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Date:	Fri, 25 Jul 2014 13:36:46 +0100
From:	Mel Gorman <>
To:	Vlastimil Babka <>
Cc:, Andrew Morton <>,
	David Rientjes <>,, Joonsoo Kim <>,
	Michal Nazarewicz <>,
	Naoya Horiguchi <>,
	Christoph Lameter <>,
	Rik van Riel <>,
	Minchan Kim <>,
	Zhang Yanfei <>
Subject: Re: [PATCH V4 11/15] mm, compaction: skip buddy pages by their order
 in the migrate scanner

On Wed, Jul 16, 2014 at 03:48:19PM +0200, Vlastimil Babka wrote:
> The migration scanner skips PageBuddy pages, but does not consider their order
> as checking page_order() is generally unsafe without holding the zone->lock,
> and acquiring the lock just for the check wouldn't be a good tradeoff.
> Still, this could avoid some iterations over the rest of the buddy page, and
> if we are careful, the race window between PageBuddy() check and page_order()
> is small, and the worst thing that can happen is that we skip too much and miss
> some isolation candidates. This is not that bad, as compaction can already fail
> for many other reasons like parallel allocations, and those have much larger
> race window.
> This patch therefore makes the migration scanner obtain the buddy page order
> and use it to skip the whole buddy page, if the order appears to be in the
> valid range.
> It's important that the page_order() is read only once, so that the value used
> in the checks and in the pfn calculation is the same. But in theory the
> compiler can replace the local variable by multiple inlines of page_order().
> Therefore, the patch introduces page_order_unsafe() that uses ACCESS_ONCE to
> prevent this.
> Testing with stress-highalloc from mmtests shows a 15% reduction in number of
> pages scanned by migration scanner. The reduction is >60% with __GFP_NO_KSWAPD
> allocations, along with success rates better by few percent.
> This change is also a prerequisite for a later patch which is detecting when
> a cc->order block of pages contains non-buddy pages that cannot be isolated,
> and the scanner should thus skip to the next block immediately.
> Signed-off-by: Vlastimil Babka <>
> Reviewed-by: Zhang Yanfei <>
> Acked-by: Minchan Kim <>

Acked-by: Mel Gorman <>

Mel Gorman
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