On Fri, 2 Apr 2010 17:02:48 +0100
Mel Gorman <mel@xxxxxxxxx> 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:
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxxx For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>