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

[Michal Hocko <mhocko@kernel.org>]

On Thu 24-05-18 14:22:53, 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.

No timeouts please! The proper way to handle this problem is to simply
teach the oom reaper to handle mlocked areas.
-- 
Michal Hocko
SUSE Labs