On 06/08/2017 12:52 AM, James Wang wrote:
> 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 <efault@xxxxxx>, give a changelog review.)
Added for 4.12, thanks.
--
Jens Axboe
