linux-kernel - Re: [overlayfs] lockdep splat after mounting overlayfs over overlayfs

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Date:	Thu, 25 Jun 2015 11:21:21 +0300
From:	Konstantin Khlebnikov <koct9i@...il.com>
To:	Xu Wang <xuw@...hat.com>
Cc:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	Miklos Szeredi <miklos@...redi.hu>,
	linux-unionfs@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [overlayfs] lockdep splat after mounting overlayfs over overlayfs

On Thu, Jun 25, 2015 at 10:24 AM, Xu Wang <xuw@...hat.com> wrote:
>> I've accidentally mounted one overlayfs over another and got obvious
>> warning from lockdep: i_mutex lockdep classes are per-fs-type.
>>
>> # mount -t overlay overlay 1 -o
>> upperdir=1_upper,workdir=1_work,lowerdir=1_lower
>> # mount -t overlay overlay 2 -o upperdir=2_upper,workdir=2_work,lowerdir=1
>> # ls 2
>
> This reporting is positive, we are not in deadlock situation actually. The "2" dir
> of overlayfs will call iterate_dir->"1" dir of overlayfs->iterate_dir, and the nest
> iterate_dir happened on the same file system, so the warning came out.
>
> We'd better make the lower and upper in different fs instance, and this warning
> will disappear.
>
> And this lockdep warning happened when the nest iterate_dir call of same fs(I
> mean the same super block). The function check_deadlock in lockdep.c will
> report the nest lock of same process. If we make the 2_upper and 2_work in
> a different fs, no warning comes out.

Yep, it's not a deadlock. As I mentioned lockdep classes are per-fs-type so
lockdep cannot see difference between i_mutexes on different sb of the
same type.
But anyway this looks messy.

Probably it's safer to forbid overlayfs as lower or upper mount for overlayfs
because this have no sense. Nesting anyway is limited by the depth of kernel
stack and sb->s_stack_depth.

Or overlayfs could detect this situation and substitute layers of underlying
overlayfs into its own lower layers in appropriate place.

>
> Thanks,
>
> --
> George Wang 王旭
>
> Kernel Quantity Engineer
> Red Hat Software (Beijing) Co.,Ltd
> IRC:xuw
> Tel:+86-010-62608041
> Phone:15901231579
> 9/F, Tower C, Raycom
> --
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