linux-kernel - Re: [PATCH 5/5] mm, vmscan: Account for skipped pages as a partial scan

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Message-ID: <20160725095251.GN10438@techsingularity.net>
Date:	Mon, 25 Jul 2016 10:52:51 +0100
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Vlastimil Babka <vbabka@...e.cz>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5] mm, vmscan: Account for skipped pages as a partial
 scan

On Mon, Jul 25, 2016 at 05:39:13PM +0900, Minchan Kim wrote:
> > @@ -1465,14 +1471,24 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
> >  	 */
> >  	if (!list_empty(&pages_skipped)) {
> >  		int zid;
> > +		unsigned long total_skipped = 0;
> >  
> > -		list_splice(&pages_skipped, src);
> >  		for (zid = 0; zid < MAX_NR_ZONES; zid++) {
> >  			if (!nr_skipped[zid])
> >  				continue;
> >  
> >  			__count_zid_vm_events(PGSCAN_SKIP, zid, nr_skipped[zid]);
> > +			total_skipped += nr_skipped[zid];
> >  		}
> > +
> > +		/*
> > +		 * Account skipped pages as a partial scan as the pgdat may be
> > +		 * close to unreclaimable. If the LRU list is empty, account
> > +		 * skipped pages as a full scan.
> > +		 */
> 
> node-lru made OOM detection lengthy because a freeing of any zone will
> reset NR_PAGES_SCANNED easily so that it's hard to meet a situation
> pgdat_reclaimable returns *false*.
> 

Your patch should go a long way to addressing that as it checks the zone
counters first before conducting the scan. Remember as well that the longer
detection of OOM only applies to zone-constrained allocations and there
is always the possibility that highmem shrinking of pages frees lowmem
memory if buffers are used.

-- 
Mel Gorman
SUSE Labs