Re: [Bug 189181] New: BUG: unable to handle kernel NULL pointer dereference in mem_cgroup_node_nr_lru_pages

[Michal Hocko <mhocko@kernel.org>]

On Wed 30-11-16 13:16:53, Johannes Weiner wrote:
> Hi Michael,
> 
> On Wed, Nov 30, 2016 at 06:00:40PM +0100, Michal Hocko wrote:
[...]
> > diff --git a/mm/workingset.c b/mm/workingset.c
> > index 617475f529f4..0f07522c5c0e 100644
> > --- a/mm/workingset.c
> > +++ b/mm/workingset.c
> > @@ -348,7 +348,7 @@ static unsigned long count_shadow_nodes(struct shrinker *shrinker,
> >  	shadow_nodes = list_lru_shrink_count(&workingset_shadow_nodes, sc);
> >  	local_irq_enable();
> >  
> > -	if (memcg_kmem_enabled()) {
> > +	if (memcg_kmem_enabled() && sc->memcg) {
> >  		pages = mem_cgroup_node_nr_lru_pages(sc->memcg, sc->nid,
> >  						     LRU_ALL_FILE);
> >  	} else {
> 
> If we do that, I'd remove the racy memcg_kmem_enabled() check
> altogether and just check for whether we have a memcg or not.

But that would make this a memcg aware shrinker even when kmem is not
enabled...

But now that I am looking into the code
shrink_slab:
		if (memcg_kmem_enabled() &&
		    !!memcg != !!(shrinker->flags & SHRINKER_MEMCG_AWARE))
			continue;

this should be taken care of already. So sc->memcg should be indeed
sufficient. So unless I am missing something I will respin my local
patch and post it later after the reporter has some time to test the
current one.
 
> What do you think, Vladimir?

-- 
Michal Hocko
SUSE Labs

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