Re: [PATCH v10 06/25] mm: make pte_unmap_same compatible with SPF

[vinayak menon <vinayakm.list@gmail.com>]

On Tue, Apr 17, 2018 at 8:03 PM, Laurent Dufour
<ldufour@linux.vnet.ibm.com> wrote:
> pte_unmap_same() is making the assumption that the page table are still
> around because the mmap_sem is held.
> This is no more the case when running a speculative page fault and
> additional check must be made to ensure that the final page table are still
> there.
>
> This is now done by calling pte_spinlock() to check for the VMA's
> consistency while locking for the page tables.
>
> This is requiring passing a vm_fault structure to pte_unmap_same() which is
> containing all the needed parameters.
>
> As pte_spinlock() may fail in the case of a speculative page fault, if the
> VMA has been touched in our back, pte_unmap_same() should now return 3
> cases :
>         1. pte are the same (0)
>         2. pte are different (VM_FAULT_PTNOTSAME)
>         3. a VMA's changes has been detected (VM_FAULT_RETRY)
>
> The case 2 is handled by the introduction of a new VM_FAULT flag named
> VM_FAULT_PTNOTSAME which is then trapped in cow_user_page().
> If VM_FAULT_RETRY is returned, it is passed up to the callers to retry the
> page fault while holding the mmap_sem.
>
> Acked-by: David Rientjes <rientjes@google.com>
> Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
> ---
>  include/linux/mm.h |  1 +
>  mm/memory.c        | 39 ++++++++++++++++++++++++++++-----------
>  2 files changed, 29 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 4d1aff80669c..714da99d77a3 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1208,6 +1208,7 @@ static inline void clear_page_pfmemalloc(struct page *page)
>  #define VM_FAULT_NEEDDSYNC  0x2000     /* ->fault did not modify page tables
>                                          * and needs fsync() to complete (for
>                                          * synchronous page faults in DAX) */
> +#define VM_FAULT_PTNOTSAME 0x4000      /* Page table entries have changed */


This has to be added to VM_FAULT_RESULT_TRACE ?