[PATCH v2 2/7] md/raid1: advertise atomic write limits and handle runtime constraints

[Abd-Alrhman Masalkhi <abd.masalkhi@gmail.com>]

Atomic writes in RAID1 must fit within a single barrier unit. Advertise
this restriction through the queue limits by setting
atomic_write_hw_unit_max to BARRIER_UNIT_SECTOR_SIZE so that bios which
would cross a barrier-unit boundary are rejected by the block layer
before reaching MD.

A bio that passes block-layer validation may still become unserviceable
within RAID1 due to bad blocks or write-behind constraints. In the former
case, complete the bio with EIO. In the latter case, disable
write-behind rather than failing the bio with EIO.

Fixes: f2a38abf5f1c ("md/raid1: Atomic write support")
Fixes: a4c55c902670 ("md/raid1: simplify raid1_write_request() error handling")
Signed-off-by: Abd-Alrhman Masalkhi <abd.masalkhi@gmail.com>
---
Changes in v2:
 - Drop the early atomic write split check from raid1_write_request().
 - Advertise the atomic write size limit via queue limits.
 - Disable write-behind instead of failing atomic writes when the
   BIO_MAX_VECS limit is encountered.
 - Link to v1: https://lore.kernel.org/linux-raid/20260623072456.333437-3-abd.masalkhi@gmail.com/
---
 drivers/md/raid1.c | 36 +++++++++++++++++++-----------------
 1 file changed, 19 insertions(+), 17 deletions(-)

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index afe2ca96ad8c..f322048ab3c2 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -1522,6 +1522,7 @@ static bool raid1_write_request(struct mddev *mddev, struct bio *bio,
 	int first_clone;
 	bool write_behind = false;
 	bool nowait = bio->bi_opf & REQ_NOWAIT;
+	bool atomic = bio->bi_opf & REQ_ATOMIC;
 	bool is_discard = op_is_discard(bio->bi_opf);
 	sector_t sector = bio->bi_iter.bi_sector;
 
@@ -1603,20 +1604,6 @@ static bool raid1_write_request(struct mddev *mddev, struct bio *bio,
 			}
 			if (is_bad) {
 				int good_sectors;
-
-				/*
-				 * We cannot atomically write this, so just
-				 * error in that case. It could be possible to
-				 * atomically write other mirrors, but the
-				 * complexity of supporting that is not worth
-				 * the benefit.
-				 */
-				if (bio->bi_opf & REQ_ATOMIC) {
-					bio->bi_status = BLK_STS_NOTSUPP;
-					bio_endio(bio);
-					goto err_dec_pending;
-				}
-
 				good_sectors = first_bad - sector;
 				if (good_sectors < max_sectors)
 					max_sectors = good_sectors;
@@ -1633,10 +1620,24 @@ static bool raid1_write_request(struct mddev *mddev, struct bio *bio,
 	 * at a time and thus needs a new bio that can fit the whole payload
 	 * this bio in page sized chunks.
 	 */
-	if (write_behind && mddev->bitmap)
-		max_sectors = min_t(int, max_sectors,
-				    BIO_MAX_VECS * (PAGE_SIZE >> 9));
+	if (write_behind && mddev->bitmap) {
+		if (atomic && max_sectors > BIO_MAX_VECS * (PAGE_SIZE >> 9))
+			/*
+			 * Atomic writes cannot be split, so disable
+			 * write-behind.
+			 */
+			write_behind = false;
+		else
+			max_sectors = min_t(int, max_sectors,
+					    BIO_MAX_VECS * (PAGE_SIZE >> 9));
+	}
+
 	if (max_sectors < bio_sectors(bio)) {
+		if (atomic) {
+			bio_io_error(bio);
+			goto err_dec_pending;
+		}
+
 		bio = bio_submit_split_bioset(bio, max_sectors,
 					      &conf->bio_split);
 		if (!bio)
@@ -3229,6 +3230,7 @@ static int raid1_set_limits(struct mddev *mddev)
 	lim.max_write_zeroes_sectors = 0;
 	lim.max_hw_wzeroes_unmap_sectors = 0;
 	lim.logical_block_size = mddev->logical_block_size;
+	lim.atomic_write_hw_unit_max = BARRIER_UNIT_SECTOR_SIZE;
 	lim.features |= BLK_FEAT_ATOMIC_WRITES;
 	lim.features |= BLK_FEAT_PCI_P2PDMA;
 	err = mddev_stack_rdev_limits(mddev, &lim, MDDEV_STACK_INTEGRITY);
-- 
2.43.0