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SubjectRe: [RFC][PATCH 3/6] mm: VMA sequence count
On Wed, Oct 22, 2014 at 01:39:51PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 22, 2014 at 02:26:57PM +0300, Kirill A. Shutemov wrote:
> > On Mon, Oct 20, 2014 at 11:56:36PM +0200, Peter Zijlstra wrote:
> > > Wrap the VMA modifications (vma_adjust/unmap_page_range) with sequence
> > > counts such that we can easily test if a VMA is changed.
> > >
> > > The unmap_page_range() one allows us to make assumptions about
> > > page-tables; when we find the seqcount hasn't changed we can assume
> > > page-tables are still valid.
> > >
> > > The flip side is that we cannot distinguish between a vma_adjust() and
> > > the unmap_page_range() -- where with the former we could have
> > > re-checked the vma bounds against the address.
> >
> > You only took care about changing size of VMA or unmap. What about other
> > aspects of VMA. How would you care about race with mprotect(2)?
> >
> > CPU0 CPU1
> > mprotect()
> > mprotect_fixup()
> > vma_merge()
> > [ maybe update vm_sequence ]
> > [ page fault kicks in ]
> > do_anonymous_page()
> > entry = mk_pte(page, fe->vma->vm_page_prot);
> > vma_set_page_prot(vma)
> > [ update vma->vm_page_prot ]
> > change_protection()
> > pte_map_lock()
> > [ vm_sequence is ok ]
> > set_pte_at(entry) // With old vm_page_prot!!!
> >
>
> This won't happen, this is be serialized by the PTL and the fault
> validates that the PTE is the 'same' it started out with after acquiring
> the PTL.

Em, no. In this case change_protection() will not touch the pte, since
it's pte_none() and the pte_same() check will pass just fine.

--
Kirill A. Shutemov


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