Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages

[Christoph Lameter <cl@linux-foundation.org>]

On Tue, 23 Mar 2010, Mel Gorman wrote:

> diff --git a/mm/migrate.c b/mm/migrate.c
> index 98eaaf2..6eb1efe 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -603,6 +603,19 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  	 */
>  	if (PageAnon(page)) {
>  		rcu_read_lock();
> +
> +		/*
> +		 * If the page has no mappings any more, just bail. An
> +		 * unmapped anon page is likely to be freed soon but worse,
> +		 * it's possible its anon_vma disappeared between when
> +		 * the page was isolated and when we reached here while
> +		 * the RCU lock was not held
> +		 */
> +		if (!page_mapcount(page)) {
> +			rcu_read_unlock();
> +			goto uncharge;
> +		}
> +
>  		rcu_locked = 1;
>  		anon_vma = page_anon_vma(page);
>  		atomic_inc(&anon_vma->migrate_refcount);

A way to make this simpler would be to move "rcu_locked = 1" before the
if statement and then do

if (!page_mapcount(page))
	goto rcu_unlock;