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Date:	Tue, 7 Oct 2014 14:10:26 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	qemu-devel@...gnu.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-api@...r.kernel.org, Robert Love <rlove@...gle.com>,
	Dave Hansen <dave@...1.net>, Jan Kara <jack@...e.cz>,
	Neil Brown <neilb@...e.de>,
	Stefan Hajnoczi <stefanha@...il.com>,
	Andrew Jones <drjones@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Michel Lespinasse <walken@...gle.com>,
	Taras Glek <tglek@...illa.com>,
	Juan Quintela <quintela@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Isaku Yamahata <yamahata@...inux.co.jp>,
	Mel Gorman <mgorman@...e.de>,
	Sasha Levin <sasha.levin@...cle.com>,
	Android Kernel Team <kernel-team@...roid.com>,
	"\\\"Dr. David Alan Gilbert\\\"" <dgilbert@...hat.com>,
	"Huangpeng (Peter)" <peter.huangpeng@...wei.com>,
	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Christopher Covington <cov@...eaurora.org>,
	Anthony Liguori <anthony@...emonkey.ws>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Keith Packard <keithp@...thp.com>,
	Wenchao Xia <wenchaoqemu@...il.com>,
	Andy Lutomirski <luto@...capital.net>,
	Minchan Kim <minchan@...nel.org>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Mike Hommey <mh@...ndium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Feiner <pfeiner@...gle.com>
Subject: Re: [Qemu-devel] [PATCH 10/17] mm: rmap preparation for
 remap_anon_pages

On Fri, Oct 03, 2014 at 07:08:00PM +0200, Andrea Arcangeli wrote:
> There's one constraint enforced to allow this simplification: the
> source pages passed to remap_anon_pages must be mapped only in one
> vma, but this is not a limitation when used to handle userland page
> faults with MADV_USERFAULT. The source addresses passed to
> remap_anon_pages should be set as VM_DONTCOPY with MADV_DONTFORK to
> avoid any risk of the mapcount of the pages increasing, if fork runs
> in parallel in another thread, before or while remap_anon_pages runs.

Have you considered triggering COW instead of adding limitation on
pages' mapcount? The limitation looks artificial from interface POV.

-- 
 Kirill A. Shutemov
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