1 files changed, 15 insertions, 4 deletions

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index acb4f7bbbd32..f94bc83011ed 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1188,14 +1188,25 @@ static inline int queue_discard_alignment(struct request_queue *q)
 
 static inline int queue_limit_discard_alignment(struct queue_limits *lim, sector_t sector)
 {
-        sector_t alignment = sector << 9;
-        alignment = sector_div(alignment, lim->discard_granularity);
+        unsigned int alignment, granularity, offset;
 
         if (!lim->max_discard_sectors)
                 return 0;
 
-        alignment = lim->discard_granularity + lim->discard_alignment - alignment;
-        return sector_div(alignment, lim->discard_granularity);
+        /* Why are these in bytes, not sectors? */
+        alignment = lim->discard_alignment >> 9;
+        granularity = lim->discard_granularity >> 9;
+        if (!granularity)
+                return 0;
+
+        /* Offset of the partition start in 'granularity' sectors */
+        offset = sector_div(sector, granularity);
+
+        /* And why do we do this modulus *again* in blkdev_issue_discard()? */
+        offset = (granularity + alignment - offset) % granularity;
+
+        /* Turn it back into bytes, gaah */
+        return offset << 9;
 }
 
 static inline int bdev_discard_alignment(struct block_device *bdev)