block: fix problem with sending down discard that isn't of correct granularity

If the queue doesn't have a limit set, or it just set UINT_MAX like
we default to, we coud be sending down a discard request that isn't
of the correct granularity if the block size is > 512b.

Fix this by adjusting max_discard_sectors down to the proper
alignment.
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

block/blk-lib.c

@@ -41,6 +41,7 @@ int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
 	struct request_queue *q = bdev_get_queue(bdev);
 	int type = flags & BLKDEV_IFL_BARRIER ?
 		DISCARD_BARRIER : DISCARD_NOBARRIER;
+	unsigned int max_discard_sectors;
 	struct bio *bio;
 	int ret = 0;
 
@@ -50,10 +51,18 @@ int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
 	if (!blk_queue_discard(q))
 		return -EOPNOTSUPP;
 
-	while (nr_sects && !ret) {
-		unsigned int max_discard_sectors =
-			min(q->limits.max_discard_sectors, UINT_MAX >> 9);
+	/*
+	 * Ensure that max_discard_sectors is of the proper
+	 * granularity
+	 */
+	max_discard_sectors = min(q->limits.max_discard_sectors, UINT_MAX >> 9);
+	if (q->limits.discard_granularity) {
+		unsigned int disc_sects = q->limits.discard_granularity >> 9;
 
+		max_discard_sectors &= ~(disc_sects - 1);
+	}
+
+	while (nr_sects && !ret) {
 		bio = bio_alloc(gfp_mask, 1);
 		if (!bio) {
 			ret = -ENOMEM;