block: Fix incorrect alignment offset reporting and update documentation

queue_sector_alignment_offset returned the wrong value which caused
partitions to report an incorrect alignment_offset.  Since offset
alignment calculation is needed several places it has been split into a
separate helper function.  The topology stacking function has been
updated accordingly.

Furthermore, comments have been added to clarify how the stacking
function works.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Tested-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 9 insertions, 2 deletions

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 784a919aa0d0..59b832be3044 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1116,11 +1116,18 @@ static inline int queue_alignment_offset(struct request_queue *q)
         return q->limits.alignment_offset;
 }
 
+static inline int queue_limit_alignment_offset(struct queue_limits *lim, sector_t offset)
+{
+        unsigned int granularity = max(lim->physical_block_size, lim->io_min);
+
+        offset &= granularity - 1;
+        return (granularity + lim->alignment_offset - offset) & (granularity - 1);
+}
+
 static inline int queue_sector_alignment_offset(struct request_queue *q,
                                                 sector_t sector)
 {
-        return ((sector << 9) - q->limits.alignment_offset)
-                & (q->limits.io_min - 1);
+        return queue_limit_alignment_offset(&q->limits, sector << 9);
 }
 
 static inline int bdev_alignment_offset(struct block_device *bdev)