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

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

On Thu, 29 Oct 2009 09:30:13 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> 
> 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 in prepare_migration when memcg is disabled
> by boot option. This causes panic in end_migration. This patch fixes it.
> 
> Reported-by: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
> Cc: Balbir Singh <balbir@in.ibm.com>
> Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> ---
>  mm/memcontrol.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> 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@mxp.nes.nec.co.jp>

And I think we should send a fix for this bug to -stable too.


Thanks,
Daisuke Nishimura.

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