[PATCH v3 2/7] md/raid1: advertise atomic write limits and handle runtime constraints

[Abd-Alrhman Masalkhi]

Atomic writes in RAID1 must fit within a single barrier unit. Set
chunk_sectors 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@xxxxxxxxx>
---
Changes in v3:
 - Set chunk_sectors to BARRIER_UNIT_SECTOR_SIZE instead of setting
   atomic_write_hw_unit_max.
 - Avoid enabling write-behind when the atomic write exceeds the
   write-behind limit.
 - Link to v2: https://lore.kernel.org/linux-raid/20260628142420.1051027-3-abd.masalkhi@xxxxxxxxx/

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@xxxxxxxxx/
---
 drivers/md/raid1.c | 25 ++++++++++---------------
 1 file changed, 10 insertions(+), 15 deletions(-)

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index afe2ca96ad8c..6c8beca995e6 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;
 
@@ -1577,7 +1578,8 @@ static bool raid1_write_request(struct mddev *mddev, struct bio *bio,
 		 * write-mostly, which means we could allocate write behind
 		 * bio later.
 		 */
-		if (!is_discard && rdev && test_bit(WriteMostly, &rdev->flags))
+		if (!is_discard && rdev && test_bit(WriteMostly, &rdev->flags) &&
+		    (!atomic || max_sectors <= BIO_MAX_VECS * (PAGE_SIZE >> 9)))
 			write_behind = true;
 
 		r1_bio->bios[i] = NULL;
@@ -1603,20 +1605,6 @@ 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;
@@ -1636,7 +1624,13 @@ static bool raid1_write_request(struct mddev *mddev, struct bio *bio,
 	if (write_behind && mddev->bitmap)
 		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)
@@ -3228,6 +3222,7 @@ static int raid1_set_limits(struct mddev *mddev)
 	md_init_stacking_limits(&lim);
 	lim.max_write_zeroes_sectors = 0;
 	lim.max_hw_wzeroes_unmap_sectors = 0;
+	lim.chunk_sectors = BARRIER_UNIT_SECTOR_SIZE;
 	lim.logical_block_size = mddev->logical_block_size;
 	lim.features |= BLK_FEAT_ATOMIC_WRITES;
 	lim.features |= BLK_FEAT_PCI_P2PDMA;
-- 
2.43.0