Re: [PATCH 4/3] mm, oom: drop the last allocation attempt before out_of_memory

[Michal Hocko]

On Thu 28-01-16 15:19:08, David Rientjes wrote:
> On Thu, 28 Jan 2016, Johannes Weiner wrote:
> 
> > The check has to happen while holding the OOM lock, otherwise we'll
> > end up killing much more than necessary when there are many racing
> > allocations.
> > 
> 
> Right, we need to try with ALLOC_WMARK_HIGH after oom_lock has been 
> acquired.
> 
> The situation is still somewhat fragile, however, but I think it's 
> tangential to this patch series.  If the ALLOC_WMARK_HIGH allocation fails 
> because an oom victim hasn't freed its memory yet, and then the TIF_MEMDIE 
> thread isn't visible during the oom killer's tasklist scan because it has 
> exited, we still end up killing more than we should.  The likelihood of 
> this happening grows with the length of the tasklist.

Yes exactly the point I made in the original thread which brought the
question about ALLOC_WMARK_HIGH originally. The race window after the
last attempt is much larger than between the last wmark check and the
attempt.

> Perhaps we should try testing watermarks after a victim has been selected 
> and immediately before killing?  (Aside: we actually carry an internal 
> patch to test mem_cgroup_margin() in the memcg oom path after selecting a 
> victim because we have been hit with this before in the memcg path.)
> 
> I would think that retrying with ALLOC_WMARK_HIGH would be enough memory 
> to deem that we aren't going to immediately reenter an oom condition so 
> the deferred killing is a waste of time.
> 
> The downside is how sloppy this would be because it's blurring the line 
> between oom killer and page allocator.  We'd need the oom killer to return 
> the selected victim to the page allocator, try the allocation, and then 
> call oom_kill_process() if necessary.

Yes the layer violation is definitely not nice.

-- 
Michal Hocko
SUSE Labs