linux-kernel - Re: [PATCH v2] fs: handle shrinker registration failure in sget

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Date:   Fri, 24 Nov 2017 06:51:09 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     mhocko@...nel.org
Cc:     viro@...IV.linux.org.uk, david@...morbit.com, jack@...e.cz,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCH v2] fs: handle shrinker registration failure in sget_userns

Al Viro wrote:
> On Fri, Nov 24, 2017 at 12:35:29AM +0900, Tetsuo Handa wrote:
> > Al Viro wrote:
> > > On Fri, Nov 24, 2017 at 12:04:23AM +0900, Tetsuo Handa wrote:
> > > > Al Viro wrote:
> > > > > On Thu, Nov 23, 2017 at 03:35:37PM +0100, Michal Hocko wrote:
> > > > > > Hopefully less screwed version.  But as I've said I am not really
> > > > > > familiar with the code and do not feel competent to change it so please
> > > > > > be very careful here. I've moved the shrinker registration to
> > > > > > alloc_super which turned out to be simpler.
> > > > > 
> > > > > I don't get it.  Why the hell do we need all that PITA in the first place?
> > > > > Just make sget_userns() end with
> > > > > 	if (unlikely(regsiter_shrinker(&s->s_shrink) != 0)) {
> > > > > 		deactivate_locked_super(s);
> > > > > 		s = ERR_PTR(-ENOMEM);
> > > > > 	}
> > > > > 	return s;
> > > > > and be done with that.  All there is to it...
> > > > > 
> > > > 
> > > > Doesn't deactivate_locked_super() call unregister_shrinker() ?
> > > 
> > > And?  unregister_shrinker() will do list_del() on empty list_head
> > > and kfree(NULL); where's the problem with that?
> > > 
> > The problem is that calling unregister_shrinker() without successful
> > register_shrinker() causes crash due to s_shrink.list.{prev,next} == NULL.
> 
> *shrug*
>         shrinker->nr_deferred = kzalloc(size, GFP_KERNEL);
>         if (!shrinker->nr_deferred) {
> 		/* make sure it's in consistent state */
> 		INIT_LIST_HEAD(&shrinker->list);
>                 return -ENOMEM;
> 	}
> 
> 

Yes, that will work.

Michal, like Al thinks, making unregister_shrinker() no-op if
register_shrinker() failed simplifies things. Can we go with
http://lkml.kernel.org/r/1511265853-15654-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp
with patch description updated to include Syzbot report?