[PATCH] md: skip bitmap accounting for empty write ranges

[Yu Kuai <yukuai@kernel.org>]

From: Yu Kuai <yukuai@fygo.io>

mkfs.ext4 can submit zero-sector flush/FUA bios. These bios are WRITE
bios for md_write_start() purposes, but they do not cover any data sector
and must not dirty bitmap bits.

md bitmap accounting currently passes such bios to bitmap start_write().
For llbitmap this reaches llbitmap_start_write() with sectors == 0,
which underflows the end chunk calculation.

The new bitmap prepare_range() hook can also turn a non-empty bio into an
empty bitmap range when the requested sectors are outside the active
bitmap geometry. Treat both cases as not started, so the completion path
will not call end_write() for an empty range.

Signed-off-by: Yu Kuai <yukuai@fygo.io>
---
 drivers/md/md.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 08eabc7e5a71..ccc4180d2c1d 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -9374,10 +9374,12 @@ static void md_bitmap_start(struct mddev *mddev,
 			   mddev->bitmap_ops->start_discard :
 			   mddev->bitmap_ops->start_write;
 
 	md_bitmap_prepare_range(mddev, &md_io_clone->offset,
 				&md_io_clone->sectors);
+	if (!md_io_clone->sectors)
+		return;
 	fn(mddev, md_io_clone->offset, md_io_clone->sectors);
 }
 
 static void md_bitmap_end(struct mddev *mddev, struct md_io_clone *md_io_clone)
 {
@@ -9394,11 +9396,12 @@ static void md_end_clone_io(struct bio *bio)
 						       bio_clone);
 	struct bio *orig_bio = md_io_clone->orig_bio;
 	struct mddev *mddev = md_io_clone->mddev;
 	struct completion *reshape_completion = bio->bi_private;
 
-	if (bio_data_dir(orig_bio) == WRITE && md_bitmap_enabled(mddev, false))
+	if (bio_data_dir(orig_bio) == WRITE && md_io_clone->sectors &&
+	    md_bitmap_enabled(mddev, false))
 		md_bitmap_end(mddev, md_io_clone);
 
 	if (bio->bi_status && !orig_bio->bi_status)
 		orig_bio->bi_status = bio->bi_status;
 
@@ -9421,14 +9424,16 @@ static void md_clone_bio(struct mddev *mddev, struct bio **bio)
 		bio_alloc_clone(bdev, *bio, GFP_NOIO, &mddev->io_clone_set);
 
 	md_io_clone = container_of(clone, struct md_io_clone, bio_clone);
 	md_io_clone->orig_bio = *bio;
 	md_io_clone->mddev = mddev;
+	md_io_clone->sectors = 0;
 	if (blk_queue_io_stat(bdev->bd_disk->queue))
 		md_io_clone->start_time = bio_start_io_acct(*bio);
 
-	if (bio_data_dir(*bio) == WRITE && md_bitmap_enabled(mddev, false)) {
+	if (bio_data_dir(*bio) == WRITE && bio_sectors(*bio) &&
+	    md_bitmap_enabled(mddev, false)) {
 		md_io_clone->offset = (*bio)->bi_iter.bi_sector;
 		md_io_clone->sectors = bio_sectors(*bio);
 		md_io_clone->rw = op_stat_group(bio_op(*bio));
 		md_bitmap_start(mddev, md_io_clone);
 	}
-- 
2.51.0