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

[Michal Hocko]

On Wed 08-01-14 21:10:29, Bob Liu wrote:
[...]
> >> > From 2d61421f26a3b63b4670d71b7adc67e2191b6157 Mon Sep 17 00:00:00 2001
> >> > From: Michal Hocko <mhocko@xxxxxxx>
> >> > 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@xxxxxxx>
> 
> Reviewed-by: Bob Liu <bob.liu@xxxxxxxxxx>

Thanks!

> >> > ---
> >> >  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)

That was meant to say BUG_ON(!vma) of course ;) but I guess your
reviewed-by still applies so I will post it to Andrew.

> >> > +               return alloc_huge_page_noerr(vma, address, 1);
> >> >         }
> >> >         /*
> >> >          * if !vma, alloc_page_vma() will use task or system default policy
> >> > --
> >> > 1.8.5.2
> >> >
> 
> -- 
> Regards,
> --Bob

-- 
Michal Hocko
SUSE Labs
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