block: Fix discard alignment calculation and printing

Discard alignment reporting for partitions was incorrect.  Update to
match the algorithm used elsewhere.

The alignment can be negative (misaligned).  Fix format string
accordingly.

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

2 files changed, 6 insertions, 3 deletions

diff --git a/block/genhd.c b/block/genhd.c
index b11a4ad7d571..d13ba76a169c 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -867,7 +867,7 @@ static ssize_t disk_discard_alignment_show(struct device *dev,
 {
         struct gendisk *disk = dev_to_disk(dev);
 
-        return sprintf(buf, "%u\n", queue_discard_alignment(disk->queue));
+        return sprintf(buf, "%d\n", queue_discard_alignment(disk->queue));
 }
 
 static DEVICE_ATTR(range, S_IRUGO, disk_range_show, NULL);
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 9b98173a8184..a41bcc8e140f 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1148,8 +1148,11 @@ static inline int queue_discard_alignment(struct request_queue *q)
 static inline int queue_sector_discard_alignment(struct request_queue *q,
                                                  sector_t sector)
 {
-        return ((sector << 9) - q->limits.discard_alignment)
-                & (q->limits.discard_granularity - 1);
+        struct queue_limits *lim = &q->limits;
+        unsigned int alignment = (sector << 9) & (lim->discard_granularity - 1);
+
+        return (lim->discard_granularity + lim->discard_alignment - alignment)
+                & (lim->discard_granularity - 1);
 }
 
 static inline unsigned int queue_discard_zeroes_data(struct request_queue *q)