Re: [patch 09/12] mm: page_alloc: private memory reserves for OOM-killing allocations

[Johannes Weiner]

On Tue, Apr 14, 2015 at 06:49:40PM +0200, Michal Hocko wrote:
> On Wed 25-03-15 02:17:13, Johannes Weiner wrote:
> > @@ -5747,17 +5765,18 @@ static void __setup_per_zone_wmarks(void)
> >  
> >  			min_pages = zone->managed_pages / 1024;
> >  			min_pages = clamp(min_pages, SWAP_CLUSTER_MAX, 128UL);
> > -			zone->watermark[WMARK_MIN] = min_pages;
> > +			zone->watermark[WMARK_OOM] = min_pages;
> >  		} else {
> >  			/*
> >  			 * If it's a lowmem zone, reserve a number of pages
> >  			 * proportionate to the zone's size.
> >  			 */
> > -			zone->watermark[WMARK_MIN] = tmp;
> > +			zone->watermark[WMARK_OOM] = tmp;
> >  		}
> >  
> > -		zone->watermark[WMARK_LOW]  = min_wmark_pages(zone) + (tmp >> 2);
> > -		zone->watermark[WMARK_HIGH] = min_wmark_pages(zone) + (tmp >> 1);
> > +		zone->watermark[WMARK_MIN]  = oom_wmark_pages(zone) + (tmp >> 3);
> > +		zone->watermark[WMARK_LOW]  = oom_wmark_pages(zone) + (tmp >> 2);
> > +		zone->watermark[WMARK_HIGH] = oom_wmark_pages(zone) + (tmp >> 1);
> 
> This will basically elevate the min watermark, right? And that might lead
> to subtle performance differences even when OOM killer is not invoked
> because the direct reclaim will start sooner.

It will move the min watermark a bit closer to the kswapd watermarks,
so I guess the risk of entering direct reclaim when kswapd won't wake
up fast enough before concurrent allocator slowpaths deplete the zone
from low to min is marginally increased.  That seems like a farfetched
worry, especially given that waking up a sleeping kswapd is not a high
frequency event in the first place.

> Shouldn't we rather give WMARK_OOM half of WMARK_MIN instead?

I guess conceptually that would work as well, since an OOM killing
task is technically reclaiming memory and this reserve is meant to
help reclaiming tasks make forward progress.

That being said, the separate OOM reserve was designed for when the
allocation can actually fail: deplete our own little reserve before
returning failure.  But it looks like neither the low-order nor the
GFP_NOFS deadlock fixes got any traction, and so right now all OOM
killing allocations still have the potential to deadlock.  Is there a
reason we shouldn't just let them do an ALLOC_NO_WATERMARK allocation
after the OOM victim exited (or timed out)?

Otherwise, I'll just do that in the next iteration.
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