[PATCH RFC] block: use discard if possible in blkdev_issue_discard()

[Theodore Ts'o <tytso@mit.edu>]

How do people think I should implement this functionality?  I see
three possible choices:

1) The patch attached below

2) Add this functionality to blk-lib.c, but with a new function name,
   such as blkdev_zero_blocks(), and keep blkdev_issue_zeroout()
   unchanged

3) This functionality shouldn't be in the block device layer; if you
   want something like this, add it to fs/ext4 instead, and other file
   systems can optimize sb_issue_zeroout() themselves if they want.

And if the answer is (1) or (2), do people mind if I carry this patch
in the ext4 tree, so I can use and test this right away, without
having to worry about merging with the block tree?

Thanks,

					- Ted


commit 1af2359e6ffca707c41ed40618773abe944d0c54
Author: Theodore Ts'o <tytso@mit.edu>
Date:   Thu Feb 13 23:27:27 2014 -0500

    block: use discard if possible in blkdev_issue_zeroout()
    
    If the block device supports discards and guarantees that subsequent
    reads will return zeros (sometimes known as DZAT, for Deterministic
    read Zeros After Trim), use this to implement blkdev_issue_zeroout()
    
    Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

diff --git a/block/blk-lib.c b/block/blk-lib.c
index 2da76c9..62cbf28 100644
--- a/block/blk-lib.c
+++ b/block/blk-lib.c
@@ -269,6 +269,32 @@ int __blkdev_issue_zeroout(struct block_device *bdev, sector_t sector,
 	return ret;
 }
 
+static int issue_zeroout_or_write_same(struct block_device *bdev,
+				       sector_t sector,
+				       sector_t nr_sects, gfp_t gfp_mask)
+{
+	if (bdev_write_same(bdev)) {
+		unsigned char bdn[BDEVNAME_SIZE];
+
+		if (!blkdev_issue_write_same(bdev, sector, nr_sects, gfp_mask,
+					     ZERO_PAGE(0)))
+			return 0;
+
+		bdevname(bdev, bdn);
+		pr_err("%s: WRITE SAME failed. Manually zeroing.\n", bdn);
+	}
+
+	return __blkdev_issue_zeroout(bdev, sector, nr_sects, gfp_mask);
+}
+
+/*
+ * Like sector_div except don't modify s.
+ */
+static unsigned int sector_mod(sector_t s, unsigned int m)
+{
+	return sector_div(s, m);
+}
+
 /**
  * blkdev_issue_zeroout - zero-fill a block range
  * @bdev:	blockdev to write
@@ -277,23 +303,49 @@ int __blkdev_issue_zeroout(struct block_device *bdev, sector_t sector,
  * @gfp_mask:	memory allocation flags (for bio_alloc)
  *
  * Description:
- *  Generate and issue number of bios with zerofiled pages.
+ *  Issues bios which zeros the requested block range.
  */
-
 int blkdev_issue_zeroout(struct block_device *bdev, sector_t sector,
 			 sector_t nr_sects, gfp_t gfp_mask)
 {
-	if (bdev_write_same(bdev)) {
-		unsigned char bdn[BDEVNAME_SIZE];
+	struct request_queue *q = bdev_get_queue(bdev);
+	unsigned int alignment, granularity;
+	unsigned int c;
+	int ret;
 
-		if (!blkdev_issue_write_same(bdev, sector, nr_sects, gfp_mask,
-					     ZERO_PAGE(0)))
-			return 0;
+	if (!q)
+		return -ENXIO;
 
-		bdevname(bdev, bdn);
-		pr_err("%s: WRITE SAME failed. Manually zeroing.\n", bdn);
+	if (!blk_queue_discard(q) || !queue_discard_zeroes_data(q) ||
+	    q->limits.discard_misaligned)
+		return issue_zeroout_or_write_same(bdev, sector,
+
+						   nr_sects, gfp_mask);
+
+	alignment = q->limits.discard_alignment >> 9;
+	granularity = q->limits.discard_granularity >> 9;
+
+	c = sector_mod(granularity + alignment - sector, granularity);
+	if (c > nr_sects)
+		c = nr_sects;
+
+	if (c) {
+		int ret = issue_zeroout_or_write_same(bdev, sector,
+						      c, gfp_mask);
+		if (ret)
+			return ret;
+		nr_sects -= c;
 	}
+	if (nr_sects == 0)
+		return 0;
 
-	return __blkdev_issue_zeroout(bdev, sector, nr_sects, gfp_mask);
+	c = sector_mod(nr_sects, granularity);
+
+	ret = blkdev_issue_discard(bdev, sector, nr_sects - c, gfp_mask, 0);
+	if (ret || c == 0)
+		return ret;
+
+	return issue_zeroout_or_write_same(bdev, sector + nr_sects - c, c,
+					   gfp_mask);
 }
 EXPORT_SYMBOL(blkdev_issue_zeroout);