Re: could you clarify mm/mempolicy: fix !vma in new_vma_page()

[Bob Liu <lliubbo@gmail.com>]

On Wed, Jan 8, 2014 at 6:08 PM, Michal Hocko <mhocko@suse.cz> wrote:

>
> If I was debugging this I would simply add printk into page_address_in_vma
> error paths.
>
> Anyway, I think that at least hugetlbfs part should be reverted because
> it might paper over real bugs. Although the migration would fail for
> such hugetlb page we should catch that a weird page was tried to be
> migrated. What about the patch below?

Looks good to me. But we need to confirm whether our assumption is right.
Sasha, could you please have a test with Michal's patch?

Thanks,
-Bob

> ---
> From 2d61421f26a3b63b4670d71b7adc67e2191b6157 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.cz>
> Date: Wed, 8 Jan 2014 10:57:41 +0100
> Subject: [PATCH] mm: new_vma_page cannot see NULL vma for hugetlb pages
>
> 11c731e81bb0 (mm/mempolicy: fix !vma in new_vma_page()) has removed
> BUG_ON(!vma) from new_vma_page which is partially correct because
> page_address_in_vma will return EFAULT for non-linear mappings and at
> least shared shmem might be mapped this way.
>
> The patch also tried to prevent NULL ptr for hugetlb pages which is not
> correct AFAICS because hugetlb pages cannot be mapped as VM_NONLINEAR
> and other conditions in page_address_in_vma seem to be legit and catch
> real bugs.
>
> This patch restores BUG_ON for PageHuge to catch potential issues when
> the to-be-migrated page is not setup properly.
>
> Signed-off-by: Michal Hocko <mhocko@suse.cz>
> ---
>  mm/mempolicy.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 9e8d2d86978a..f3f51464a23b 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1199,10 +1199,8 @@ static struct page *new_vma_page(struct page *page, unsigned long private, int *
>         }
>
>         if (PageHuge(page)) {
> -               if (vma)
> -                       return alloc_huge_page_noerr(vma, address, 1);
> -               else
> -                       return NULL;
> +               BUG_ON(vma)
> +               return alloc_huge_page_noerr(vma, address, 1);
>         }
>         /*
>          * if !vma, alloc_page_vma() will use task or system default policy
> --
> 1.8.5.2
>
> --
> Michal Hocko
> SUSE Labs

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