Re: [patch] mm, oom: prevent additional oom kills before memory is freed

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Michal Hocko wrote:
> On Wed 14-06-17 16:43:03, David Rientjes wrote:
> > If mm->mm_users is not incremented because it is already zero by the oom
> > reaper, meaning the final refcount has been dropped, do not set
> > MMF_OOM_SKIP prematurely.
> > 
> > __mmput() may not have had a chance to do exit_mmap() yet, so memory from
> > a previous oom victim is still mapped.
> 
> true and do we have a _guarantee_ it will do it? E.g. can somebody block
> exit_aio from completing? Or can somebody hold mmap_sem and thus block
> ksm_exit resp. khugepaged_exit from completing? The reason why I was
> conservative and set such a mm as MMF_OOM_SKIP was because I couldn't
> give a definitive answer to those questions. And we really _want_ to
> have a guarantee of a forward progress here. Killing an additional
> proecess is a price to pay and if that doesn't trigger normall it sounds
> like a reasonable compromise to me.

Right. If you want this patch, __oom_reap_task_mm() must not return true without
setting MMF_OOM_SKIP (in other words, return false if __oom_reap_task_mm()
does not set MMF_OOM_SKIP). The most important role of the OOM reaper is to
guarantee that the OOM killer is re-enabled within finite time, for __mmput()
cannot guarantee that MMF_OOM_SKIP is set within finite time.

> 
> > __mput() naturally requires no
> > references on mm->mm_users to do exit_mmap().
> > 
> > Without this, several processes can be oom killed unnecessarily and the
> > oom log can show an abundance of memory available if exit_mmap() is in
> > progress at the time the process is skipped.
> 
> Have you seen this happening in the real life?
> 
> > Signed-off-by: David Rientjes <rientjes@google.com>