Re: list corruption in deferred_split_scan()

[Shakeel Butt <shakeelb@google.com>]

On Tue, Jul 16, 2019 at 5:12 PM Yang Shi <yang.shi@linux.alibaba.com> wrote:
>
>
>
> On 7/16/19 4:36 PM, Shakeel Butt wrote:
> > 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.
>
> I'm surprised too. A quick test with drgn shows root memcg is definitely
> allocated:
>
>  >>> prog['root_mem_cgroup']
> *(struct mem_cgroup *)0xffff8902cf058000 = {
> [snip]
>
> But, isn't this a bug?

It can be treated as a bug as this is not expected but we can discuss
and take care of it later. I think we need your patch urgently as
memory reclaim and /proc/sys/vm/drop_caches is broken for
"cgroup_disabled=memory" kernel. So, please send your patch asap.

thanks,
Shakeel