Re: [BUGFIX][PATCH] memcg: fix wrong pointer initialization at pagemigration when memcg is disabled.

[KAMEZAWA Hiroyuki]

On Thu, 29 Oct 2009 09:50:51 +0900
Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:
> > Index: linux-2.6.32-rc5/mm/memcontrol.c
> > ===================================================================
> > --- linux-2.6.32-rc5.orig/mm/memcontrol.c
> > +++ linux-2.6.32-rc5/mm/memcontrol.c
> > @@ -1990,7 +1990,8 @@ int mem_cgroup_prepare_migration(struct 
> >  	struct page_cgroup *pc;
> >  	struct mem_cgroup *mem = NULL;
> >  	int ret = 0;
> > -
> > +	/* this pointer will be checked at end_migration */
> > +	*ptr = NULL;
> >  	if (mem_cgroup_disabled())
> >  		return 0;
> >  
> > 
> I thought unmap_and_move() itself initializes "mem" to NULL, but it doesn't...
> I personaly prefer initializing "mem" to NULL in unmap_and_move(), but anyway
> I think this patch is also correct.
> 
> 	Reviewed-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
> 
Ok, here
> And I think we should send a fix for this bug to -stable too.
I think so, too.


==
Lee Schermerhorn reported that he saw bad pointer dereference
in mem_cgroup_end_migration() when he disabled memcg by boot option.

memcg's page migration logic works as

	mem_cgroup_prepare_migration(page, &ptr);
	do page migration
	mem_cgroup_end_migration(page, ptr);

Now, ptr is not initialized when memcg is disabled by boot option.
This patch fixes it.

Changelog: 2009/10/29
 - modified "fix" from memcontrol.c to migrate.c


Reported-by: Lee Schermerhorn <Lee.Schermerhorn@xxxxxx>
Cc: Balbir Singh <balbir@xxxxxxxxxx>
Cc: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
Reviewed-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
 mm/migrate.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-2.6.32-rc5/mm/migrate.c
===================================================================
--- linux-2.6.32-rc5.orig/mm/migrate.c
+++ linux-2.6.32-rc5/mm/migrate.c
@@ -602,7 +602,7 @@ static int unmap_and_move(new_page_t get
 	struct page *newpage = get_new_page(page, private, &result);
 	int rcu_locked = 0;
 	int charge = 0;
-	struct mem_cgroup *mem;
+	struct mem_cgroup *mem = NULL;
 
 	if (!newpage)
 		return -ENOMEM;

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