Re: [PATCH 14/14] mm,migration: Allow the migration of PageSwapCache pages

[Andrew Morton <akpm@linux-foundation.org>]

On Fri,  2 Apr 2010 17:02:48 +0100
Mel Gorman <mel@csn.ul.ie> wrote:

> PageAnon pages that are unmapped may or may not have an anon_vma so are
> not currently migrated. However, a swap cache page can be migrated and
> fits this description. This patch identifies page swap caches and allows
> them to be migrated but ensures that no attempt to made to remap the pages
> would would potentially try to access an already freed anon_vma.
> 
> ...
>
> @@ -484,7 +484,8 @@ static int fallback_migrate_page(struct address_space *mapping,
>   *   < 0 - error code
>   *  == 0 - success
>   */
> -static int move_to_new_page(struct page *newpage, struct page *page)
> +static int move_to_new_page(struct page *newpage, struct page *page,
> +						int remap_swapcache)

You're not a fan of `bool'.

>  {
>  	struct address_space *mapping;
>  	int rc;
> @@ -519,10 +520,12 @@ static int move_to_new_page(struct page *newpage, struct page *page)
>  	else
>  		rc = fallback_migrate_page(mapping, newpage, page);
>  
> -	if (!rc)
> -		remove_migration_ptes(page, newpage);
> -	else
> +	if (rc) {
>  		newpage->mapping = NULL;
> +	} else {
> +		if (remap_swapcache) 
> +			remove_migration_ptes(page, newpage);
> +	}
>  
>  	unlock_page(newpage);
>  
> @@ -539,6 +542,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  	int rc = 0;
>  	int *result = NULL;
>  	struct page *newpage = get_new_page(page, private, &result);
> +	int remap_swapcache = 1;
>  	int rcu_locked = 0;
>  	int charge = 0;
>  	struct mem_cgroup *mem = NULL;
> @@ -600,18 +604,27 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  		rcu_read_lock();
>  		rcu_locked = 1;
>  
> -		/*
> -		 * 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_mapped(page))
> -			goto rcu_unlock;
> +		/* Determine how to safely use anon_vma */
> +		if (!page_mapped(page)) {
> +			if (!PageSwapCache(page))
> +				goto rcu_unlock;
>  
> -		anon_vma = page_anon_vma(page);
> -		atomic_inc(&anon_vma->external_refcount);
> +			/*
> +			 * We cannot be sure that the anon_vma of an unmapped
> +			 * swapcache page is safe to use.

Why not?  A full explanation here would be nice.

> 			   In this case, the
> +			 * swapcache page gets migrated but the pages are not
> +			 * remapped
> +			 */
> +			remap_swapcache = 0;
> +		} else { 
> +			/*
> +			 * Take a reference count on the anon_vma if the
> +			 * page is mapped so that it is guaranteed to
> +			 * exist when the page is remapped later
> +			 */
> +			anon_vma = page_anon_vma(page);
> +			atomic_inc(&anon_vma->external_refcount);
> +		}
>  	}
>  
>  	/*
> @@ -646,9 +659,9 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  
>  skip_unmap:
>  	if (!page_mapped(page))
> -		rc = move_to_new_page(newpage, page);
> +		rc = move_to_new_page(newpage, page, remap_swapcache);
>  
> -	if (rc)
> +	if (rc && remap_swapcache)
>  		remove_migration_ptes(page, page);
>  rcu_unlock: