Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

[Laurent Dufour]

On 03/04/2018 21:10, Jerome Glisse wrote:
> On Tue, Mar 13, 2018 at 06:59:36PM +0100, Laurent Dufour 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.
>>
>> Signed-off-by: Laurent Dufour 
>> ---
>>  include/linux/mm.h |  1 +
>>  mm/memory.c| 29 +++--
>>  2 files changed, 20 insertions(+), 10 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 2f3e98edc94a..b6432a261e63 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -1199,6 +1199,7 @@ 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 */
>>  
>>  #define VM_FAULT_ERROR  (VM_FAULT_OOM | VM_FAULT_SIGBUS | 
>> VM_FAULT_SIGSEGV | \
>>   VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE | \
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 21b1212a0892..4bc7b0bdcb40 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -2309,21 +2309,29 @@ static bool pte_map_lock(struct vm_fault *vmf)
>>   * parts, do_swap_page must check under lock before unmapping the pte and
>>   * proceeding (but do_wp_page is only called after already making such a 
>> check;
>>   * and do_anonymous_page can safely check later on).
>> + *
>> + * pte_unmap_same() returns:
>> + *  0   if the PTE are the same
>> + *  VM_FAULT_PTNOTSAME  if the PTE are different
>> + *  VM_FAULT_RETRY  if the VMA has changed in our back during
>> + *  a speculative page fault handling.
>>   */
>> -static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
>> -pte_t *page_table, pte_t orig_pte)
>> +static inline int pte_unmap_same(struct vm_fault *vmf)
>>  {
>> -int same = 1;
>> +int ret = 0;
>> +
>>  #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
>>  if (sizeof(pte_t) > sizeof(unsigned long)) {
>> -spinlock_t *ptl = pte_lockptr(mm, pmd);
>> -spin_lock(ptl);
>> -same = pte_same(*page_table, orig_pte);
>> -spin_unlock(ptl);
>> +if (pte_spinlock(vmf)) {
>> +if (!pte_same(*vmf->pte, vmf->orig_pte))
>> +ret = VM_FAULT_PTNOTSAME;
>> +spin_unlock(vmf->ptl);
>> +} else
>> +ret = VM_FAULT_RETRY;
>>  }
>>  #endif
>> -pte_unmap(page_table);
>> -return same;
>> +pte_unmap(vmf->pte);
>> +return ret;
>>  }
>>  
>>  static inline void cow_user_page(struct page *dst, struct page *src, 
>> unsigned long va, struct vm_area_struct *vma)
>> @@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
>>  int exclusive = 0;
>>  int ret = 0;
>>  
>> -if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
>> +ret = pte_unmap_same(vmf);
>> +if (ret)
>>  goto out;
>>  
> 
> This change what do_swap_page() returns ie before it was returning 0
> when locked pte lookup was different from orig_pte. After this patch
> it returns VM_FAULT_PTNOTSAME but this is a new return value for
> handle_mm_fault() (the do_swap_page() return value is what ultimately
> get return by handle_mm_fault())
> 
> Do we really want that ? This might confuse some existing user of
> handle_mm_fault() and i am not sure of the value of that information
> to caller.
> 
> Note i do understand that you want to return retry if anything did
> change from underneath and thus need to differentiate from when the
> pte value are not the same.

You're right, do_swap_page() should still return 0 in the case the lookup pte
is different from orig_pte, assuming that the swap 

Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

[Laurent Dufour]

