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SubjectRe: [RFC][PATCH 3/6] mm: VMA sequence count
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.


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