Re: [patch -mm 8/9 v2] oom: avoid oom killer for lowmem allocations

[David Rientjes <rientjes@google.com>]

On Tue, 23 Feb 2010, Balbir Singh wrote:

> > out_of_memory() doesn't return a value to specify whether the page 
> > allocator should retry the allocation or just return NULL, all that policy 
> > is kept in mm/page_alloc.c.  For highzone_idx < ZONE_NORMAL, we want to 
> > fail the allocation when !(gfp_mask & __GFP_NOFAIL) and call the oom 
> > killer when it's __GFP_NOFAIL.
> > ---
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1696,6 +1696,9 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
> >  		/* The OOM killer will not help higher order allocs */
> >  		if (order > PAGE_ALLOC_COSTLY_ORDER)
> >  			goto out;
> > +		/* The OOM killer does not needlessly kill tasks for lowmem */
> > +		if (high_zoneidx < ZONE_NORMAL)
> > +			goto out;
> 
> I am not sure if this is a good idea, ZONE_DMA could have a lot of
> memory on some architectures. IIUC, we return NULL for allocations
> from ZONE_DMA? What is the reason for the heuristic?
> 

As the patch description says, we would otherwise needlessly kill tasks 
that may not be consuming any lowmem since there is no way to determine 
its usage and typically the memory in lowmem will either be reclaimable 
(or migratable via memory compaction) if it is not pinned for I/O in which 
case we shouldn't kill for it anyway at this point.

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