[PATCH v2] block: fix discard alignment for non-power-of-two granularities

[Hungyu Lin <dennylin0707@gmail.com>]

bio_discard_limit() uses round_up() and round_down(), which require a
power-of-two alignment. However, discard_granularity need not be a power
of two. For example, dm-thin supports data block sizes that are arbitrary
multiples of 64 KiB and advertises the block size as its granularity.

With a 192 KiB granularity, discarding one complete block at offset
384 KiB is split into 128 KiB and 64 KiB bios. Neither covers a complete
thin block, so dm-thin ignores both and the mapping remains allocated.

Use sector_div() to find the distance to the next granularity boundary,
and rounddown() to align the maximum bio size. This handles non-power-of-
two granularities while preserving the alignment for power-of-two ones.

When discard is unsupported or disabled, discard_granularity can be zero.
Return BIO_MAX_SECTORS in that case so the existing bio submission checks
can handle unsupported discard requests without dividing by zero.

Fixes: 9b15d109a6b2 ("block: improve discard bio alignment in __blkdev_issue_discard()")
Reported-by: Ming-Hung Tsai <mtsai@redhat.com>
Link: https://lore.kernel.org/r/CALjSBEvjbiGDVewuapWKhGDtaxZwcn4=22raUiBEOyj22rY7-g@mail.gmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
---
Changes in v2:
- Handle zero discard_granularity before dividing, as pointed out by
  Sashiko. Leave unsupported discard handling to the bio submission path.
 block/blk-lib.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/block/blk-lib.c b/block/blk-lib.c
index 688bc67cbf73..5649e4d3b5bf 100644
--- a/block/blk-lib.c
+++ b/block/blk-lib.c
@@ -13,26 +13,28 @@
 static sector_t bio_discard_limit(struct block_device *bdev, sector_t sector)
 {
 	unsigned int discard_granularity = bdev_discard_granularity(bdev);
-	sector_t granularity_aligned_sector;
+	unsigned int remainder;
+
+	if (!discard_granularity)
+		return BIO_MAX_SECTORS;
 
 	if (bdev_is_partition(bdev))
 		sector += bdev->bd_start_sect;
 
-	granularity_aligned_sector =
-		round_up(sector, discard_granularity >> SECTOR_SHIFT);
+	remainder = sector_div(sector, discard_granularity >> SECTOR_SHIFT);
 
 	/*
 	 * Make sure subsequent bios start aligned to the discard granularity if
 	 * it needs to be split.
 	 */
-	if (granularity_aligned_sector != sector)
-		return granularity_aligned_sector - sector;
+	if (remainder)
+		return (discard_granularity >> SECTOR_SHIFT) - remainder;
 
 	/*
 	 * Align the bio size to the discard granularity to make splitting the bio
 	 * at discard granularity boundaries easier in the driver if needed.
 	 */
-	return round_down(BIO_MAX_SIZE, discard_granularity) >> SECTOR_SHIFT;
+	return rounddown(BIO_MAX_SIZE, discard_granularity) >> SECTOR_SHIFT;
 }
 
 struct bio *blk_alloc_discard_bio(struct block_device *bdev,
-- 
2.43.0