Re: [PATCH v2] fs: handle shrinker registration failure in sget_userns

[Al Viro <viro@ZenIV.linux.org.uk>]

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;
	}