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

[Rik van Riel <riel@redhat.com>]

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.