linux-kernel - Re: [patch] mm, compaction: abort free scanner if split fails

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Date:	Wed, 22 Jun 2016 14:59:02 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Rientjes <rientjes@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Minchan Kim <minchan@...nel.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, stable@...r.kernel.org
Subject: Re: [patch] mm, compaction: abort free scanner if split fails

On Wed, 22 Jun 2016 14:56:17 -0700 Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Tue, 21 Jun 2016 18:22:49 -0700 (PDT) David Rientjes <rientjes@...gle.com> wrote:
> 
> > If the memory compaction free scanner cannot successfully split a free
> > page (only possible due to per-zone low watermark), terminate the free 
> > scanner rather than continuing to scan memory needlessly.  If the 
> > watermark is insufficient for a free page of order <= cc->order, then 
> > terminate the scanner since all future splits will also likely fail.
> > 
> > This prevents the compaction freeing scanner from scanning all memory on 
> > very large zones (very noticeable for zones > 128GB, for instance) when 
> > all splits will likely fail while holding zone->lock.
> > 
> 
> This collides pretty heavily with Joonsoo's "mm/compaction: split
> freepages without holding the zone lock".
> 
> I ended up with this, in isolate_freepages_block():
> 
> 		/* Found a free page, will break it into order-0 pages */
> 		order = page_order(page);
> 		isolated = __isolate_free_page(page, page_order(page));
> 		set_page_private(page, order);
> 
> 		total_isolated += isolated;
> 		cc->nr_freepages += isolated;
> 		list_add_tail(&page->lru, freelist);
> 
> 		if (!strict && cc->nr_migratepages <= cc->nr_freepages) {
> 			blockpfn += isolated;
> 			break;
> 		}
> 		/* Advance to the end of split page */
> 		blockpfn += isolated - 1;
> 		cursor += isolated - 1;
> 		continue;
> 
> isolate_fail:
> 

And
mm-compaction-split-freepages-without-holding-the-zone-lock-fix.patch
churns things around some more.  Now this:


		/* Found a free page, will break it into order-0 pages */
		order = page_order(page);
		isolated = __isolate_free_page(page, order);
		set_page_private(page, order);
		total_isolated += isolated;
		list_add_tail(&page->lru, freelist);
		cc->nr_freepages += isolated;
		if (!strict && cc->nr_migratepages <= cc->nr_freepages) {
			blockpfn += isolated;
			break;
		}
		/* Advance to the end of split page */
		blockpfn += isolated - 1;
		cursor += isolated - 1;
		continue;

isolate_fail:

and things are looking a bit better...