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

[Hungyu Lin]

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.

Fixes: 9b15d109a6b2 ("block: improve discard bio alignment in 
__blkdev_issue_discard()")
Reported-by: Ming-Hung Tsai <mt...@redhat.com>
Link: 
https://lore.kernel.org/r/CALjSBEvjbiGDVewuapWKhGDtaxZwcn4=22rauibeoyj22ry...@mail.gmail.com
Cc: sta...@vger.kernel.org
Signed-off-by: Hungyu Lin <dennylin0...@gmail.com>
---
 block/blk-lib.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/block/blk-lib.c b/block/blk-lib.c
index 688bc67cbf73..e0de5bb4d969 100644
--- a/block/blk-lib.c
+++ b/block/blk-lib.c
@@ -13,26 +13,25 @@
 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 (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