Re: [patch] mm, oom: fix unnecessary killing of additional processes

From: Michal Hocko
Date: Fri Jun 15 2018 - 02:55:49 EST

On Thu 14-06-18 13:42:59, David Rientjes wrote:
> The oom reaper ensures forward progress by setting MMF_OOM_SKIP itself if
> it cannot reap an mm. This can happen for a variety of reasons,
> including:
> - the inability to grab mm->mmap_sem in a sufficient amount of time,
> - when the mm has blockable mmu notifiers that could cause the oom reaper
> to stall indefinitely,
> but we can also add a third when the oom reaper can "reap" an mm but doing
> so is unlikely to free any amount of memory:
> - when the mm's memory is fully mlocked.
> When all memory is mlocked, the oom reaper will not be able to free any
> substantial amount of memory. It sets MMF_OOM_SKIP before the victim can
> unmap and free its memory in exit_mmap() and subsequent oom victims are
> chosen unnecessarily. This is trivial to reproduce if all eligible
> processes on the system have mlocked their memory: the oom killer calls
> panic() even though forward progress can be made.
> This is the same issue where the exit path sets MMF_OOM_SKIP before
> unmapping memory and additional processes can be chosen unnecessarily
> because the oom killer is racing with exit_mmap().
> We can't simply defer setting MMF_OOM_SKIP, however, because if there is
> a true oom livelock in progress, it never gets set and no additional
> killing is possible.
> To fix this, this patch introduces a per-mm reaping timeout, initially set
> at 10s. It requires that the oom reaper's list becomes a properly linked
> list so that other mm's may be reaped while waiting for an mm's timeout to
> expire.
> This replaces the current timeouts in the oom reaper: (1) when trying to
> grab mm->mmap_sem 10 times in a row with HZ/10 sleeps in between and (2)
> a HZ sleep if there are blockable mmu notifiers. It extends it with
> timeout to allow an oom victim to reach exit_mmap() before choosing
> additional processes unnecessarily.
> The exit path will now set MMF_OOM_SKIP only after all memory has been
> freed, so additional oom killing is justified, and rely on MMF_UNSTABLE to
> determine when it can race with the oom reaper.
> The oom reaper will now set MMF_OOM_SKIP only after the reap timeout has
> lapsed because it can no longer guarantee forward progress.
> The reaping timeout is intentionally set for a substantial amount of time
> since oom livelock is a very rare occurrence and it's better to optimize
> for preventing additional (unnecessary) oom killing than a scenario that
> is much more unlikely.
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>

Nacked-by: Michal Hocko <mhocko@xxxxxxxx>
as already explained elsewhere in this email thread.

> ---
> Note: I understand there is an objection based on timeout based delays.
> This is currently the only possible way to avoid oom killing important
> processes completely unnecessarily. If the oom reaper can someday free
> all memory, including mlocked memory and those mm's with blockable mmu
> notifiers, and is guaranteed to always be able to grab mm->mmap_sem,
> this can be removed. I do not believe any such guarantee is possible
> and consider the massive killing of additional processes unnecessarily
> to be a regression introduced by the oom reaper and its very quick
> setting of MMF_OOM_SKIP to allow additional processes to be oom killed.

If you find oom reaper more harmful than useful I would be willing to
ack a comman line option to disable it. Especially when you keep
claiming that the lockups are not really happening in your environment.

Other than that I've already pointed to a more robust solution. If you
are reluctant to try it out I will do, but introducing a timeout is just
papering over the real problem. Maybe we will not reach the state that
_all_ the memory is reapable but we definitely should try to make as
much as possible to be reapable and I do not see any fundamental
problems in that direction.
Michal Hocko