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

[David Rientjes <rientjes@google.com>]

On Thu, 15 Jun 2017, Michal Hocko 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.
> 

I have not seen any issues where __mmput() stalls and exit_mmap() fails to 
free its mapped memory once mm->mm_users has dropped to 0.

> > __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?
> 

Yes, quite a bit in testing.

One oom kill shows the system to be oom:

[22999.488705] Node 0 Normal free:90484kB min:90500kB ...
[22999.488711] Node 1 Normal free:91536kB min:91948kB ...

followed up by one or more unnecessary oom kills showing the oom killer 
racing with memory freeing of the victim:

[22999.510329] Node 0 Normal free:229588kB min:90500kB ...
[22999.510334] Node 1 Normal free:600036kB min:91948kB ...

The patch is absolutely required for us to prevent continuous oom killing 
of processes after a single process has been oom killed and its memory is 
in the process of being freed.

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