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Date: Tue, 23 Mar 2010 20:06:30 +0100
From: Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH 00 of 34] Transparent Hugepage support #14
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To: Christoph Lameter <cl@linux-foundation.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>, linux-mm@kvack.org, Marcelo Tosatti <mtosatti@redhat.com>, Adam Litke <agl@us.ibm.com>, Avi Kivity <avi@redhat.com>, Izik Eidus <ieidus@redhat.com>, Hugh Dickins <hugh.dickins@tiscali.co.uk>, Nick Piggin <npiggin@suse.de>, Rik van Riel <riel@redhat.com>, Mel Gorman <mel@csn.ul.ie>, Dave Hansen <dave@linux.vnet.ibm.com>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Ingo Molnar <mingo@elte.hu>, Mike Travis <travis@sgi.com>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, Chris Wright <chrisw@sous-sol.org>, bpicco@redhat.com, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, Balbir Singh <balbir@linux.vnet.ibm.com>, Arnd Bergmann <arnd@arndb.de>, "Michael S. Tsirkin" <mst@redhat.com>, Peter Zijlstra <peterz@infradead.org>
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On Tue, Mar 23, 2010 at 12:11:01PM -0500, Christoph Lameter wrote:
> No it does not happen on the fly. It happens only when you can account for
> all references that current exist for the page to be migrated. Then you
> change all the references at once. The mechanism ensures that there is no
> one else operating on the page.

And if one of the references is taken by GUP migration fails and you
can try later or wait for I/O. This is not the case for
split_huge_page, we don't fail or wait there.

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