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Date: Wed, 22 Jun 2016 14:56:17 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: David Rientjes <rientjes@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>, Minchan Kim <minchan@kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Hugh Dickins <hughd@google.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, stable@vger.kernel.org
Subject: Re: [patch] mm, compaction: abort free scanner if split fails
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On Tue, 21 Jun 2016 18:22:49 -0700 (PDT) David Rientjes <rientjes@google.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: