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

[Peter Zijlstra]

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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