linux-kernel - Re: [PATCH 02 of 66] mm, migration: Fix race between shift_arg_pages and rmap_walk by guaranteeing rmap_walk finds PTEs created within the temporary stack

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Message-ID: <AANLkTin76ajKd5Dc1fYwibqmL9yd6MtEvCWxCYYwnqOU@mail.gmail.com>
Date:	Fri, 19 Nov 2010 09:54:27 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Mel Gorman <mel@....ul.ie>, linux-mm@...ck.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,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.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 02 of 66] mm, migration: Fix race between shift_arg_pages
 and rmap_walk by guaranteeing rmap_walk finds PTEs created within the
 temporary stack

On Fri, Nov 19, 2010 at 9:38 AM, Andrea Arcangeli <aarcange@...hat.com> wrote:
>
> We decided the kmalloc was ok, but Linus didn't like this approach. I
> kept it in my tree because I didn't want to remember when/if to add the
> special check in the accurate rmap walks. I find it simpler if all
> rmap walks are accurate by default.

Why isn't the existing cheap solution sufficient?

My opinion is still that we shouldn't add the expense to the common
case, and it's the uncommon case (migration) that should just handle
it.

                Linus
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