Re: list corruption in deferred_split_scan()

Shakeel Butt
2019-07-16T23:36:32Z

Adding related people.

The thread starts at:
http://lkml.kernel.org/r/1562795006.8510.19.camel@lca.pw

On Mon, Jul 15, 2019 at 8:01 PM Yang Shi <yang.shi@linux.alibaba.com> wrote:
>
>
>
> On 7/15/19 6:36 PM, Qian Cai wrote:
> >
> >> On Jul 15, 2019, at 8:22 PM, Yang Shi <yang.shi@linux.alibaba.com> wrote:
> >>
> >>
> >>
> >> On 7/15/19 2:23 PM, Qian Cai wrote:
> >>> On Fri, 2019-07-12 at 12:12 -0700, Yang Shi wrote:
> >>>>> Another possible lead is that without reverting the those commits below,
> >>>>> kdump
> >>>>> kernel would always also crash in shrink_slab_memcg() at this line,
> >>>>>
> >>>>> map = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_map, true);
> >>>> This looks a little bit weird. It seems nodeinfo[nid] is NULL? I didn't
> >>>> think of where nodeinfo was freed but memcg was still online. Maybe a
> >>>> check is needed:
> >>> Actually, "memcg" is NULL.
> >> It sounds weird. shrink_slab() is called in mem_cgroup_iter which does pin the memcg. So, the memcg should not go away.
> > Well, the commit “mm: shrinker: make shrinker not depend on memcg kmem” changed this line in shrink_slab_memcg(),
> >
> > - if (!memcg_kmem_enabled() || !mem_cgroup_online(memcg))
> > + if (!mem_cgroup_online(memcg))
> > return 0;
> >
> > Since the kdump kernel has the parameter “cgroup_disable=memory”, shrink_slab_memcg() will no longer be able to handle NULL memcg from mem_cgroup_iter() as,
> >
> > if (mem_cgroup_disabled())
> > return NULL;
>
> Aha, yes. memcg_kmem_enabled() implicitly checks !mem_cgroup_disabled().
> Thanks for figuring this out. I think we need add mem_cgroup_dsiabled()
> check before calling shrink_slab_memcg() as below:
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a0301ed..2f03c61 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -701,7 +701,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int
> nid,
> unsigned long ret, freed = 0;
> struct shrinker *shrinker;
>
> - if (!mem_cgroup_is_root(memcg))
> + if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg))
> return shrink_slab_memcg(gfp_mask, nid, memcg, priority);
>
> if (!down_read_trylock(&shrinker_rwsem))
>

We were seeing unneeded oom-kills on kernels with
"cgroup_disabled=memory" and Yang's patch series basically expose the
bug to crash. I think the commit aeed1d325d42 ("mm/vmscan.c:
generalize shrink_slab() calls in shrink_node()") missed the case for
"cgroup_disabled=memory". However I am surprised that root_mem_cgroup
is allocated even for "cgroup_disabled=memory" and it seems like
css_alloc() is called even before checking if the corresponding
controller is disabled.

Yang, can you please send the above change with signed-off and CC to
stable as well?

thanks,
Shakeel