On 03/04/2018 21:10, Jerome Glisse wrote:
> On Tue, Mar 13, 2018 at 06:59:36PM +0100, Laurent Dufour 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.
>>
>> Signed-off-by: Laurent Dufour 
>> ---
>>  include/linux/mm.h |  1 +
>>  mm/memory.c| 29 +++--
>>  2 files changed, 20 insertions(+), 10 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 2f3e98edc94a..b6432a261e63 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -1199,6 +1199,7 @@ 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 */
>>  
>>  #define VM_FAULT_ERROR  (VM_FAULT_OOM | VM_FAULT_SIGBUS | 
>> VM_FAULT_SIGSEGV | \
>>   VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE | \
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 21b1212a0892..4bc7b0bdcb40 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -2309,21 +2309,29 @@ static bool pte_map_lock(struct vm_fault *vmf)
>>   * parts, do_swap_page must check under lock before unmapping the pte and
>>   * proceeding (but do_wp_page is only called after already making such a 
>> check;
>>   * and do_anonymous_page can safely check later on).
>> + *
>> + * pte_unmap_same() returns:
>> + *  0   if the PTE are the same
>> + *  VM_FAULT_PTNOTSAME  if the PTE are different
>> + *  VM_FAULT_RETRY  if the VMA has changed in our back during
>> + *  a speculative page fault handling.
>>   */
>> -static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
>> -pte_t *page_table, pte_t orig_pte)
>> +static inline int pte_unmap_same(struct vm_fault *vmf)
>>  {
>> -int same = 1;
>> +int ret = 0;
>> +
>>  #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
>>  if (sizeof(pte_t) > sizeof(unsigned long)) {
>> -spinlock_t *ptl = pte_lockptr(mm, pmd);
>> -spin_lock(ptl);
>> -same = pte_same(*page_table, orig_pte);
>> -spin_unlock(ptl);
>> +if (pte_spinlock(vmf)) {
>> +if (!pte_same(*vmf->pte, vmf->orig_pte))
>> +ret = VM_FAULT_PTNOTSAME;
>> +spin_unlock(vmf->ptl);
>> +} else
>> +ret = VM_FAULT_RETRY;
>>  }
>>  #endif
>> -pte_unmap(page_table);
>> -return same;
>> +pte_unmap(vmf->pte);
>> +return ret;
>>  }
>>  
>>  static inline void cow_user_page(struct page *dst, struct page *src, 
>> unsigned long va, struct vm_area_struct *vma)
>> @@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
>>  int exclusive = 0;
>>  int ret = 0;
>>  
>> -if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
>> +ret = pte_unmap_same(vmf);
>> +if (ret)
>>  goto out;
>>  
> 
> This change what do_swap_page() returns ie before it was returning 0
> when locked pte lookup was different from orig_pte. After this patch
> it returns VM_FAULT_PTNOTSAME but this is a new return value for
> handle_mm_fault() (the do_swap_page() return value is what ultimately
> get return by handle_mm_fault())
> 
> Do we really want that ? This might confuse some existing user of
> handle_mm_fault() and i am not sure of the value of that information
> to caller.
> 
> Note i do understand that you want to return retry if anything did
> change from underneath and thus need to differentiate from when the
> pte value are not the same.

You're right, do_swap_page() should still return 0 in the case the lookup pte
is different from orig_pte, assuming that the swap operation has been handled
in 

Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

[Jerome Glisse]

On Tue, Apr 03, 2018 at 01:40:18PM -0700, David Rientjes wrote:
> On Tue, 3 Apr 2018, Jerome Glisse wrote:
> 
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index 21b1212a0892..4bc7b0bdcb40 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -2309,21 +2309,29 @@ static bool pte_map_lock(struct vm_fault *vmf)
> > >   * parts, do_swap_page must check under lock before unmapping the pte and
> > >   * proceeding (but do_wp_page is only called after already making such a 
> > > check;
> > >   * and do_anonymous_page can safely check later on).
> > > + *
> > > + * pte_unmap_same() returns:
> > > + *   0   if the PTE are the same
> > > + *   VM_FAULT_PTNOTSAME  if the PTE are different
> > > + *   VM_FAULT_RETRY  if the VMA has changed in our back 
> > > during
> > > + *   a speculative page fault handling.
> > >   */

[...]

> > >  
> > 
> > This change what do_swap_page() returns ie before it was returning 0
> > when locked pte lookup was different from orig_pte. After this patch
> > it returns VM_FAULT_PTNOTSAME but this is a new return value for
> > handle_mm_fault() (the do_swap_page() return value is what ultimately
> > get return by handle_mm_fault())
> > 
> > Do we really want that ? This might confuse some existing user of
> > handle_mm_fault() and i am not sure of the value of that information
> > to caller.
> > 
> > Note i do understand that you want to return retry if anything did
> > change from underneath and thus need to differentiate from when the
> > pte value are not the same.
> > 
> 
> I think VM_FAULT_RETRY should be handled appropriately for any user of 
> handle_mm_fault() already, and would be surprised to learn differently.  
> Khugepaged has the appropriate handling.  I think the concern is whether a 
> user is handling anything other than VM_FAULT_RETRY and VM_FAULT_ERROR 
> (which VM_FAULT_PTNOTSAME is not set in)?  I haven't done a full audit of 
> the users.

