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

Tetsuo Handa
2018-04-18T04:47:56Z

David Rientjes wrote:
> 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().

"we have moved"? No, not yet. Your patch is about to move it.

My question is: is it guaranteed that munlock_vma_pages_all()/unmap_vmas()/free_pgtables()
by exit_mmap() are never blocked for memory allocation. Note that exit_mmap() tries to unmap
all pages while the OOM reaper tries to unmap only safe pages. If there is possibility that
munlock_vma_pages_all()/unmap_vmas()/free_pgtables() by exit_mmap() are blocked for memory
allocation, your patch will introduce an OOM livelock.

> 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.
>