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

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Date:	Mon, 26 Apr 2010 20:30:41 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mel@....ul.ie>
CC:	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>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing
 the wrong VMA information

On 04/26/2010 06:37 PM, Mel Gorman wrote:

> diff --git a/mm/rmap.c b/mm/rmap.c
> index 85f203e..bc313a6 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1368,15 +1368,31 @@ 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
>   	 */
> +retry:
>   	anon_vma = page_anon_vma(page);
>   	if (!anon_vma)
>   		return ret;
>   	spin_lock(&anon_vma->lock);
>   	list_for_each_entry(avc,&anon_vma->head, same_anon_vma) {
>   		struct vm_area_struct *vma = avc->vma;
> -		unsigned long address = vma_address(page, vma);
> -		if (address == -EFAULT)
> -			continue;
> +		unsigned long address;
> +
> +		/*
> +		 * Guard against deadlocks by not spinning against
> +		 * vma->anon_vma->lock. If contention is found, release our
> +		 * lock and try again until VMA list can be traversed without
> +		 * contention.
> +		 */
> +		if (anon_vma != vma->anon_vma) {
> +			if (!spin_trylock(&vma->anon_vma->lock)) {
> +				spin_unlock(&anon_vma->lock);
> +				goto retry;
> +			}
> +		}

If you're part way down the list, surely you'll need to
unlock multiple anon_vmas here before going to retry?

Otherwise you could forget to unlock one that you already
locked, and live-lock here.

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