work_struct *work;
 	struct list_head *cmd_list;
 
+	/* always use the first bio's css */
+	cmd->blkcg_css = NULL;
+	cmd->memcg_css = NULL;
+#ifdef CONFIG_BLK_CGROUP
+	if (rq->bio) {
+		cmd->blkcg_css = bio_blkcg_css(rq->bio);
+#ifdef CONFIG_MEMCG
+		if (cmd->blkcg_css) {
+			cmd->memcg_css =
+				cgroup_get_e_css(cmd->blkcg_css->cgroup,
+						&memory_cgrp_subsys);
+		}
+#endif
+	}
+#endif
+
 	spin_lock_irq(&lo->lo_work_lock);
 
 	if (queue_on_root_worker(cmd->blkcg_css))
@@ -1903,21 +1920,6 @@ static blk_status_t loop_queue_rq(struct blk_mq_hw_ctx *hctx,
 		break;
 	}
 
-	/* always use the first bio's css */
-	cmd->blkcg_css = NULL;
-	cmd->memcg_css = NULL;
-#ifdef CONFIG_BLK_CGROUP
-	if (rq->bio) {
-		cmd->blkcg_css = bio_blkcg_css(rq->bio);
-#ifdef CONFIG_MEMCG
-		if (cmd->blkcg_css) {
-			cmd->memcg_css =
-				cgroup_get_e_css(cmd->blkcg_css->cgroup,
-						&memory_cgrp_subsys);
-		}
-#endif
-	}
-#endif
 	loop_queue_work(lo, cmd);
 
 	return BLK_STS_OK;
-- 
2.47.0

[PATCH V5 5/6] loop: try to handle loop aio command via NOWAIT IO first

[Ming Lei]

Try to handle loop aio command via NOWAIT IO first, then we can avoid to
queue the aio command into workqueue. This is usually one big win in
case that FS block mapping is stable, Mikulas verified [1] that this way
improves IO perf by close to 5X in 12jobs sequential read/write test,
in which FS block mapping is just stable.

Fallback to workqueue in case of -EAGAIN. This way may bring a little
cost from the 1st retry, but when running the following write test over
loop/sparse_file, the actual effect on randwrite is obvious:

```
truncate -s 4G 1.img    #1.img is created on XFS/virtio-scsi
losetup -f 1.img --direct-io=on
fio --direct=1 --bs=4k --runtime=40 --time_based --numjobs=1 --ioengine=libaio \
	--iodepth=16 --group_reporting=1 --filename=/dev/loop0 -name=job --rw=$RW
```

- RW=randwrite: obvious IOPS drop observed
- RW=write: a little drop(%5 - 10%)

This perf drop on randwrite over sparse file will be addressed in the
following patch.

BLK_MQ_F_BLOCKING has to be set for calling into .read_iter() or .write_iter()
which might sleep even though it is NOWAIT, and the only effect is that rcu read
lock is replaced with srcu read lock.

Link: https://lore.kernel.org/linux-block/a8e5c76a-231f-07d1-a394-847de930f638@redhat.com/ [1]
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 drivers/block/loop.c | 68 ++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 63 insertions(+), 5 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 99eec0a25dbc..f752942d0889 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -90,6 +90,8 @@ struct loop_cmd {
 #define LOOP_IDLE_WORKER_TIMEOUT (60 * HZ)
 #define LOOP_DEFAULT_HW_Q_DEPTH 128
 
+static void loop_queue_work(struct loop_device *lo, struct loop_cmd *cmd);
+
 static DEFINE_IDR(loop_index_idr);
 static DEFINE_MUTEX(loop_ctl_mutex);
 static DEFINE_MUTEX(loop_validate_mutex);
@@ -321,6 +323,15 @@ static void lo_rw_aio_do_completion(struct loop_cmd *cmd)
 
 	if (!atomic_dec_and_test(&cmd->ref))
 		return;
+
+	/* -EAGAIN could be returned from bdev's ->ki_complete */
+	if (cmd->ret == -EAGAIN) {
+		struct loop_device *lo = rq->q->queuedata;
+
+		loop_queue_work(lo, cmd);
+		return;
+	}
+
 	kfree(cmd->bvec);
 	cmd->bvec = NULL;
 	if (req_op(rq) == REQ_OP_WRITE)
@@ -430,22 +441,51 @@ static int lo_submit_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
 	return ret;
 }
 
