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

[Michal Hocko]

On Thu 15-06-17 14:26:26, David Rientjes wrote:
> 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.

OK, could you play with the patch/idea suggested in
http://lkml.kernel.org/r/20170615122031.GL1486@xxxxxxxxxxxxxx?

-- 
Michal Hocko
SUSE Labs