Re: [PATCH] mm, memcg: avoid oom if cgroup is not populated

[Yafang Shao <laoar.shao@gmail.com>]

On Tue, Nov 26, 2019 at 9:16 PM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Tue 26-11-19 08:02:49, Yafang Shao wrote:
> > There's one case that the processes in a memcg are all exit (due to OOM
> > group or some other reasons), but the file page caches are still exist.
> > These file page caches may be protected by memory.min so can't be
> > reclaimed. If we can't success to restart the processes in this memcg or
> > don't want to make this memcg offline, then we want to drop the file page
> > caches.
> > The advantage of droping this file caches is it can avoid the reclaimer
> > (either kswapd or direct) scanning and reclaiming pages from all memcgs
> > exist in this system, because currently the reclaimer will fairly reclaim
> > pages from all memcgs if the system is under memory pressure.
> > The possible method to drop these file page caches is setting the
> > hard limit of this memcg to 0. Unfortunately this may invoke the OOM killer
> > and generates lots of misleading outputs, that should not happen.
>
> I disagree that the output is misleading. Quite contrary, it provides a
> useful lead on the unreclaimable memory.
>

We can show the unreclaimable memory independently, rather than print
the full oom output.
OOM killer is used to kill process, why do we invoke it when there's
no process ?
What's the advantage of doing it ?

> > One misleading output is "Out of memory and no killable processes...",
> > while really there is no tasks rather than no killable tasks.
>
> Again, this is nothing misleading. No task is a trivial subset of no
> killable task. I do not see why we should treat one differently than the
> other.
>

No killable tasks means  there's task and the OOM killer may be invoked.
While no tasks means the OOM killer is useless.

> > Furthermore,
> > the OOM output is not expected by the admin if he or she only wants to drop
> > the cahes and knows there're no processes running in this memcg.
>
> But this is not what hard limit reduced to 0 really does. No matter
> whether there is some task or not. It simply reclaims _all_ the memory
> as explained in other email.
>

Are there any way to reclaim page cache only ?
No.

I know it will relcaim all the memory.
If you really think this expression is a prolem,  but does it
improtant that we should distingush between  caches (both page caches
and kmem) and _all_ memory, especially when there's no processes ?

> > If memcg is not populated, we should not invoke the OOM killer.
>
> I have already explained why I believe this is not correct in other
> email and this description doesn't provide any real justification. It is
> merely your intepretation of what should happen and I believe you
> haven't thought through it really.
>
> > Fixes: b6e6edcf ("mm: memcontrol: reclaim and OOM kill when shrinking memory.max below usage")
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: Michal Hocko <mhocko@suse.com>
> > ---
> >  mm/memcontrol.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 1c4c08b..4e08905 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -6139,9 +6139,13 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
> >                       continue;
> >               }
> >
> > -             memcg_memory_event(memcg, MEMCG_OOM);
> > -             if (!mem_cgroup_out_of_memory(memcg, GFP_KERNEL, 0))
> > +             if (cgroup_is_populated(memcg->css.cgroup)) {
> > +                     memcg_memory_event(memcg, MEMCG_OOM);
> > +                     if (!mem_cgroup_out_of_memory(memcg, GFP_KERNEL, 0))
> > +                             break;
> > +             } else  {
> >                       break;
> > +             }
> >       }
> >
> >       memcg_wb_domain_size_changed(memcg);
> > --
> > 1.8.3.1
>
> --
> Michal Hocko
> SUSE Labs