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 Thu 01-12-16 03:24:54, Marek Marczykowski-Gorecki wrote:
> On Wed, Nov 30, 2016 at 07:30:17PM +0100, Michal Hocko wrote:
> > 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.
> 
> The above patch seems to help. At least the problem haven't occurred for
> the last ~40 VM startups.

I will consider this as
Tested-by: Marek Marczykowski-Gorecki <marmarek@mimuw.edu.pl>

OK? Thanks for the report and testing!

-- 
Michal Hocko
SUSE Labs

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