Re: [PATCH v2 1/2] memcg: remove unnecessary BUG_ON

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Feb 21, 2011 at 12:17:17AM +0900, Minchan Kim wrote:
> Now memcg in unmap_and_move checks BUG_ON of charge.
> But mem_cgroup_prepare_migration returns either 0 or -ENOMEM.
> If it returns -ENOMEM, it jumps out unlock without the check.
> If it returns 0, it can pass BUG_ON. So it's meaningless.
> Let's remove it.
> 
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
> Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
> ---
>  mm/migrate.c |    1 -
>  1 files changed, 0 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index eb083a6..2abc9c9 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -683,7 +683,6 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>  		rc = -ENOMEM;
>  		goto unlock;
>  	}
> -	BUG_ON(charge);

You remove this assertion of the mem_cgroup_prepare_migration() return
value but only add a comment about the expectations in the next patch.

Could you write a full-blown kerneldoc on mem_cgroup_prepare_migration
and remove this BUG_ON() in the same patch?