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

[Linus Torvalds <torvalds@linux-foundation.org>]

On Wed, 5 May 2010, Mel Gorman wrote:
> 
> If the same_vma list is properly ordered then maybe something like the
> following is allowed?

Heh. This is the same logic I just sent out. However:

> +	anon_vma = page_rmapping(page);
> +	if (!anon_vma)
> +		return NULL;
> +
> +	spin_lock(&anon_vma->lock);

RCU should guarantee that this spin_lock() is valid, but:

> +	/*
> +	 * Get the oldest anon_vma on the list by depending on the ordering
> +	 * of the same_vma list setup by __page_set_anon_rmap
> +	 */
> +	avc = list_entry(&anon_vma->head, struct anon_vma_chain, same_anon_vma);

We're not guaranteed that the 'anon_vma->head' list is non-empty.

Somebody could have freed the list and the anon_vma and we have a stale 
'page->anon_vma' (that has just not been _released_ yet). 

And shouldn't that be 'list_first_entry'? Or &anon_vma->head.next?

How did that line actually work for you? Or was it just a "it boots", but 
no actual testing of the rmap walk?

			Linus

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