linux-kernel - Re: [PATCH 55 of 66] select CONFIG_COMPACTION if TRANSPARENT

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Date:	Thu, 9 Dec 2010 20:04:07 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm@...ck.org, Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Rik van Riel <riel@...hat.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Ingo Molnar <mingo@...e.hu>, Mike Travis <travis@....com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Chris Wright <chrisw@...s-sol.org>, bpicco@...hat.com,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Chris Mason <chris.mason@...cle.com>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 55 of 66] select CONFIG_COMPACTION if
 TRANSPARENT_HUGEPAGE enabled

On Thu, Nov 18, 2010 at 04:22:45PM +0000, Mel Gorman wrote:
> Just to confirm - by hang, you mean grinds to a slow pace as opposed to
> coming to a complete stop and having to restart?

Hmm it's like if you're gigabytes in swap and apps hangs for a while
and system is not really usable and it swaps for most new memory
allocations despite there's plenty of memory free, but it's not a
deadlock of course.

BTW, alternatively I could:

 unsigned long transparent_hugepage_flags __read_mostly =
        (1<<TRANSPARENT_HUGEPAGE_FLAG)|
+#ifdef CONFIG_COMPACTION
+       (1<<TRANSPARENT_HUGEPAGE_DEFRAG_FLAG)|
+#endif
        (1<<TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG);

That would adds GFP_ATOMIC to THP allocation if compaction wasn't
selected, but I think having compaction enabled diminish the risk of
misconfigured kernels leading to unexpected measurements and behavior,
so I feel much safer to keep the select COMPACTION in this patch.

> Acked-by: Mel Gorman <mel@....ul.ie>

Added.

Thanks,
Andrea
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