block: Add discard flag to blkdev_issue_zeroout() function

blkdev_issue_discard() will zero a given block range. This is done by
way of explicit writing, thus provisioning or allocating the blocks on
disk.

There are use cases where the desired behavior is to zero the blocks but
unprovision them if possible. The blocks must deterministically contain
zeroes when they are subsequently read back.

This patch adds a flag to blkdev_issue_zeroout() that provides this
variant. If the discard flag is set and a block device guarantees
discard_zeroes_data we will use REQ_DISCARD to clear the block range. If
the device does not support discard_zeroes_data or if the discard
request fails we will fall back to first REQ_WRITE_SAME and then a
regular REQ_WRITE.

Also update the callers of blkdev_issue_zero() to reflect the new flag
and make sb_issue_zeroout() prefer the discard approach.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index e9086be6d9a0..4c4b732d7556 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1162,7 +1162,7 @@ extern int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
 extern int blkdev_issue_write_same(struct block_device *bdev, sector_t sector,
                 sector_t nr_sects, gfp_t gfp_mask, struct page *page);
 extern int blkdev_issue_zeroout(struct block_device *bdev, sector_t sector,
-                        sector_t nr_sects, gfp_t gfp_mask);
+                sector_t nr_sects, gfp_t gfp_mask, bool discard);
 static inline int sb_issue_discard(struct super_block *sb, sector_t block,
                 sector_t nr_blocks, gfp_t gfp_mask, unsigned long flags)
 {
@@ -1176,7 +1176,7 @@ static inline int sb_issue_zeroout(struct super_block *sb, sector_t block,
         return blkdev_issue_zeroout(sb->s_bdev,
                                     block << (sb->s_blocksize_bits - 9),
                                     nr_blocks << (sb->s_blocksize_bits - 9),
-                                    gfp_mask);
+                                    gfp_mask, true);
 }
 
 extern int blk_verify_command(unsigned char *cmd, fmode_t has_write_perm);