+static bool lo_backfile_support_nowait(const struct loop_device *lo)
+{
+	return lo->lo_backing_file->f_mode & FMODE_NOWAIT;
+}
+
 static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
 		     loff_t pos, int rw)
 {
 	int nr_bvec = lo_cmd_nr_bvec(cmd);
 	int ret;
 
-	ret = lo_rw_aio_prep(lo, cmd, nr_bvec, pos);
-	if (unlikely(ret))
-		return ret;
+	/* prepared already if we have tried nowait */
+	if (!cmd->use_aio || !lo_backfile_support_nowait(lo)) {
+		ret = lo_rw_aio_prep(lo, cmd, nr_bvec, pos);
+		if (unlikely(ret))
+			goto fail;
+	}
 
+	cmd->iocb.ki_flags &= ~IOCB_NOWAIT;
 	ret = lo_submit_rw_aio(lo, cmd, nr_bvec, rw);
+fail:
 	if (ret != -EIOCBQUEUED)
 		lo_rw_aio_complete(&cmd->iocb, ret);
 	return -EIOCBQUEUED;
 }
 
+static int lo_rw_aio_nowait(struct loop_device *lo, struct loop_cmd *cmd,
+			    int rw)
+{
+	struct request *rq = blk_mq_rq_from_pdu(cmd);
+	loff_t pos = ((loff_t) blk_rq_pos(rq) << 9) + lo->lo_offset;
+	int nr_bvec = lo_cmd_nr_bvec(cmd);
+	int ret = lo_rw_aio_prep(lo, cmd, nr_bvec, pos);
+
+	if (unlikely(ret))
+		goto fail;
+
+	cmd->iocb.ki_flags |= IOCB_NOWAIT;
+	ret = lo_submit_rw_aio(lo, cmd, nr_bvec, rw);
+fail:
+	if (ret != -EIOCBQUEUED && ret != -EAGAIN)
+		lo_rw_aio_complete(&cmd->iocb, ret);
+	return ret;
+}
+
 static int do_req_filebacked(struct loop_device *lo, struct request *rq)
 {
 	struct loop_cmd *cmd = blk_mq_rq_to_pdu(rq);
@@ -1903,6 +1943,7 @@ static blk_status_t loop_queue_rq(struct blk_mq_hw_ctx *hctx,
 	struct request *rq = bd->rq;
 	struct loop_cmd *cmd = blk_mq_rq_to_pdu(rq);
 	struct loop_device *lo = rq->q->queuedata;
+	int rw = 0;
 
 	blk_mq_start_request(rq);
 
@@ -1915,9 +1956,25 @@ static blk_status_t loop_queue_rq(struct blk_mq_hw_ctx *hctx,
 	case REQ_OP_WRITE_ZEROES:
 		cmd->use_aio = false;
 		break;
-	default:
+	case REQ_OP_READ:
+		rw = ITER_DEST;
 		cmd->use_aio = lo->lo_flags & LO_FLAGS_DIRECT_IO;
 		break;
+	case REQ_OP_WRITE:
+		rw = ITER_SOURCE;
+		cmd->use_aio = lo->lo_flags & LO_FLAGS_DIRECT_IO;
+		break;
+	default:
+		return BLK_STS_IOERR;
+	}
+
+	/* try NOWAIT if the backing file supports the mode */
+	if (cmd->use_aio && lo_backfile_support_nowait(lo)) {
+		int res = lo_rw_aio_nowait(lo, cmd, rw);
+
+		if (res != -EAGAIN && res != -EOPNOTSUPP)
+			return BLK_STS_OK;
+		/* fallback to workqueue for handling aio */
 	}
 
 	loop_queue_work(lo, cmd);
@@ -2069,7 +2126,8 @@ static int loop_add(int i)
 	lo->tag_set.queue_depth = hw_queue_depth;
 	lo->tag_set.numa_node = NUMA_NO_NODE;
 	lo->tag_set.cmd_size = sizeof(struct loop_cmd);
-	lo->tag_set.flags = BLK_MQ_F_STACKING | BLK_MQ_F_NO_SCHED_BY_DEFAULT;
+	lo->tag_set.flags = BLK_MQ_F_STACKING | BLK_MQ_F_NO_SCHED_BY_DEFAULT |
+		BLK_MQ_F_BLOCKING;
 	lo->tag_set.driver_data = lo;
 
 	err = blk_mq_alloc_tag_set(&lo->tag_set);
-- 
2.47.0

[PATCH V5 6/6] loop: add hint for handling aio via IOCB_NOWAIT

[Ming Lei]

