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

Hi John,

On Mon, Jun 29, 2026 at 15:48 +0100, John Garry wrote:

On 28/06/2026 15:24, Abd-Alrhman Masalkhi wrote:

quoted

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 <redacted>
---
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://urldefense.com/v3/__https://lore.kernel.org/linux-raid/20260623072456.333437-3-abd.masalkhi@gmail.com/__;!!ACWV5N9M2RV99hQ!LbMSGSClRi0PNBqQti5ZNWGDVjDd34-7saYEAwNyBNjpNTjEA7veqM5RHG8KB1QiscarW4UaIefjm19ywSImtIgh$
---
  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))

where does BIO_MAX_VECS * (PAGE_SIZE >> 9) even come from?

BIO_MAX_VECS * (PAGE_SIZE >> 9) defines the maximum size supported by
write-behind. The write-behind copy (alloc_behind_master_bio) uses a
single bio, which can hold at most BIO_MAX_VECS pages, making this the
largest payload it can carry. With a 4 KiB PAGE_SIZE, that corresponds
to 256 pages, or 1 MiB (2048 sectors).
This patch changes the behavior for atomic writes that exceed this
limit. Instead of failing the write with -EIO when the number of sectors
must be reduced, it disables write-behind and proceeds with the atomic
write.

quoted

+			/*
+			 * 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;

This BARRIER_UNIT_SECTOR_SIZE is a bit like chunk sectors, no? I am just 
wondering if we just should set it to chunk sectors = 
BARRIER_UNIT_SECTOR_SIZE

I assume that it affects more than Reads and writes, e.g. discard also.

BARRIER_UNIT_SECTOR_SIZE is the resync barrier-bucket, not the layout
chunk size. Unless I'm missing something, using
atomic_write_hw_unit_max seems more appropriate than using the chunk
size. That way, the limit only applies to atomic writes instead of
affecting other operations such.

quoted

  	lim.features |= BLK_FEAT_ATOMIC_WRITES;
  	lim.features |= BLK_FEAT_PCI_P2PDMA;
  	err = mddev_stack_rdev_limits(mddev, &lim, MDDEV_STACK_INTEGRITY);

-- 
Best Regards,
Abd-Alrhman