I am not worried about VM_FAULT_RETRY and barely have any worry about
VM_FAULT_PTNOTSAME either as they are other comparable new return value
(VM_FAULT_NEEDDSYNC for instance which is quite recent).

I wonder if adding a new value is really needed here. I don't see any
value to it for caller of handle_mm_fault() except for stats.

Note that I am not oppose, but while today we have free bits, maybe
tomorrow we will run out, i am always worried about thing like that :)

Cheers,
Jérôme

Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

[Jerome Glisse]

On Tue, Apr 03, 2018 at 01:40:18PM -0700, David Rientjes wrote:
> On Tue, 3 Apr 2018, Jerome Glisse wrote:
> 
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index 21b1212a0892..4bc7b0bdcb40 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -2309,21 +2309,29 @@ static bool pte_map_lock(struct vm_fault *vmf)
> > >   * parts, do_swap_page must check under lock before unmapping the pte and
> > >   * proceeding (but do_wp_page is only called after already making such a 
> > > check;
> > >   * and do_anonymous_page can safely check later on).
> > > + *
> > > + * pte_unmap_same() returns:
> > > + *   0   if the PTE are the same
> > > + *   VM_FAULT_PTNOTSAME  if the PTE are different
> > > + *   VM_FAULT_RETRY  if the VMA has changed in our back 
> > > during
> > > + *   a speculative page fault handling.
> > >   */

[...]

> > >  
> > 
> > This change what do_swap_page() returns ie before it was returning 0
> > when locked pte lookup was different from orig_pte. After this patch
> > it returns VM_FAULT_PTNOTSAME but this is a new return value for
> > handle_mm_fault() (the do_swap_page() return value is what ultimately
> > get return by handle_mm_fault())
> > 
> > Do we really want that ? This might confuse some existing user of
> > handle_mm_fault() and i am not sure of the value of that information
> > to caller.
> > 
> > Note i do understand that you want to return retry if anything did
> > change from underneath and thus need to differentiate from when the
> > pte value are not the same.
> > 
> 
> I think VM_FAULT_RETRY should be handled appropriately for any user of 
> handle_mm_fault() already, and would be surprised to learn differently.  
> Khugepaged has the appropriate handling.  I think the concern is whether a 
> user is handling anything other than VM_FAULT_RETRY and VM_FAULT_ERROR 
> (which VM_FAULT_PTNOTSAME is not set in)?  I haven't done a full audit of 
> the users.

I am not worried about VM_FAULT_RETRY and barely have any worry about
VM_FAULT_PTNOTSAME either as they are other comparable new return value
(VM_FAULT_NEEDDSYNC for instance which is quite recent).

I wonder if adding a new value is really needed here. I don't see any
value to it for caller of handle_mm_fault() except for stats.

Note that I am not oppose, but while today we have free bits, maybe
tomorrow we will run out, i am always worried about thing like that :)

Cheers,
Jérôme

Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

[David Rientjes]

On Tue, 3 Apr 2018, Jerome Glisse wrote:

