linux-kernel - Re: [PATCH -mm v2] mm: have order > 0 compaction start off where it left

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Date:	Wed, 4 Jul 2012 10:57:58 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Minchan Kim <minchan@...nel.org>, Rik van Riel <riel@...hat.com>,
	Sasha Levin <levinsasha928@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, jaschut@...dia.gov,
	kamezawa.hiroyu@...fujitsu.com, Dave Jones <davej@...hat.com>
Subject: Re: [PATCH -mm v2] mm: have order > 0 compaction start off where it
 left

On Wed, Jul 04, 2012 at 12:42:19AM -0700, Andrew Morton wrote:
> On Wed, 04 Jul 2012 11:34:09 +0900 Minchan Kim <minchan@...nel.org> wrote:
> 
> > > The rest of this patch takes care to ensure that
> > > ->compact_cached_free_pfn is aligned to pageblock_nr_pages.  But it now
> > > appears that this particular site will violate that.
> > > 
> > > What's up?  Do we need to fix this site, or do we remove all that
> > > make-compact_cached_free_pfn-aligned code?
> > 
> > 
> > I vote removing the warning because it doesn't related to Rik's incremental compaction.
> > Let's see. 
> > 
> > high_pfn = min(low_pfn, pfn) = cc->migrate_pfn + pageblock_nr_pages.
> > In here, cc->migrate_pfn isn't necessarily pageblock aligined.
> > So if we don't consider compact_cached_free_pfn, it can hit.
> > 
> > static void isolate_freepages()
> > {
> > 	high_pfn = min(low_pfn, pfn) = cc->migrate_pfn + pageblock_nr_pages;
> > 	for (..) {
> > 		...
> > 		 WARN_ON_ONCE(high_pfn & (pageblock_nr_pages - 1));
> > 		
> > 	}
> > }
> 
> Please, look at the patch.  In numerous places it is aligning
> compact_cached_free_pfn to a multiple of pageblock_nr_pages.  But in
> one place it doesn't do that.  So are all those alignment operations
> necessary?
> 

I don't think the alignments are necessary. The main importance is that
it does not leave the zone.

-- 
Mel Gorman
SUSE Labs
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