linux-kernel - Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon

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Date:	Mon, 12 Apr 2010 14:40:22 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Borislav Petkov <bp@...en8.de>,
	Johannes Weiner <hannes@...xchg.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	sgunderson@...foot.com
Subject: Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon_vmas
 of a mergeable VMA

On 04/12/2010 12:26 PM, Linus Torvalds wrote:

> But there is a _much_ more subtle case that involved swapping.
>
> So guys, here's my fairly simple theory on what happens:

That bug looks entirely possible.  Given that Borislav
has heavy swapping going on, it is quite possible that
this is the bug he has been triggering.

> The patch below is my largely mindless try at fixing this. It's untested.
> I'm not entirely sure that it actually works. But it makes some amount of
> conceptual sense. No?

The patch would help avoid the bug you described.

It does have the drawback of moving all the pages of
child processes back into the anon_vma of the parent
process after swapin, even if they are privately owned
pages by the child process.

I am guessing it may need a check to see whether the
page and swap slot are exclusively owned by the current
process.

Page or swap slot shared?      => oldest anon_vma
Page and swap slot exclusive?  => newest anon_vma

I suspect the easiest way to achieve this would be to
pass a flag in from do_swap_page, where we already
check this, a few lines above calling page_add_anon_rmap:

         if ((flags & FAULT_FLAG_WRITE) && reuse_swap_page(page)) {
                 pte = maybe_mkwrite(pte_mkdirty(pte), vma);
                 flags &= ~FAULT_FLAG_WRITE;
         }



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