Re: [PATCH] mm: softdirty: respect VM_SOFTDIRTY in PTE holes

[Cyrill Gorcunov <gorcunov@gmail.com>]

On Thu, Jul 31, 2014 at 06:43:25PM -0400, Peter Feiner wrote:
> After a VMA is created with the VM_SOFTDIRTY flag set,
> /proc/pid/pagemap should report that the VMA's virtual pages are
> soft-dirty until VM_SOFTDIRTY is cleared (i.e., by the next write of
> "4" to /proc/pid/clear_refs). However, pagemap ignores the
> VM_SOFTDIRTY flag for virtual addresses that fall in PTE holes (i.e.,
> virtual addresses that don't have a PMD, PUD, or PGD allocated yet).
> 
> To observe this bug, use mmap to create a VMA large enough such that
> there's a good chance that the VMA will occupy an unused PMD, then
> test the soft-dirty bit on its pages. In practice, I found that a VMA
> that covered a PMD's worth of address space was big enough.
> 
> This patch adds the necessary VMA lookup to the PTE hole callback in
> /proc/pid/pagemap's page walk and sets soft-dirty according to the
> VMAs' VM_SOFTDIRTY flag.
> 
> Signed-off-by: Peter Feiner <pfeiner@google.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>

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