Add hint for using IOCB_NOWAIT to handle loop aio command for avoiding
to cause write(especially randwrite) perf regression on sparse backed file.

Try IOCB_NOWAIT in the following situations:

- backing file is block device

OR

- READ aio command

OR

- there isn't any queued blocking async WRITEs, because NOWAIT won't cause
contention with blocking WRITE, which often implies exclusive lock

With this simple policy, perf regression of randwrite/write on sparse
backing file is fixed.

Link: https://lore.kernel.org/dm-devel/7d6ae2c9-df8e-50d0-7ad6-b787cb3cfab4@redhat.com/
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 drivers/block/loop.c | 61 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index f752942d0889..e42bdfc73c20 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -68,6 +68,7 @@ struct loop_device {
 	struct rb_root          worker_tree;
 	struct timer_list       timer;
 	bool			sysfs_inited;
+	unsigned 		lo_nr_blocking_writes;
 
 	struct request_queue	*lo_queue;
 	struct blk_mq_tag_set	tag_set;
@@ -467,6 +468,33 @@ static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
 	return -EIOCBQUEUED;
 }
 
+static inline bool lo_aio_try_nowait(struct loop_device *lo,
+		struct loop_cmd *cmd)
+{
+	struct file *file = lo->lo_backing_file;
+	struct inode *inode = file->f_mapping->host;
+	struct request *rq = blk_mq_rq_from_pdu(cmd);
+
+	/* NOWAIT works fine for backing block device */
+	if (S_ISBLK(inode->i_mode))
+		return true;
+
+	/*
+	 * NOWAIT is supposed to be fine for READ without contending with
+	 * blocking WRITE
+	 */
+	if (req_op(rq) == REQ_OP_READ)
+		return true;
+
+	/*
+	 * If there is any queued non-NOWAIT async WRITE , don't try new
+	 * NOWAIT WRITE for avoiding contention
+	 *
+	 * Here we focus on handling stable FS block mapping via NOWAIT
+	 */
+	return READ_ONCE(lo->lo_nr_blocking_writes) == 0;
+}
+
 static int lo_rw_aio_nowait(struct loop_device *lo, struct loop_cmd *cmd,
 			    int rw)
 {
@@ -478,6 +506,9 @@ static int lo_rw_aio_nowait(struct loop_device *lo, struct loop_cmd *cmd,
 	if (unlikely(ret))
 		goto fail;
 
+	if (!lo_aio_try_nowait(lo, cmd))
+		return -EAGAIN;
+
 	cmd->iocb.ki_flags |= IOCB_NOWAIT;
 	ret = lo_submit_rw_aio(lo, cmd, nr_bvec, rw);
 fail:
@@ -778,12 +809,19 @@ static ssize_t loop_attr_dio_show(struct loop_device *lo, char *buf)
 	return sysfs_emit(buf, "%s\n", dio ? "1" : "0");
 }
 
+static ssize_t loop_attr_nr_blocking_writes_show(struct loop_device *lo,
+						 char *buf)
+{
+	return sysfs_emit(buf, "%u\n", lo->lo_nr_blocking_writes);
+}
+
 LOOP_ATTR_RO(backing_file);
 LOOP_ATTR_RO(offset);
 LOOP_ATTR_RO(sizelimit);
 LOOP_ATTR_RO(autoclear);
 LOOP_ATTR_RO(partscan);
 LOOP_ATTR_RO(dio);
+LOOP_ATTR_RO(nr_blocking_writes);
 
 static struct attribute *loop_attrs[] = {
 	&loop_attr_backing_file.attr,
@@ -792,6 +830,7 @@ static struct attribute *loop_attrs[] = {
 	&loop_attr_autoclear.attr,
 	&loop_attr_partscan.attr,
 	&loop_attr_dio.attr,
+	&loop_attr_nr_blocking_writes.attr,
 	NULL,
 };
 
