linux-kernel - Re: [PATCH 1/5] mm, compaction: pass classzone_idx and alloc

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Date:	Wed, 29 Oct 2014 14:51:11 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
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 1/5] mm, compaction: pass classzone_idx and alloc_flags
 to watermark checking

On 10/28/2014 08:16 AM, Joonsoo Kim wrote:> On Mon, Oct 27, 2014 at 
10:11:31AM +0100, Vlastimil Babka wrote:
 >> On 10/27/2014 07:46 AM, Joonsoo Kim wrote:
 >>> On Tue, Oct 07, 2014 at 05:33:35PM +0200, Vlastimil Babka wrote:
 >>>
 >>> Hello,
 >>>
 >>> compaction_suitable() has one more zone_watermark_ok(). Why is it
 >>> unchanged?
 >>
 >> Hi,
 >>
 >> it's a check whether there are enough free pages to perform compaction,
 >> which means enough migration targets and temporary copies during
 >> migration. These allocations are not affected by the flags of the
 >> process that makes the high-order allocation.
 >
 > Hmm...
 >
 > To check whether enough free page is there or not needs zone index and
 > alloc flag. What we need to ignore is just order information, IMO.
 > If there is not enough free page in that zone, compaction progress
 > doesn't have any meaning. It will fail due to shortage of free page
 > after successful compaction.

I thought that the second check in compaction_suitable() makes sure of 
this, but now I see it's in fact not.
But i'm not sure if we should just put the flags in the first check, as 
IMHO the flags should only affect the final high-order allocation, not 
also the temporary pages needed for migration?

BTW now I'm not even sure that the 2UL << order part makes sense 
anymore. The number of pages migrated at once is always restricted by 
COMPACT_CLUSTER_MAX, so why would we need more than that to cover migration?
Also the order of checks seems wrong. It should return COMPACT_PARTIAL 
"If the allocation would succeed without compaction" but that only can 
happen after passing the check if the zone has the extra 1UL << order 
for migration. Do you agree?

 > I guess that __isolate_free_page() is also good candidate to need this
 > information in order to prevent compaction from isolating too many
 > freepage in low memory condition.

I don't see how it would help here. It's temporary allocations for page 
migration. How would passing classzone_idx and alloc_flags prevent 
isolating too many?

 > Thanks.
 >

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