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Date:   Thu, 23 Nov 2017 22:57:06 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     mhocko@...nel.org
Cc:     jack@...e.cz, viro@...iv.linux.org.uk, david@...morbit.com,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCH] fs: handle shrinker registration failure in sget_userns

Michal Hocko wrote:
> > @@ -260,9 +261,8 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
> >  	s->s_shrink.count_objects = super_cache_count;
> >  	s->s_shrink.batch = 1024;
> >  	s->s_shrink.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE;
> > -	INIT_LIST_HEAD(&s->s_shrink.list);
> > -	return s;
> > -
> > +	if (register_shrinker(&s->s_shrink) == 0)
> > +		return s;
> >  fail:
> >  	destroy_unused_super(s);
> >  	return NULL;
> 
> But I am not sure this is correct. So what protects shrinker invocation
> while the object is not initialized yet?

Then, what protects shrinker invocation in your patch?
Your patch is calling register_shrinker() immediately after
alloc_super() succeeds. My patch just moved register_shrinker()
into alloc_super().

@@ -503,6 +512,10 @@ struct super_block *sget_userns(struct file_system_type *type,
 		s = alloc_super(type, (flags & ~SB_SUBMOUNT), user_ns);
 		if (!s)
 			return ERR_PTR(-ENOMEM);
+		if (register_shrinker(&s->s_shrink)) {
+			destroy_unused_super(s);
+			return ERR_PTR(-ENOMEM);
+		}
 		goto retry;
 	}
 

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