Re: [patch v2] mm, oom: fix concurrent munlock and oom reaper unmap

[David Rientjes <rientjes@google.com>]

On Wed, 18 Apr 2018, Tetsuo Handa wrote:

> > Fix this by reusing MMF_UNSTABLE to specify that an mm should not be
> > reaped.  This prevents the concurrent munlock_vma_pages_range() and
> > unmap_page_range().  The oom reaper will simply not operate on an mm that
> > has the bit set and leave the unmapping to exit_mmap().
> 
> This change assumes that munlock_vma_pages_all()/unmap_vmas()/free_pgtables()
> are never blocked for memory allocation. Is that guaranteed? For example,
> i_mmap_lock_write() from unmap_single_vma() from unmap_vmas() is never blocked
> for memory allocation? Commit 97b1255cb27c551d ("mm,oom_reaper: check for
> MMF_OOM_SKIP before complaining") was waiting for i_mmap_lock_write() from
> unlink_file_vma() from free_pgtables(). Is it really guaranteed that somebody
> else who is holding that lock is never waiting for memory allocation?
> 

Commit 97b1255cb27c is referencing MMF_OOM_SKIP already being set by 
exit_mmap().  The only thing this patch changes is where that is done: 
before or after free_pgtables().  We can certainly move it to before 
free_pgtables() at the risk of subsequent (and eventually unnecessary) oom 
kills.  It's not exactly the point of this patch.

I have thousands of real-world examples where additional processes were 
oom killed while the original victim was in free_pgtables().  That's why 
we've moved the MMF_OOM_SKIP to after free_pgtables().  I'm not sure how 
likely your scenario is in the real world, but if it poses a problem then 
I believe it should be fixed by eventually deferring previous victims as a 
change to oom_evaluate_task(), not exit_mmap().  If you'd like me to fix 
that, please send along your test case that triggers it and I will send a 
patch.