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

[Bob Liu <lliubbo@gmail.com>]

On Wed, Jan 8, 2014 at 8:42 PM, Michal Hocko <mhocko@suse.cz> wrote:
> On Wed 08-01-14 20:09:30, Bob Liu wrote:
>> 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.
>
> Which assumption you have in mind? non-linear mapping or failing on
> anon_vma or f_mapping checks?
>

The assumption that the original BUG_ON(!vma) was triggered by
non-linear mapping.

>> Sasha, could you please have a test with Michal's patch?
>
> I obviously doesn't have anything against testing but we should really
> focus on the original issue. This patch simply restores hugetlb code

Oh, I see your point.
Yes, I agree that your patch should be merged and if the BUG_ON() is
triggered(which is unlikely) again.
We can open another thread and analysis the root cause.

>> > ---
>> > 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>

Reviewed-by: Bob Liu <bob.liu@oracle.com>

>> > ---
>> >  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
>> >

-- 
Regards,
--Bob

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