Re: [PATCH 01/14] mm,migration: Take a reference to the anon_vma before migrating

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Tue, 20 Apr 2010 22:01:03 +0100
Mel Gorman <mel@csn.ul.ie> wrote:

> rmap_walk_anon() does not use page_lock_anon_vma() for looking up and
> locking an anon_vma and it does not appear to have sufficient locking to
> ensure the anon_vma does not disappear from under it.
> 
> This patch copies an approach used by KSM to take a reference on the
> anon_vma while pages are being migrated.  This should prevent rmap_walk()
> running into nasty surprises later because anon_vma has been freed.
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> Acked-by: Rik van Riel <riel@redhat.com>

Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Hmm, a nitpick..


>  static inline struct anon_vma *page_anon_vma(struct page *page)
>  {
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 5938db5..b768a1d 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -543,6 +543,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  	int rcu_locked = 0;
>  	int charge = 0;
>  	struct mem_cgroup *mem = NULL;
> +	struct anon_vma *anon_vma = NULL;
>  
>  	if (!newpage)
>  		return -ENOMEM;
> @@ -599,6 +600,8 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  	if (PageAnon(page)) {
>  		rcu_read_lock();
>  		rcu_locked = 1;
> +		anon_vma = page_anon_vma(page);
> +		atomic_inc(&anon_vma->migrate_refcount);
>  	}
>  
>  	/*
> @@ -638,6 +641,15 @@ skip_unmap:
>  	if (rc)
>  		remove_migration_ptes(page, page);
>  rcu_unlock:
> +
> +	/* Drop an anon_vma reference if we took one */
> +	if (anon_vma && atomic_dec_and_lock(&anon_vma->migrate_refcount, &anon_vma->lock)) {
> +		int empty = list_empty(&anon_vma->head);
> +		spin_unlock(&anon_vma->lock);
> +		if (empty)
> +			anon_vma_free(anon_vma);
> +	}
> +

Hmm. Can't we move this part to mm/rmap.c ?

	as
		anon_vma_drop_external_reference()
or some.

Thanks,
-Kame

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