linux-kernel - Re: [PATCH 1/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information

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Message-ID: <4BE2E3F9.9090708@redhat.com>
Date:	Thu, 06 May 2010 11:44:57 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mel@....ul.ie>
CC:	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>,
	Christoph Lameter <cl@...ux.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing
 the wrong VMA information

On 05/06/2010 11:33 AM, Mel Gorman wrote:

> @@ -1368,16 +1424,25 @@ static int rmap_walk_anon(struct page *page, int (*rmap_one)(struct page *,
>   	 * are holding mmap_sem. Users without mmap_sem are required to
>   	 * take a reference count to prevent the anon_vma disappearing
>   	 */
> -	anon_vma = page_anon_vma(page);
> +	anon_vma = page_anon_vma_lock_root(page);
>   	if (!anon_vma)
>   		return ret;
> -	spin_lock(&anon_vma->lock);
>   	list_for_each_entry(avc,&anon_vma->head, same_anon_vma) {

One conceptual problem here.  By taking the oldest anon_vma,
instead of the anon_vma of the page, we may end up searching
way too many processes.

Eg. if the page is the page of a child process in a forking
server workload, the above code will end up searching the
parent and all of the siblings - even for a private page, in
the child process's private anon_vma.

For an Apache or Oracle system with 1000 clients (and child
processes), that could be quite a drag - searching 1000 times
as many processes as we should.
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