[PATCH v2] loop: fix no-unmap write-zeroes request behavior

["Darrick J. Wong" <darrick.wong@oracle.com>]

From: Darrick J. Wong <darrick.wong@oracle.com>

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 <darrick.wong@oracle.com>
---
v2: reorganize a little according to hch feedback
---
 drivers/block/loop.c |   31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index f6f77eaa7217..4943d0c5c61c 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -441,6 +441,28 @@ 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;
+
+	/*
+	 * Ask the fs to zero out the blocks, which is supposed to 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;
@@ -596,8 +618,15 @@ static int do_req_filebacked(struct loop_device *lo, struct request *rq)
 	switch (req_op(rq)) {
 	case REQ_OP_FLUSH:
 		return lo_req_flush(lo, rq);
-	case REQ_OP_DISCARD:
 	case REQ_OP_WRITE_ZEROES:
+		/*
+		 * If the caller doesn't want deallocation, call zeroout to
+		 * write zeroes the range.  Otherwise, punch them out.
+		 */
+		if (rq->cmd_flags & REQ_NOUNMAP)
+			return lo_zeroout(lo, rq, pos);
+		/* fall through */
+	case REQ_OP_DISCARD:
 		return lo_discard(lo, rq, pos);
 	case REQ_OP_WRITE:
 		if (lo->transfer)