[PATCH 1/2] block: tweak rounding of max_discard_sectors

[Paolo Bonzini <pbonzini@redhat.com>]

Mostly a preparation for the next patch.

In principle this fixes an infinite loop if max_discard_sectors < granularity,
but that really shouldn't happen.

Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 block/blk-lib.c    |    9 +++++----
 1 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/block/blk-lib.c b/block/blk-lib.c
index 2b461b4..d4bb160 100644
--- a/block/blk-lib.c
+++ b/block/blk-lib.c
@@ -44,6 +44,7 @@ int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
 	struct request_queue *q = bdev_get_queue(bdev);
 	int type = REQ_WRITE | REQ_DISCARD;
 	unsigned int max_discard_sectors;
+	unsigned int granularity;
 	struct bio_batch bb;
 	struct bio *bio;
 	int ret = 0;
@@ -53,18 +54,18 @@ int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
 	if (!blk_queue_discard(q))
 		return -EOPNOTSUPP;
 
+	/* Zero-sector (unknown) and one-sector granularities are the same.  */
+	granularity = max(q->limits.discard_granularity >> 9, 1U);
+
 	/*
 	 * Ensure that max_discard_sectors is of the proper
 	 * granularity
 	 */
 	max_discard_sectors = min(q->limits.max_discard_sectors, UINT_MAX >> 9);
+	max_discard_sectors = round_down(max_discard_sectors, granularity);
 	if (unlikely(!max_discard_sectors)) {
 		/* Avoid infinite loop below. Being cautious never hurts. */
 		return -EOPNOTSUPP;
-	} else if (q->limits.discard_granularity) {
-		unsigned int disc_sects = q->limits.discard_granularity >> 9;
-
-		max_discard_sectors &= ~(disc_sects - 1);
 	}
 
 	if (flags & BLKDEV_DISCARD_SECURE) {
-- 
1.7.7.6