[PATCH v4 8/8] block: implement async write zero pages command

[Pavel Begunkov <asml.silence@gmail.com>]

Add a command that writes the zero page to the drive. Apart from passing
the zero page instead of actual data it uses the normal write path and
doesn't do any further acceleration, nor it requires any special
hardware support. The indended use is to have a fallback when
BLOCK_URING_CMD_WRITE_ZEROES is not supported.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 block/ioctl.c           | 24 +++++++++++++++++++++---
 include/uapi/linux/fs.h |  1 +
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/block/ioctl.c b/block/ioctl.c
index ef4b2a90ad79..3cb479192023 100644
--- a/block/ioctl.c
+++ b/block/ioctl.c
@@ -774,7 +774,8 @@ static void bio_cmd_bio_end_io(struct bio *bio)
 
 static int blkdev_cmd_write_zeroes(struct io_uring_cmd *cmd,
 				   struct block_device *bdev,
-				   uint64_t start, uint64_t len, bool nowait)
+				   uint64_t start, uint64_t len,
+				   bool nowait, bool zero_pages)
 {
 
 	sector_t bs_mask = (bdev_logical_block_size(bdev) >> SECTOR_SHIFT) - 1;
@@ -793,6 +794,20 @@ static int blkdev_cmd_write_zeroes(struct io_uring_cmd *cmd,
 	if (err)
 		return err;
 
+	if (zero_pages) {
+		struct blk_iou_cmd *bic = io_uring_cmd_to_pdu(cmd,
+						struct blk_iou_cmd);
+
+		err = blkdev_issue_zero_pages_bio(bdev, sector, nr_sects,
+						  gfp, &prev,
+						  BLKDEV_ZERO_PAGES_NOWAIT);
+		if (!prev)
+			return -EAGAIN;
+		if (err)
+			bic->res = err;
+		goto out_submit;
+	}
+
 	if (!limit)
 		return -EOPNOTSUPP;
 	/*
@@ -826,7 +841,7 @@ static int blkdev_cmd_write_zeroes(struct io_uring_cmd *cmd,
 	}
 	if (!prev)
 		return -EAGAIN;
-
+out_submit:
 	prev->bi_private = cmd;
 	prev->bi_end_io = bio_cmd_bio_end_io;
 	submit_bio(prev);
@@ -904,7 +919,10 @@ int blkdev_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags)
 		return blkdev_cmd_discard(cmd, bdev, start, len, bic->nowait);
 	case BLOCK_URING_CMD_WRITE_ZEROES:
 		return blkdev_cmd_write_zeroes(cmd, bdev, start, len,
-					       bic->nowait);
+					       bic->nowait, false);
+	case BLOCK_URING_CMD_WRITE_ZERO_PAGE:
+		return blkdev_cmd_write_zeroes(cmd, bdev, start, len,
+					       bic->nowait, true);
 	}
 	return -EINVAL;
 }
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 68b0fccebf92..f4337b87d846 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -210,6 +210,7 @@ struct fsxattr {
 
 #define BLOCK_URING_CMD_DISCARD			_IO(0x12,137)
 #define BLOCK_URING_CMD_WRITE_ZEROES		_IO(0x12,138)
+#define BLOCK_URING_CMD_WRITE_ZERO_PAGE		_IO(0x12,139)
 
 #define BMAP_IOCTL 1		/* obsolete - kept for compatibility */
 #define FIBMAP	   _IO(0x00,1)	/* bmap access */
-- 
2.45.2