linux-kernel - Re: [PATCH 2/2] mm,migration: Avoid race between shift_arg_pages() and rmap_walk() during migration by not migrating temporary stacks

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Message-ID: <20100511162909.GI10631@random.random>
Date:	Tue, 11 May 2010 18:29:09 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Christoph Lameter <cl@...ux.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 2/2] mm,migration: Avoid race between shift_arg_pages()
 and rmap_walk() during migration by not migrating temporary stacks

On Tue, May 11, 2010 at 05:15:16PM +0100, Mel Gorman wrote:
> On Tue, May 11, 2010 at 10:56:00AM -0500, Christoph Lameter wrote:
> > On Tue, 11 May 2010, Mel Gorman wrote:
> > 
> > > Or just identify the temporary stack from the migration side instead of
> > > adding to the cost of exec?
> > 
> > Adding one off checks to a generic mechanism isnt really clean
> > programming. Using the provided means of disabling a generic mechanism is.
> > 
> 
> Andrea's solution is likely lighter than yours as it is one kmalloc and
> an insertion into the VM as opposed to a page table walk with reference
> counting. Better yet, it exists as a patch that has been tested and it
> fits in with the generic mechanism by guaranteeing that rmap_walk finds
> all the migration PTEs during the second walk.
> 
> The problem remains the same - that class of solution increases the cost of
> a common operation (exec) to keep a much less operation (migration) happy.

page table walk adding reference counting is still a one off check,
the generic rmap_walk mechanism won't care about the reference
counting, still only migrate checks the page count... so it doesn't
move the needle in clean programming terms.
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