I've sent the same patch and has been merged into -mm tree by Andrew Morton.

Thanks
Zhang

2013/4/7 k80c <k80ck80c@gmail.com>

> As per commit 841e31e5cc6219d62054788faa289b6ed682d068,
> we dont really need this if(mm) check anymore.
>
> A WARN_ON_ONCE was added just for safety, but there have been no bug
> reports about this so far.
>
> Removing this if(mm) check.
>
> Signed-off-by: k80c <k80ck80c@gmail.com>
> ---
>  mm/mmap.c |    3 ---
>  1 files changed, 0 insertions(+), 3 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 0db0de1..b2c363f 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1935,9 +1935,6 @@ struct vm_area_struct *find_vma(struct mm_struct
> *mm, unsigned long addr)
>  {
>         struct vm_area_struct *vma = NULL;
>
> -       if (WARN_ON_ONCE(!mm))          /* Remove this in linux-3.6 */
> -               return NULL;
> -
>         /* Check the cache first. */
>         /* (Cache hit rate is typically around 35%.) */
>         vma = ACCESS_ONCE(mm->mmap_cache);
> --
> 1.7.5.4
>
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