> > diff --git a/mm/memory.c b/mm/memory.c
> > index 21b1212a0892..4bc7b0bdcb40 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -2309,21 +2309,29 @@ static bool pte_map_lock(struct vm_fault *vmf)
> >   * parts, do_swap_page must check under lock before unmapping the pte and
> >   * proceeding (but do_wp_page is only called after already making such a 
> > check;
> >   * and do_anonymous_page can safely check later on).
> > + *
> > + * pte_unmap_same() returns:
> > + * 0   if the PTE are the same
> > + * VM_FAULT_PTNOTSAME  if the PTE are different
> > + * VM_FAULT_RETRY  if the VMA has changed in our back during
> > + * a speculative page fault handling.
> >   */
> > -static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
> > -   pte_t *page_table, pte_t orig_pte)
> > +static inline int pte_unmap_same(struct vm_fault *vmf)
> >  {
> > -   int same = 1;
> > +   int ret = 0;
> > +
> >  #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
> > if (sizeof(pte_t) > sizeof(unsigned long)) {
> > -   spinlock_t *ptl = pte_lockptr(mm, pmd);
> > -   spin_lock(ptl);
> > -   same = pte_same(*page_table, orig_pte);
> > -   spin_unlock(ptl);
> > +   if (pte_spinlock(vmf)) {
> > +   if (!pte_same(*vmf->pte, vmf->orig_pte))
> > +   ret = VM_FAULT_PTNOTSAME;
> > +   spin_unlock(vmf->ptl);
> > +   } else
> > +   ret = VM_FAULT_RETRY;
> > }
> >  #endif
> > -   pte_unmap(page_table);
> > -   return same;
> > +   pte_unmap(vmf->pte);
> > +   return ret;
> >  }
> >  
> >  static inline void cow_user_page(struct page *dst, struct page *src, 
> > unsigned long va, struct vm_area_struct *vma)
> > @@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
> > int exclusive = 0;
> > int ret = 0;
> >  
> > -   if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
> > +   ret = pte_unmap_same(vmf);
> > +   if (ret)
> > goto out;
> >  
> 
> This change what do_swap_page() returns ie before it was returning 0
> when locked pte lookup was different from orig_pte. After this patch
> it returns VM_FAULT_PTNOTSAME but this is a new return value for
> handle_mm_fault() (the do_swap_page() return value is what ultimately
> get return by handle_mm_fault())
> 
> Do we really want that ? This might confuse some existing user of
> handle_mm_fault() and i am not sure of the value of that information
> to caller.
> 
> Note i do understand that you want to return retry if anything did
> change from underneath and thus need to differentiate from when the
> pte value are not the same.
> 

I think VM_FAULT_RETRY should be handled appropriately for any user of 
handle_mm_fault() already, and would be surprised to learn differently.  
Khugepaged has the appropriate handling.  I think the concern is whether a 
user is handling anything other than VM_FAULT_RETRY and VM_FAULT_ERROR 
(which VM_FAULT_PTNOTSAME is not set in)?  I haven't done a full audit of 
the users.

Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

[David Rientjes]

On Tue, 3 Apr 2018, Jerome Glisse wrote:

> > diff --git a/mm/memory.c b/mm/memory.c
> > index 21b1212a0892..4bc7b0bdcb40 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -2309,21 +2309,29 @@ static bool pte_map_lock(struct vm_fault *vmf)
> >   * parts, do_swap_page must check under lock before unmapping the pte and
> >   * proceeding (but do_wp_page is only called after already making such a 
> > check;
> >   * and do_anonymous_page can safely check later on).
> > + *
> > + * pte_unmap_same() returns:
> > + * 0   if the PTE are the same
> > + * VM_FAULT_PTNOTSAME  if the PTE are different
> > + * VM_FAULT_RETRY  if the VMA has changed in our back during
> > + * a speculative page fault handling.
> >   */
> > -static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
> > -   pte_t *page_table, pte_t orig_pte)
> > +static inline int pte_unmap_same(struct vm_fault *vmf)
> >  {
> > -   int same = 1;
> > +   int ret = 0;
> > +
> >  #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
> > if (sizeof(pte_t) > sizeof(unsigned long)) {
> > -   spinlock_t *ptl = pte_lockptr(mm, pmd);
> > -   spin_lock(ptl);
> > -   same = pte_same(*page_table, orig_pte);
> > -   spin_unlock(ptl);
> > +   if (pte_spinlock(vmf)) {
> > +   if (!pte_same(*vmf->pte, vmf->orig_pte))
> > +   ret = VM_FAULT_PTNOTSAME;
> > +   spin_unlock(vmf->ptl);
> > +   } else
> > +   ret = VM_FAULT_RETRY;
> > }
> >  #endif
> > -   pte_unmap(page_table);
> > -   return same;
> > +   pte_unmap(vmf->pte);
> > +   return ret;
> >  }
> >  
> >  static inline void cow_user_page(struct page *dst, struct page *src, 
> > unsigned long va, struct vm_area_struct *vma)
> > @@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
> > int exclusive = 0;
> > int ret = 0;
> >  
> > -   if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
> > +   ret = pte_unmap_same(vmf);
> > +   if (ret)
> > goto out;
> >  
> 
> This change what do_swap_page() returns ie before it was returning 0
> when locked pte lookup was different from orig_pte. After this patch
> it returns VM_FAULT_PTNOTSAME but this is a new return value for
> handle_mm_fault() (the do_swap_page() return value is what ultimately
> get return by handle_mm_fault())
> 
> Do we really want that ? This might confuse some existing user of
> handle_mm_fault() and i am not sure of the value of that information
> to caller.
> 
> Note i do understand that you want to return retry if anything did
> change from underneath and thus need to differentiate from when the
> pte value are not the same.
> 

