[PATCH v3 01/21] md/raid5: round bitmap stripes with sector division

[Yu Kuai]

From: Yu Kuai <yukuai@xxxxxxx>

raid5_bitmap_sector_map() aligns the array range to full RAID5 stripe
widths before converting it to component sectors. That width is
chunk_sectors multiplied by the number of data disks, and it is not
always a power of two.

Reproduce with a 4-disk RAID5, 1024-sector chunks, and three data disks.
The full-stripe width is 3072 sectors. For a one-sector write at array
sector 3072, correct rounding gives array range [3072, 6144), which maps
to component range [1024, 2048). The old round_down()/round_up() logic
instead gives [1024, 4096), which maps to [0, 1024).

Use sector_div() based arithmetic so the rounded range is aligned to the
actual RAID5 stripe width.

The deterministic mapper test now reports the fixed component range as
[1024, 2048), while the old mask-based range was [0, 1024).

Fixes: 9c89f604476c ("md/raid5: implement pers->bitmap_sector()")
Reported-by: Mykola Marzhan <mykola@xxxxxxxxxxx>
Link: https://lore.kernel.org/all/20260726185916.2223460-1-mykola@xxxxxxxxxxx/
Signed-off-by: Yu Kuai <yukuai@xxxxxxx>
---
 drivers/md/raid5.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 0c5c9fb0606e..35910bc6683b 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -5956,8 +5956,11 @@ static void raid5_bitmap_sector(struct mddev *mddev, sector_t *offset,
 
 	sectors_per_chunk = conf->chunk_sectors *
 		(conf->raid_disks - conf->max_degraded);
-	start = round_down(start, sectors_per_chunk);
-	end = round_up(end, sectors_per_chunk);
+	sector_div(start, sectors_per_chunk);
+	start *= sectors_per_chunk;
+	if (sector_div(end, sectors_per_chunk))
+		end++;
+	end *= sectors_per_chunk;
 
 	start = raid5_compute_sector(conf, start, 0, &dd_idx, NULL);
 	end = raid5_compute_sector(conf, end, 0, &dd_idx, NULL);
@@ -5975,8 +5978,10 @@ static void raid5_bitmap_sector(struct mddev *mddev, sector_t *offset,
 
 	sectors_per_chunk = conf->prev_chunk_sectors *
 		(conf->previous_raid_disks - conf->max_degraded);
-	prev_start = round_down(prev_start, sectors_per_chunk);
-	prev_end = round_down(prev_end, sectors_per_chunk);
+	sector_div(prev_start, sectors_per_chunk);
+	prev_start *= sectors_per_chunk;
+	sector_div(prev_end, sectors_per_chunk);
+	prev_end *= sectors_per_chunk;
 
 	prev_start = raid5_compute_sector(conf, prev_start, 1, &dd_idx, NULL);
 	prev_end = raid5_compute_sector(conf, prev_end, 1, &dd_idx, NULL);
-- 
2.51.0