linux-kernel - Re: [PATCH] Fix loop device flush before configure

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Date:   Wed, 7 Jun 2017 17:25:12 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     James Wang <jnwang@...e.com>
Cc:     axboe@...com, hare@...e.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, mgorman@...e.com
Subject: Re: [PATCH] Fix loop device flush before configure

On Wed, Jun 07, 2017 at 05:01:04PM +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 devcies.
> 
> When the loop device didn't configure backing file or Request Queue, we
> shouldn't to cost a lot of time to flush it.
> 
> Testing steps are following:
> 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
> 
> Testing data is following:
> /dev/loop0	<rpm-4.4.68-2> <+patched>
> 		8.1217e-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
> 
> From /dev/loop10 start, loop isn't mounted. but it take more time than
> mounted devices. And The data differ by several orders of magnitude.
> 
> Reviewed-by: Hannes Reinecke <hare@...e.com>
> 
> Signed-off-by: James Wang <jnwang@...e.com>
> ---
>  drivers/block/loop.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 48f6fa6f810e..c1807e91db08 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->worker_task)
> +		return 0;
>  	return loop_switch(lo, NULL);
>  }

Good catch!

But looks better to add check like the following

	if (lo->lo_state != Lo_bound)
		return 0;

because we don't clear lo->worker_task in loop_unprepare_queue()
and it is more readable to check on lo->lo_state.


thanks,
Ming