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: <20100511091022.347785db.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Tue, 11 May 2010 09:10:22 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Christoph Lameter <cl@...ux.com>, Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Minchan Kim <minchan.kim@...il.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 Mon, 10 May 2010 21:05:59 +0200
Andrea Arcangeli <aarcange@...hat.com> wrote:

> On Mon, May 10, 2010 at 12:41:07PM -0500, Christoph Lameter wrote:
> > A simple way to disallow migration of pages is to increment the refcount
> > of a page.
> 
> Ok for migrate but it won't prevent to crash in split_huge_page rmap
> walk, nor the PG_lock. Why for a rmap bug have a migrate specific fix?
> The fix that makes execve the only special place to handle in every
> rmap walk, is at least more maintainable than a fix that makes one of
> the rmap walk users special and won't fix the others, as there will be
> more than just 1 user that requires this. My fix didn't make execve
> special and it didn't require execve knowledge into the every rmap
> walk like migrate (split_huge_page etc...) but as long as the kernel
> doesn't crash I'm fine ;).
> 

At first, I like step-by-step approach even if it makes our cost double
because it's easy to understand and makes chasing change-log easy.

Ok, your split_huge_page() has some problems with current rmap+migration.
But I don't like a patch for never-happen-now bug in change-log.

I believe it can be fixed by the same approach for execs.
Renaming 
#define VM_STACK_INCOMPLETE_SETUP
to be
#define VM_TEMPORARY_INCONSITENT_RMAP
in _your_ patch series and add some check in rmap_walk() seems enough.

Of course, I may misunderstand your problem. Could you show your patch
which meets the problem with rmap+migration ?

Thanks,
-Kame




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