@@ -867,6 +906,24 @@ static inline int queue_on_root_worker(struct cgroup_subsys_state *css)
 }
 #endif
 
+static inline void loop_inc_blocking_writes(struct loop_device *lo,
+		struct loop_cmd *cmd)
+{
+	lockdep_assert_held(&lo->lo_work_lock);
+
+	if (req_op(blk_mq_rq_from_pdu(cmd)) == REQ_OP_WRITE)
+		lo->lo_nr_blocking_writes += 1;
+}
+
+static inline void loop_dec_blocking_writes(struct loop_device *lo,
+		struct loop_cmd *cmd)
+{
+	lockdep_assert_held(&lo->lo_work_lock);
+
+	if (req_op(blk_mq_rq_from_pdu(cmd)) == REQ_OP_WRITE)
+		lo->lo_nr_blocking_writes -= 1;
+}
+
 static void loop_queue_work(struct loop_device *lo, struct loop_cmd *cmd)
 {
 	struct request __maybe_unused *rq = blk_mq_rq_from_pdu(cmd);
@@ -949,6 +1006,8 @@ static void loop_queue_work(struct loop_device *lo, struct loop_cmd *cmd)
 		work = &lo->rootcg_work;
 		cmd_list = &lo->rootcg_cmd_list;
 	}
+	if (cmd->use_aio)
+		loop_inc_blocking_writes(lo, cmd);
 	list_add_tail(&cmd->list_entry, cmd_list);
 	queue_work(lo->workqueue, work);
 	spin_unlock_irq(&lo->lo_work_lock);
@@ -2048,6 +2107,8 @@ static void loop_process_work(struct loop_worker *worker,
 		cond_resched();
 
 		spin_lock_irq(&lo->lo_work_lock);
+		if (cmd->use_aio)
+			loop_dec_blocking_writes(lo, cmd);
 	}
 
 	/*
-- 
2.47.0

Re: [PATCH V5 1/6] loop: add helper lo_cmd_nr_bvec()

[Bart Van Assche]

On 10/15/25 4:07 AM, Ming Lei wrote:
> +static inline unsigned lo_cmd_nr_bvec(struct loop_cmd *cmd)
> +{
> +	struct request *rq = blk_mq_rq_from_pdu(cmd);
> +	struct req_iterator rq_iter;
> +	struct bio_vec tmp;
> +	int nr_bvec = 0;
> +
> +	rq_for_each_bvec(tmp, rq, rq_iter)
> +		nr_bvec++;
> +
> +	return nr_bvec;
> +}

'cmd' is not used in this function other than in the conversion to a
struct request. Has it been considered to change the argument type of
this function from 'struct loop_cmd *' into 'struct request *'? That
will allow to leave out the blk_mq_rq_from_pdu() from this function.
Otherwise this patch looks good to me.

Thanks,

Bart.

Re: [PATCH V5 1/6] loop: add helper lo_cmd_nr_bvec()

[Ming Lei]

On Wed, Oct 15, 2025 at 08:49:45AM -0700, Bart Van Assche wrote:
> On 10/15/25 4:07 AM, Ming Lei wrote:
> > +static inline unsigned lo_cmd_nr_bvec(struct loop_cmd *cmd)
> > +{
> > +	struct request *rq = blk_mq_rq_from_pdu(cmd);
> > +	struct req_iterator rq_iter;
> > +	struct bio_vec tmp;
> > +	int nr_bvec = 0;
> > +
> > +	rq_for_each_bvec(tmp, rq, rq_iter)
> > +		nr_bvec++;
> > +
> > +	return nr_bvec;
> > +}
> 
> 'cmd' is not used in this function other than in the conversion to a
> struct request. Has it been considered to change the argument type of
> this function from 'struct loop_cmd *' into 'struct request *'? That
> will allow to leave out the blk_mq_rq_from_pdu() from this function.
> Otherwise this patch looks good to me.

It isn't one big deal, I can switch to `request *` if respin is needed,
otherwise I'd leave it as it is. 


Thanks,
Ming