[PATCH v2] block: fix alignment_offset math that assumes io_min is a power-of-2

[PATCH v2] block: fix alignment_offset math that assumes io_min is a power-of-2

[Mike Snitzer]

The math in both blk_stack_limits() and queue_limit_alignment_offset()
assume that a block device's io_min (aka minimum_io_size) is always a
power-of-2.  Fix the math such that it works for non-power-of-2 io_min.

This issue (of alignment_offset != 0) became apparent when testing
dm-thinp with a thinp blocksize that matches a RAID6 stripesize of
1280K.  Commit fdfb4c8c1 ("dm thin: set minimum_io_size to pool's data
block size") unlocked the potential for alignment_offset != 0 due to
the dm-thin-pool's io_min possibly being a non-power-of-2.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org
---
 block/blk-settings.c   | 4 ++--
 include/linux/blkdev.h | 5 ++---
 2 files changed, 4 insertions(+), 5 deletions(-)

v2: fix bug in queue_limit_alignment_offset's math conversion (missed trailing << 9)

diff --git a/block/blk-settings.c b/block/blk-settings.c
index f1a1795..aa02247 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -574,7 +574,7 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
 		bottom = max(b->physical_block_size, b->io_min) + alignment;
 
 		/* Verify that top and bottom intervals line up */
-		if (max(top, bottom) & (min(top, bottom) - 1)) {
+		if (max(top, bottom) % min(top, bottom)) {
 			t->misaligned = 1;
 			ret = -1;
 		}
@@ -619,7 +619,7 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
 
 	/* Find lowest common alignment_offset */
 	t->alignment_offset = lcm(t->alignment_offset, alignment)
-		& (max(t->physical_block_size, t->io_min) - 1);
+		% max(t->physical_block_size, t->io_min);
 
 	/* Verify that new alignment_offset is on a logical block boundary */
 	if (t->alignment_offset & (t->logical_block_size - 1)) {
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 038b40f..5546392 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1279,10 +1279,9 @@ static inline int queue_alignment_offset(struct request_queue *q)
 static inline int queue_limit_alignment_offset(struct queue_limits *lim, sector_t sector)
 {
 	unsigned int granularity = max(lim->physical_block_size, lim->io_min);
-	unsigned int alignment = (sector << 9) & (granularity - 1);
+	unsigned int alignment = sector_div(sector, granularity >> 9) << 9;
 
-	return (granularity + lim->alignment_offset - alignment)
-		& (granularity - 1);
+	return (granularity + lim->alignment_offset - alignment) % granularity;
 }
 
 static inline int bdev_alignment_offset(struct block_device *bdev)
-- 
1.8.3.1

Re: [PATCH v2] block: fix alignment_offset math that assumes io_min is a power-of-2

[Martin K. Petersen]

>>>>> "Mike" == Mike Snitzer <snitzer@redhat.com> writes:

Mike> The math in both blk_stack_limits() and
Mike> queue_limit_alignment_offset() assume that a block device's io_min
Mike> (aka minimum_io_size) is always a power-of-2.  Fix the math such
Mike> that it works for non-power-of-2 io_min.

Mike> This issue (of alignment_offset != 0) became apparent when testing
Mike> dm-thinp with a thinp blocksize that matches a RAID6 stripesize of
Mike> 1280K.  Commit fdfb4c8c1 ("dm thin: set minimum_io_size to pool's
Mike> data block size") unlocked the potential for alignment_offset != 0
Mike> due to the dm-thin-pool's io_min possibly being a non-power-of-2.

Looks good to me, Mike.

Acked-by: Martin K. Petersen <martin.petersen@oracle.com>

-- 
Martin K. Petersen	Oracle Linux Engineering