[PATCH AUTOSEL 5.4 187/350] loop: fix no-unmap write-zeroes request behavior

Sasha Levin
2019-12-10T21:24:58Z

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

[ Upstream commit efcfec579f6139528c9e6925eca2bc4a36da65c6 ]

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 <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 f6f77eaa7217e..ef6e251857c8c 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 mode = FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE;
int ret;

+ mode |= FALLOC_FL_KEEP_SIZE;
+
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);
- case REQ_OP_DISCARD:
case REQ_OP_WRITE_ZEROES:
- 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) ?
+ FALLOC_FL_ZERO_RANGE :
+ FALLOC_FL_PUNCH_HOLE);
+ case REQ_OP_DISCARD:
+ return lo_fallocate(lo, rq, pos, FALLOC_FL_PUNCH_HOLE);
case REQ_OP_WRITE:
if (lo->transfer)
return lo_write_transfer(lo, rq, pos);
--
2.20.1