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

[Hugh Dickins <hughd@google.com>]

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On Fri, 4 Nov 2011, Nai Xia wrote:
> On Fri, Nov 4, 2011 at 3:31 PM, Hugh Dickins <hughd@google.com> wrote:
> > On Mon, 31 Oct 2011, Andrea Arcangeli wrote:
> >> @@ -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.
> 
> At the first glance that rmap_walk does travel this merged VMA
> once...
> But, Now, Wait...., I am actually really puzzled that this case can really
> happen at all, you see that vma_merge() does not break the validness
> between page->index and its VMA. So if this can really happen,
> a page->index should be valid in both areas in a same VMA.
> It's strange to imagine that a PTE is copy inside a _same_ VMA
> and page->index is valid at both old and new places.

Yes, I think you are right, thank you for elucidating it.

That was a real case when we wrote copy_vma(), when rmap was using
pte_chains; but once anon_vma came in, and imposed vm_pgoff matching
on anonymous mappings too, it became dead code.  With linear vm_pgoff
matching, you cannot fit a range in two places within the same vma.
(And even the non-linear case relies upon vm_pgoff defaults.)

So we could simplify the copy_vma() interface a little now (get rid of
that nasty **vmap): I'm not quite sure whether we ought to do that,
but certainly Andrea's comment there should be updated (if he also
agrees with your analysis).

> 
> IMO, the only case that src VMA can be merged by the new
> is that src VMA hasn't been faulted yet and the pgoff
> is recalculated. And if my reasoning is true, this place
> does not need to be worried about.

I don't see a place where "the pgoff is recalculated" (except in
the consistent way when expanding or splitting or merging vma), nor
where vma merge would allow for variable pgoff.  I agree that we
could avoid finalizing vm_pgoff for an anonymous area until its
anon_vma is assigned: were you imagining doing that in future,
or am I overlooking something already there?

Hugh