[SRU][B][PATCH 1/1] loop: fix I/O error on fsync() in detached loop devices

[Jorge Merlino]

From: Mauricio Faria de Oliveira <mfo at canonical.com>

BugLink: https://bugs.launchpad.net/bugs/1856871

There's an I/O error on fsync() in a detached loop device
if it has been previously attached.

The issue is write cache is enabled in the attach path in
loop_configure() but it isn't disabled in the detach path;
thus it remains enabled in the block device regardless of
whether it is attached or not.

Now fsync() can get an I/O request that will just be failed
later in loop_queue_rq() as device's state is not 'Lo_bound'.

So, disable write cache in the detach path.

Do so based on the queue flag, not the loop device flag for
read-only (used to enable) as the queue flag can be changed
via sysfs even on read-only loop devices (e.g., losetup -r.)

Test-case:

    # DEV=/dev/loop7

    # IMG=/tmp/image
    # truncate --size 1M $IMG

    # losetup $DEV $IMG
    # losetup -d $DEV

Before:

    # strace -e fsync parted -s $DEV print 2>&1 | grep fsync
    fsync(3)                                = -1 EIO (Input/output error)
    Warning: Error fsyncing/closing /dev/loop7: Input/output error
    [  982.529929] blk_update_request: I/O error, dev loop7, sector 0 op 0x1:(WRITE) flags 0x800 phys_seg 0 prio class 0

After:

    # strace -e fsync parted -s $DEV print 2>&1 | grep fsync
    fsync(3)                                = 0

Co-developed-by: Eric Desrochers <eric.desrochers at canonical.com>
Signed-off-by: Eric Desrochers <eric.desrochers at canonical.com>
Signed-off-by: Mauricio Faria de Oliveira <mfo at canonical.com>
Tested-by: Gabriel Krisman Bertazi <krisman at collabora.com>
Reviewed-by: Ming Lei <ming.lei at redhat.com>
Signed-off-by: Jens Axboe <axboe at kernel.dk>
(backported from commit 4ceddce55eb35d15b0f87f5dcf6f0058fd15d3a4)
[Jorge Merlino: move patch to loop_clr_fd function]
Signed-off-by: Jorge Merlino <jorge.merlino at canonical.com>
---
 drivers/block/loop.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 22f1575feb10..66c468de84fa 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1113,6 +1113,9 @@ static int loop_clr_fd(struct loop_device *lo)
 	if (filp == NULL)
 		return -EINVAL;
 
+	if (test_bit(QUEUE_FLAG_WC, &lo->lo_queue->queue_flags))
+		blk_queue_write_cache(lo->lo_queue, false, false);
+
 	/* freeze request queue during the transition */
 	blk_mq_freeze_queue(lo->lo_queue);
 
-- 
2.17.1