md: add support for REQ_NOWAIT

commit 021a244 ("block: add QUEUE_FLAG_NOWAIT") added support for checking whether a given bdev supports handling of REQ_NOWAIT or not. Since then commit 6abc494 ("dm: add support for REQ_NOWAIT and enable it for linear target") added support for REQ_NOWAIT for dm. This uses a similar approach to incorporate REQ_NOWAIT for md based bios. This patch was tested using t/io_uring tool within FIO. A nvme drive was partitioned into 2 partitions and a simple raid 0 configuration /dev/md0 was created. md0 : active raid0 nvme4n1p1[1] nvme4n1p2[0] 937423872 blocks super 1.2 512k chunks Before patch: $ ./t/io_uring /dev/md0 -p 0 -a 0 -d 1 -r 100 Running top while the above runs: $ ps -eL | grep $(pidof io_uring) 38396 38396 pts/2 00:00:00 io_uring 38396 38397 pts/2 00:00:15 io_uring 38396 38398 pts/2 00:00:13 iou-wrk-38397 We can see iou-wrk-38397 io worker thread created which gets created when io_uring sees that the underlying device (/dev/md0 in this case) doesn't support nowait. After patch: $ ./t/io_uring /dev/md0 -p 0 -a 0 -d 1 -r 100 Running top while the above runs: $ ps -eL | grep $(pidof io_uring) 38341 38341 pts/2 00:10:22 io_uring 38341 38342 pts/2 00:10:37 io_uring After running this patch, we don't see any io worker thread being created which indicated that io_uring saw that the underlying device does support nowait. This is the exact behaviour noticed on a dm device which also supports nowait. For all the other raid personalities except raid0, we would need to train pieces which involves make_request fn in order for them to correctly handle REQ_NOWAIT. Reviewed-by: Jens Axboe <axboe@kernel.dk> Signed-off-by: Vishal Verma <vverma@digitalocean.com> Signed-off-by: Song Liu <song@kernel.org>

Author: Vishal Verma

drivers/md/md.c

@@ -418,6 +418,12 @@ void md_handle_request(struct mddev *mddev, struct bio *bio)
 rcu_read_lock();
 if (is_suspended(mddev, bio)) {
 DEFINE_WAIT(__wait);
+/* Bail out if REQ_NOWAIT is set for the bio */
+if (bio->bi_opf & REQ_NOWAIT) {
+rcu_read_unlock();
+bio_wouldblock_error(bio);
+return;
+}
 for (;;) {
 prepare_to_wait(&mddev->sb_wait, &__wait,
 TASK_UNINTERRUPTIBLE);
@@ -5787,6 +5793,7 @@ int md_run(struct mddev *mddev)
 int err;
 struct md_rdev *rdev;
 struct md_personality *pers;
+bool nowait = true;
 
 if (list_empty(&mddev->disks))
 /* cannot run an array with no devices.. */
@@ -5857,8 +5864,13 @@ int md_run(struct mddev *mddev)
 }
 }
 sysfs_notify_dirent_safe(rdev->sysfs_state);
+nowait = nowait && blk_queue_nowait(bdev_get_queue(rdev->bdev));
 }
 
+/* Set the NOWAIT flags if all underlying devices support it */
+if (nowait)
+blk_queue_flag_set(QUEUE_FLAG_NOWAIT, mddev->queue);
+
 if (!bioset_initialized(&mddev->bio_set)) {
 err = bioset_init(&mddev->bio_set, BIO_POOL_SIZE, 0, BIOSET_NEED_BVECS);
 if (err)
@@ -7002,6 +7014,15 @@ static int hot_add_disk(struct mddev *mddev, dev_t dev)
 set_bit(MD_SB_CHANGE_DEVS, &mddev->sb_flags);
 if (!mddev->thread)
 md_update_sb(mddev, 1);
+/*
+ * If the new disk does not support REQ_NOWAIT,
+ * disable on the whole MD.
+ */
+if (!blk_queue_nowait(bdev_get_queue(rdev->bdev))) {
+pr_info("%s: Disabling nowait because %s does not support nowait\n",
+mdname(mddev), bdevname(rdev->bdev, b));
+blk_queue_flag_clear(QUEUE_FLAG_NOWAIT, mddev->queue);
+}
 /*
  * Kick recovery, maybe this spare has to be added to the
  * array immediately.