[dm-devel] [PATCH 3/3] brd: implement write zeroes

[Mikulas Patocka]

This patch implements REQ_OP_WRITE_ZEROES on brd. Write zeroes will free
the pages just like discard, but the difference is that it writes zeroes
to the preceding and following page if the range is not aligned on page
boundary.

Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>

---
 drivers/block/brd.c |   22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

Index: linux-2.6/drivers/block/brd.c
===================================================================
--- linux-2.6.orig/drivers/block/brd.c
+++ linux-2.6/drivers/block/brd.c
@@ -272,7 +272,8 @@ out:
 
 void brd_do_discard(struct brd_device *brd, struct bio *bio)
 {
-	sector_t sector, len, front_pad;
+	bool zero_padding = bio_op(bio) == REQ_OP_WRITE_ZEROES;
+	sector_t sector, len, front_pad, end_pad;
 
 	if (unlikely(!discard)) {
 		bio->bi_status = BLK_STS_NOTSUPP;
@@ -282,11 +283,22 @@ void brd_do_discard(struct brd_device *b
 	sector = bio->bi_iter.bi_sector;
 	len = bio_sectors(bio);
 	front_pad = -sector & (PAGE_SECTORS - 1);
+
+	if (zero_padding && unlikely(front_pad != 0))
+		copy_to_brd(brd, page_address(ZERO_PAGE(0)),
+			    sector, min(len, front_pad) << SECTOR_SHIFT);
+
 	sector += front_pad;
 	if (unlikely(len <= front_pad))
 		return;
 	len -= front_pad;
-	len = round_down(len, PAGE_SECTORS);
+
+	end_pad = len & (PAGE_SECTORS - 1);
+	if (zero_padding && unlikely(end_pad != 0))
+		copy_to_brd(brd, page_address(ZERO_PAGE(0)),
+			    sector + len - end_pad, end_pad << SECTOR_SHIFT);
+	len -= end_pad;
+
 	while (len) {
 		brd_free_page(brd, sector);
 		sector += PAGE_SECTORS;
@@ -302,7 +314,8 @@ static void brd_submit_bio(struct bio *b
 	struct bio_vec bvec;
 	struct bvec_iter iter;
 
-	if (bio_op(bio) == REQ_OP_DISCARD) {
+	if (bio_op(bio) == REQ_OP_DISCARD ||
+	    bio_op(bio) == REQ_OP_WRITE_ZEROES) {
 		brd_do_discard(brd, bio);
 		goto endio;
 	}
@@ -355,7 +368,7 @@ MODULE_PARM_DESC(max_part, "Num Minors t
 
 static bool discard = false;
 module_param(discard, bool, 0444);
-MODULE_PARM_DESC(discard, "Support discard");
+MODULE_PARM_DESC(discard, "Support discard and write zeroes");
 
 MODULE_LICENSE("GPL");
 MODULE_ALIAS_BLOCKDEV_MAJOR(RAMDISK_MAJOR);
@@ -425,6 +438,7 @@ static int brd_alloc(int i)
 	if (discard) {
 		disk->queue->limits.discard_granularity = PAGE_SIZE;
 		blk_queue_max_discard_sectors(disk->queue, round_down(UINT_MAX, PAGE_SECTORS));
+		blk_queue_max_write_zeroes_sectors(disk->queue, round_down(UINT_MAX, PAGE_SECTORS));
 	}
 
 	/* Tell the block layer that this is not a rotational device */