4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: James Wang <jnw...@suse.com>


[ Upstream commit 6460495709aeb651896bc8e5c134b2e4ca7d34a8 ]

While installing SLES-12 (based on v4.4), I found that the installer
will stall for 60+ seconds during LVM disk scan.  The root cause was
determined to be the removal of a bound device check in loop_flush()
by commit b5dd2f6047ca ("block: loop: improve performance via blk-mq").

Restoring this check, examining ->lo_state as set by loop_set_fd()
eliminates the bad behavior.

Test method:
modprobe loop max_loop=64
dd if=/dev/zero of=disk bs=512 count=200K
for((i=0;i<4;i++))do losetup -f disk; done
mkfs.ext4 -F /dev/loop0
for((i=0;i<4;i++))do mkdir t$i; mount /dev/loop$i t$i;done
for f in `ls /dev/loop[0-9]*|sort`; do \
        echo $f; dd if=$f of=/dev/null  bs=512 count=1; \
        done

Test output:  stock          patched
/dev/loop0    18.1217e-05    8.3842e-05
/dev/loop1     6.1114e-05    0.000147979
/dev/loop10    0.414701      0.000116564
/dev/loop11    0.7474        6.7942e-05
/dev/loop12    0.747986      8.9082e-05
/dev/loop13    0.746532      7.4799e-05
/dev/loop14    0.480041      9.3926e-05
/dev/loop15    1.26453       7.2522e-05

Note that from loop10 onward, the device is not mounted, yet the
stock kernel consumes several orders of magnitude more wall time
than it does for a mounted device.
(Thanks for Mike Galbraith <efa...@gmx.de>, give a changelog review.)

Reviewed-by: Hannes Reinecke <h...@suse.com>
Reviewed-by: Ming Lei <ming....@redhat.com>
Signed-off-by: James Wang <jnw...@suse.com>
Fixes: b5dd2f6047ca ("block: loop: improve performance via blk-mq")
Signed-off-by: Jens Axboe <ax...@fb.com>
Signed-off-by: Sasha Levin <alexander.le...@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
 drivers/block/loop.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -612,6 +612,9 @@ static int loop_switch(struct loop_devic
  */
 static int loop_flush(struct loop_device *lo)
 {
+       /* loop not yet configured, no running thread, nothing to flush */
+       if (lo->lo_state != Lo_bound)
+               return 0;
        return loop_switch(lo, NULL);
 }
 


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