[BUGFIX][PATCH v3] memcg: fix memory migration of shmem swapcache

[Daisuke Nishimura]

> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 159a076..cc5a8fd 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -93,7 +93,7 @@ extern int
> > Âmem_cgroup_prepare_migration(struct page *page,
> > Â Â Â Âstruct page *newpage, struct mem_cgroup **ptr);
> > Âextern void mem_cgroup_end_migration(struct mem_cgroup *mem,
> > - Â Â Â struct page *oldpage, struct page *newpage);
> > + Â Â Â struct page *oldpage, struct page *newpage, bool success);
> 
> The term "success" implies present or future tense.
> The event this variable cares about in the past so "succeed" might be
> a more appropriate term.
> Sorry to be picky about the English but there is an important
> distinction here since we don't have any comment about the variable.
> 
> Am I being too fussy?
Not at all. Your comments are very helpful to make the code more readable :)

===
From: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>

In current implimentation, mem_cgroup_end_migration() decides whether the page
migration has succeeded or not by checking "oldpage->mapping".

But if we are tring to migrate a shmem swapcache, the page->mapping of it is
NULL from the begining, so the check would be invalid.
As a result, mem_cgroup_end_migration() assumes the migration has succeeded
even if it's not, so "newpage" would be freed while it's not uncharged.

This patch fixes it by passing mem_cgroup_end_migration() the result of the
page migration.

Signed-off-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
Reviewed-by: Minchan Kim <minchan.kim@xxxxxxxxx>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
v2->v3
  - s/success/succeed

v1->v2
  - pass mem_cgroup_end_migration() "bool" instead of "int".

 include/linux/memcontrol.h |    5 ++---
 mm/memcontrol.c            |    5 ++---
 mm/migrate.c               |    2 +-
 3 files changed, 5 insertions(+), 7 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 159a076..9f52b57 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -93,7 +93,7 @@ extern int
 mem_cgroup_prepare_migration(struct page *page,
 	struct page *newpage, struct mem_cgroup **ptr);
 extern void mem_cgroup_end_migration(struct mem_cgroup *mem,
-	struct page *oldpage, struct page *newpage);
+	struct page *oldpage, struct page *newpage, bool succeed);
 
 /*
  * For memory reclaim.
@@ -231,8 +231,7 @@ mem_cgroup_prepare_migration(struct page *page, struct page *newpage,
 }
 
 static inline void mem_cgroup_end_migration(struct mem_cgroup *mem,
-					struct page *oldpage,
-					struct page *newpage)
+		struct page *oldpage, struct page *newpage, bool succeed)
 {
 }
 
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 61678be..71a39bc 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2856,7 +2856,7 @@ int mem_cgroup_prepare_migration(struct page *page,
 
 /* remove redundant charge if migration failed*/
 void mem_cgroup_end_migration(struct mem_cgroup *mem,
-	struct page *oldpage, struct page *newpage)
+	struct page *oldpage, struct page *newpage, bool succeed)
 {
 	struct page *used, *unused;
 	struct page_cgroup *pc;
@@ -2865,8 +2865,7 @@ void mem_cgroup_end_migration(struct mem_cgroup *mem,
 		return;
 	/* blocks rmdir() */
 	cgroup_exclude_rmdir(&mem->css);
-	/* at migration success, oldpage->mapping is NULL. */
-	if (oldpage->mapping) {
+	if (!succeed) {
 		used = oldpage;
 		unused = newpage;
 	} else {
diff --git a/mm/migrate.c b/mm/migrate.c
index 6ae8a66..be66b23 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -756,7 +756,7 @@ rcu_unlock:
 		rcu_read_unlock();
 uncharge:
 	if (!charge)
-		mem_cgroup_end_migration(mem, page, newpage);
+		mem_cgroup_end_migration(mem, page, newpage, rc == 0);
 unlock:
 	unlock_page(page);
 
-- 
1.7.1

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