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

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Message-Id: <20120704004219.47d0508d.akpm@linux-foundation.org>
Date:	Wed, 4 Jul 2012 00:42:19 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Mel Gorman <mel@....ul.ie>, 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, 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?

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