On Tue, Apr 06, 2010 at 05:06:23PM -0700, Andrew Morton wrote:
> 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'.
>
This function existed before compaction and returns an error code rather
than a true/false value.
> > {
> > 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.
Patch below.
>
> > 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:
>
Patch that updates the comment if you prefer it is as follows
==== CUT HERE ====
mm,compaction: Expand comment on unmapped page swap cache
The comment on the handling of anon_vma for unmapped pages is a bit
sparse. Expand it.
This is a fix to the patch "mm,migration: Allow the migration of
PageSwapCache pages"
Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
---
mm/migrate.c | 12 +++++++++---
1 files changed, 9 insertions(+), 3 deletions(-)
diff --git a/mm/migrate.c b/mm/migrate.c
index 0356e64..281a239 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -611,9 +611,15 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
/*
* We cannot be sure that the anon_vma of an unmapped
- * swapcache page is safe to use. In this case, the
- * swapcache page gets migrated but the pages are not
- * remapped
+ * swapcache page is safe to use because we don't
+ * know in advance if the VMA that this page belonged
+ * to still exists. If the VMA and others sharing the
+ * data have been freed, then the anon_vma could
+ * already be invalid.
+ *
+ * To avoid this possibility, swapcache pages get
+ * migrated but are not remapped when migration
+ * completes
*/
remap_swapcache = 0;
} else {
--
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