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From: "Darrick J. Wong" <>
To: Jens Axboe <>, Christoph Hellwig <>
	linux-fsdevel <>,
	xfs <>
Subject: [PATCH v3] loop: fix no-unmap write-zeroes request behavior
Date: Mon, 14 Oct 2019 08:50:30 -0700	[thread overview]
Message-ID: <20191014155030.GS13108@magnolia> (raw)
In-Reply-To: <20191010170239.GC13098@magnolia>

From: Darrick J. Wong <>

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 doesn't want us to free 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 <>
v3: refactor into a single fallocate function
v2: reorganize a little according to hch feedback
 drivers/block/loop.c |   26 ++++++++++++++++++--------
 1 file changed, 18 insertions(+), 8 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index f6f77eaa7217..ef6e251857c8 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -417,18 +417,20 @@ static int lo_read_transfer(struct loop_device *lo, struct request *rq,
 	return ret;
-static int lo_discard(struct loop_device *lo, struct request *rq, loff_t pos)
+static int lo_fallocate(struct loop_device *lo, struct request *rq, loff_t pos,
+			int mode)
-	 * We use punch hole to reclaim the free space used by the
-	 * image a.k.a. discard. However we do not support discard if
-	 * encryption is enabled, because it may give an attacker
-	 * useful information.
+	 * We use fallocate to manipulate the space mappings used by the image
+	 * a.k.a. discard/zerorange. However we do not support this if
+	 * encryption is enabled, because it may give an attacker useful
+	 * information.
 	struct file *file = lo->lo_backing_file;
 	int ret;
 	if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
 		ret = -EOPNOTSUPP;
 		goto out;
@@ -596,9 +598,17 @@ 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);
-		return lo_discard(lo, rq, pos);
+		/*
+		 * If the caller doesn't want deallocation, call zeroout to
+		 * write zeroes the range.  Otherwise, punch them out.
+		 */
+		return lo_fallocate(lo, rq, pos,
+			(rq->cmd_flags & REQ_NOUNMAP) ?
+		return lo_fallocate(lo, rq, pos, FALLOC_FL_PUNCH_HOLE);
 	case REQ_OP_WRITE:
 		if (lo->transfer)
 			return lo_write_transfer(lo, rq, pos);

  parent reply	other threads:[~2019-10-14 15:52 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-10 17:02 [PATCH] loop: fix no-unmap write-zeroes request behavior Darrick J. Wong
2019-10-11  7:51 ` Christoph Hellwig
2019-10-11 16:05 ` [PATCH v2] " Darrick J. Wong
2019-10-14  7:28   ` Christoph Hellwig
2019-10-14 15:50 ` Darrick J. Wong [this message]
2019-10-14 16:39   ` [PATCH v3] " Eric Sandeen
2019-10-14 17:00     ` Darrick J. Wong
2019-10-15  7:58   ` Christoph Hellwig

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