Re: [patch 00/12] mm: page_alloc: improve OOM mechanism and policy

[Michal Hocko]

On Sat 11-04-15 16:29:26, Tetsuo Handa wrote:
> Johannes Weiner wrote:
> > The argument here was always that NOFS allocations are very limited in
> > their reclaim powers and will trigger OOM prematurely.  However, the
> > way we limit dirty memory these days forces most cache to be clean at
> > all times, and direct reclaim in general hasn't been allowed to issue
> > page writeback for quite some time.  So these days, NOFS reclaim isn't
> > really weaker than regular direct reclaim.  The only exception is that
> > it might block writeback, so we'd go OOM if the only reclaimables left
> > were dirty pages against that filesystem.  That should be acceptable.
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 47981c5e54c3..fe3cb2b0b85b 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2367,16 +2367,6 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order, int alloc_flags,
> >  		/* The OOM killer does not needlessly kill tasks for lowmem */
> >  		if (ac->high_zoneidx < ZONE_NORMAL)
> >  			goto out;
> > -		/* The OOM killer does not compensate for IO-less reclaim */
> > -		if (!(gfp_mask & __GFP_FS)) {
> > -			/*
> > -			 * XXX: Page reclaim didn't yield anything,
> > -			 * and the OOM killer can't be invoked, but
> > -			 * keep looping as per tradition.
> > -			 */
> > -			*did_some_progress = 1;
> > -			goto out;
> > -		}
> >  		if (pm_suspended_storage())
> >  			goto out;
> >  		/* The OOM killer may not free memory on a specific node */
> > 
> 
> I think this change will allow calling out_of_memory() which results in
> "oom_kill_process() is trivially called via pagefault_out_of_memory()"
> problem described in https://lkml.org/lkml/2015/3/18/219 .
> 
> I myself think that we should trigger OOM killer for !__GFP_FS allocation
> in order to make forward progress in case the OOM victim is blocked.
> So, my question about this change is whether we can accept involving OOM
> killer from page fault, no matter how trivially OOM killer will kill some
> process?

We trigger OOM killer from the page fault path for ages. In fact the memcg
will trigger memcg OOM killer _only_ from the page fault path because
this context is safe as we do not sit on any locks at the time.
-- 
Michal Hocko
SUSE Labs
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