Re: [PATCH] Fix loop device flush before configure v2

[James Wang <jnwang@suse.com>]

On 06/08/2017 01:06 PM, Mike Galbraith wrote:
> On Thu, 2017-06-08 at 10:17 +0800, James Wang wrote:
>> This condition check was exist at before commit b5dd2f6047ca ("block: loop:
>> improve performance via blk-mq") When add MQ support to loop device, it be
>> removed because the member of '->lo_thread' be removed. And then upstream
>> add '->worker_task', I think they forget add it to here.
>>
>> When I install SLES-12 product is base on 4.4 kernel I found installer will
>> hang +60 second at scan disks. and I found LVM tools would take this action.
>> finally I found this problem is more obvious on AMD platform. This problem
>> will impact all scenarios that scan loop devcie.
>>
>> When the loop device didn't configure backing file or Request Queue, we
>> shouldn't to cost a lot of time to flush it.
> The changelog sounds odd to me, perhaps reword/condense a bit?...
>
> 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.
Thank you sir. I will rewrite this changelog.
> 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.
>
> Reviewed-by: Hannes Reinecke <hare@suse.com>
> Signed-off-by: James Wang <jnwang@suse.com>
> Fixes: b5dd2f6047ca ("block: loop: improve performance via blk-mq")
> ---
>>  drivers/block/loop.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
>> index 48f6fa6f810e..2e5b8538760c 100644
>> --- a/drivers/block/loop.c
>> +++ b/drivers/block/loop.c
>> @@ -625,6 +625,9 @@ static int loop_switch(struct loop_device *lo, struct file *file)
>>   */
>>  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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