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

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

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 the mm's memory is fully mlocked (needs privilege) or
     fully shared (does not need privilege)

> 
> 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.

s/mlocked/mlocked or shared/g

> 
> 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.

I already proposed more simpler one at https://patchwork.kernel.org/patch/9877991/ .

> 
> 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.

But before thinking about your proposal, please think about how to guarantee
that the OOM reaper and the exit path can run discussed at
http://lkml.kernel.org/r/201805122318.HJG81246.MFVFLFJOOQtSHO@I-love.SAKURA.ne.jp .