linux-kernel - Re: [PATCH 2/6] mm, compaction: simplify handling restart position in free pages scanner

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Message-ID: <20150616053807.GB12641@js1304-P5Q-DELUXE>
Date:	Tue, 16 Jun 2015 14:38:07 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Minchan Kim <minchan@...nel.org>,
	Mel Gorman <mgorman@...e.de>,
	Michal Nazarewicz <mina86@...a86.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH 2/6] mm, compaction: simplify handling restart position
 in free pages scanner

On Wed, Jun 10, 2015 at 11:32:30AM +0200, Vlastimil Babka wrote:
> Handling the position where compaction free scanner should restart (stored in
> cc->free_pfn) got more complex with commit e14c720efdd7 ("mm, compaction:
> remember position within pageblock in free pages scanner"). Currently the
> position is updated in each loop iteration of isolate_freepages(), although it
> should be enough to update it only when breaking from the loop. There's also
> an extra check outside the loop updates the position in case we have met the
> migration scanner.
> 
> This can be simplified if we move the test for having isolated enough from the
> for loop header next to the test for contention, and determining the restart
> position only in these cases. We can reuse the isolate_start_pfn variable for
> this instead of setting cc->free_pfn directly. Outside the loop, we can simply
> set cc->free_pfn to current value of isolate_start_pfn without any extra check.
> 
> Also add a VM_BUG_ON to catch possible mistake in the future, in case we later
> add a new condition that terminates isolate_freepages_block() prematurely
> without also considering the condition in isolate_freepages().
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Acked-by: Joonsoo Kim <iamjoonsoo.kim@....com>

Thanks.
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