loop: fix no-unmap write-zeroes request behavior

Commit Message

Darrick J. Wong Oct. 10, 2019, 5:02 p.m. UTC

From: Darrick J. Wong <[e-mail]>

Currently, if the loop device receives a WRITE_ZEROES request, it asks
the underlying filesystem to punch out the range.  This behavior is
correct if unmapping is allowed.  However, a NOUNMAP request means that
the caller forbids us from freeing the storage backing the range, so
punching out the range is incorrect behavior.

To satisfy a NOUNMAP | WRITE_ZEROES request, loop should ask the
underlying filesystem to FALLOC_FL_ZERO_RANGE, which is (according to
the fallocate documentation) required to ensure that the entire range is
backed by real storage, which suffices for our purposes.

Fixes: 19372e2769179dd ("loop: implement REQ_OP_WRITE_ZEROES")
Signed-off-by: Darrick J. Wong <[e-mail]>
---
 drivers/block/loop.c |   32 +++++++++++++++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)

Comments

Christoph Hellwig Oct. 11, 2019, 7:51 a.m. UTC | #1

On Thu, Oct 10, 2019 at 10:02:39AM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <[e-mail]>
> 
> Currently, if the loop device receives a WRITE_ZEROES request, it asks
> the underlying filesystem to punch out the range.  This behavior is
> correct if unmapping is allowed.  However, a NOUNMAP request means that
> the caller forbids us from freeing the storage backing the range, so
> punching out the range is incorrect behavior.

It doesn't really forbid, as most protocols don't have a way for forbid
deallocation.  It requests not to.

Otherwise this looks fine, although I would have implemented it slightly
differently:

>  	case REQ_OP_FLUSH:
>  		return lo_req_flush(lo, rq);
>  	case REQ_OP_DISCARD:
> -	case REQ_OP_WRITE_ZEROES:
>  		return lo_discard(lo, rq, pos);
> +	case REQ_OP_WRITE_ZEROES:
> +		return lo_zeroout(lo, rq, pos);

This could just become:

	case REQ_OP_WRITE_ZEROES:
		if (rq->cmd_flags & REQ_NOUNMAP))
			return lo_zeroout(lo, rq, pos);
		/*FALLTHRU*/
	case REQ_OP_DISCARD:
		return lo_discard(lo, rq, pos);

Patch

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index f6f77eaa7217..0dc981e94bf0 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -441,6 +441,35 @@  static int lo_discard(struct loop_device *lo, struct request *rq, loff_t pos)
 	return ret;
 }
 
+static int lo_zeroout(struct loop_device *lo, struct request *rq, loff_t pos)
+{
+	struct file *file = lo->lo_backing_file;
+	int mode = FALLOC_FL_ZERO_RANGE | FALLOC_FL_KEEP_SIZE;
+	int ret;
+
+	/* If we're allowed to unmap the blocks, ask the fs to punch them. */
+	if (!(rq->cmd_flags & REQ_NOUNMAP)) {
+		ret = lo_discard(lo, rq, pos);
+		if (!ret)
+			return 0;
+	}
+
+	/*
+	 * Otherwise, ask the fs to zero out the blocks, which will result in
+	 * space being allocated to the file.
+	 */
+	if (!file->f_op->fallocate) {
+		ret = -EOPNOTSUPP;
+		goto out;
+	}
+
+	ret = file->f_op->fallocate(file, mode, pos, blk_rq_bytes(rq));
+	if (unlikely(ret && ret != -EINVAL && ret != -EOPNOTSUPP))
+		ret = -EIO;
+ out:
+	return ret;
+}
+
 static int lo_req_flush(struct loop_device *lo, struct request *rq)
 {
 	struct file *file = lo->lo_backing_file;
@@ -597,8 +626,9 @@  static int do_req_filebacked(struct loop_device *lo, struct request *rq)
 	case REQ_OP_FLUSH:
 		return lo_req_flush(lo, rq);
 	case REQ_OP_DISCARD:
-	case REQ_OP_WRITE_ZEROES:
 		return lo_discard(lo, rq, pos);
+	case REQ_OP_WRITE_ZEROES:
+		return lo_zeroout(lo, rq, pos);
 	case REQ_OP_WRITE:
 		if (lo->transfer)
 			return lo_write_transfer(lo, rq, pos);