Re: [PATCH] mremap: enforce rmap src/dst vma ordering in case of vma_merge succeeding in copy_vma

[Nai Xia <nai.xia@gmail.com>]

On Sat, Nov 5, 2011 at 8:21 AM, Nai Xia <nai.xia@gmail.com> wrote:
> On Sat, Nov 5, 2011 at 7:56 AM, Andrea Arcangeli <aarcange@redhat.com> wrote:
>> On Fri, Nov 04, 2011 at 12:31:04AM -0700, Hugh Dickins wrote:
>>> On Mon, 31 Oct 2011, Andrea Arcangeli wrote:
>>> > diff --git a/mm/mmap.c b/mm/mmap.c
>>> > index a65efd4..a5858dc 100644
>>> > --- a/mm/mmap.c
>>> > +++ b/mm/mmap.c
>>> > @@ -2339,7 +2339,15 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
>>> >              */
>>> >             if (vma_start >= new_vma->vm_start &&
>>> >                 vma_start < new_vma->vm_end)
>>> > +                   /*
>>> > +                    * No need to call anon_vma_order_tail() in
>>> > +                    * this case because the same PT lock will
>>> > +                    * serialize the rmap_walk against both src
>>> > +                    * and dst vmas.
>>> > +                    */
>>>
>>> Really?  Please convince me: I just do not see what ensures that
>>> the same pt lock covers both src and dst areas in this case.
>>
>> Right, vma being the same for src/dst doesn't mean the PT lock is the
>> same, it might be if source pte entry fit in the same pagetable but
>> maybe not if the vma is >2M (the max a single pagetable can point to).
>>
>>> >                     *vmap = new_vma;
>>> > +           else
>>> > +                   anon_vma_order_tail(new_vma);
>>>
>>> And if this puts new_vma in the right position for the normal
>>> move_page_tables(), as anon_vma_clone() does in the block below,
>>> aren't they both in exactly the wrong position for the abnormal
>>> move_page_tables(), called to put ptes back where they were if
>>> the original move_page_tables() fails?
>>
>> Failure paths. Good point, they'd need to be reversed again in that
>> case.
>>
>>> It might be possible to argue that move_page_tables() can only
>>> fail by failing to allocate memory for pud or pmd, and that (perhaps)
>>> could only happen if the task was being OOM-killed and ran out of
>>> reserves at this point, and if it's being OOM-killed then we don't
>>> mind losing a migration entry for a moment... perhaps.
>>
>> Hmm no it wouldn't be ok, or I wouldn't want to risk that.
>>
>>> Certainly I'd agree that it's a very rare case.  But it feels wrong
>>> to be attempting to fix the already unlikely issue, while ignoring
>>> this aspect, or relying on such unrelated implementation details.
>>
>> Agreed.
>>
>>> Perhaps some further anon_vma_ordering could fix it up,
>>> but that would look increasingly desperate.
>>
>> I think what Nai didn't consider in explaining this theoretical race
>> that I noticed now is the anon_vma root lock taken by adjust_vma.
>>
>> If the merge succeeds adjust_vma will take the lock and flush away
>> from all others CPUs any sign of rmap_walk before the move_page_tables
>> can start.
>>
>> So it can't happen that you do rmap_walk, check vma1, mremap moves
>> stuff from vma2 to vma1 (wrong order), and then rmap_walk continues
>> checking vma2 where the pte won't be there anymore. It can't happen
>> because mremap would block in vma_merge waiting the rmap_walk to
>> complete. Before proceeding moving any pte. Thanks to the anon_vma
>> lock already taken by adjust_vma.
>
> Still,  I think it's not rmap_walk() ---> mremap() --> rmap_walk() that trigger
> the bug,  but this events would:
>
> copy_vma() ---> rmap_walk() scan dst VMA --> move_page_tables() moves src to dst
> --->  rmap_walk() scan src VMA.  :D

OK, I think I need to be more concise: Your last reasoning only
ensures that mremap
as a whole entity cannot interleave with  rmap_walk(). But I think
nothing can prevent
move_page_tables() from doing this. As long as copy_vma() gives an
wrong ordering,
the racing between  rmap_walk() & move_page_tables() afterwards may
trigger the bug.

Do you agree?



>
> I might be wrong. But thank you all for the time and patience for
> playing this racing game
> with me. It's really an honor to exhaust my mind on a daunting thing
> with you. :)
>
>
> Best Regards,
>
> Nai
>

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