I think VM_FAULT_RETRY should be handled appropriately for any user of 
handle_mm_fault() already, and would be surprised to learn differently.  
Khugepaged has the appropriate handling.  I think the concern is whether a 
user is handling anything other than VM_FAULT_RETRY and VM_FAULT_ERROR 
(which VM_FAULT_PTNOTSAME is not set in)?  I haven't done a full audit of 
the users.

Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

[Jerome Glisse]

On Tue, Mar 13, 2018 at 06:59:36PM +0100, Laurent Dufour 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.
> 
> Signed-off-by: Laurent Dufour 
> ---
>  include/linux/mm.h |  1 +
>  mm/memory.c| 29 +++--
>  2 files changed, 20 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 2f3e98edc94a..b6432a261e63 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1199,6 +1199,7 @@ 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 */
>  
>  #define VM_FAULT_ERROR   (VM_FAULT_OOM | VM_FAULT_SIGBUS | 
> VM_FAULT_SIGSEGV | \
>VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE | \
> diff --git a/mm/memory.c b/mm/memory.c
> index 21b1212a0892..4bc7b0bdcb40 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2309,21 +2309,29 @@ static bool pte_map_lock(struct vm_fault *vmf)
>   * parts, do_swap_page must check under lock before unmapping the pte and
>   * proceeding (but do_wp_page is only called after already making such a 
> check;
>   * and do_anonymous_page can safely check later on).
> + *
> + * pte_unmap_same() returns:
> + *   0   if the PTE are the same
> + *   VM_FAULT_PTNOTSAME  if the PTE are different
> + *   VM_FAULT_RETRY  if the VMA has changed in our back during
> + *   a speculative page fault handling.
>   */
> -static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
> - pte_t *page_table, pte_t orig_pte)
> +static inline int pte_unmap_same(struct vm_fault *vmf)
>  {
> - int same = 1;
> + int ret = 0;
> +
>  #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
>   if (sizeof(pte_t) > sizeof(unsigned long)) {
> - spinlock_t *ptl = pte_lockptr(mm, pmd);
> - spin_lock(ptl);
> - same = pte_same(*page_table, orig_pte);
> - spin_unlock(ptl);
> + if (pte_spinlock(vmf)) {
> + if (!pte_same(*vmf->pte, vmf->orig_pte))
> + ret = VM_FAULT_PTNOTSAME;
> + spin_unlock(vmf->ptl);
> + } else
> + ret = VM_FAULT_RETRY;
>   }
>  #endif
> - pte_unmap(page_table);
> - return same;
> + pte_unmap(vmf->pte);
> + return ret;
>  }
>  
>  static inline void cow_user_page(struct page *dst, struct page *src, 
> unsigned long va, struct vm_area_struct *vma)
> @@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
>   int exclusive = 0;
>   int ret = 0;
>  
> - if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
> + ret = pte_unmap_same(vmf);
> + if (ret)
>   goto out;
>  

This change what do_swap_page() returns ie before it was returning 0
when locked pte lookup was different from orig_pte. After this patch
it returns VM_FAULT_PTNOTSAME but this is a new return value for
handle_mm_fault() (the do_swap_page() return value is what ultimately
get return by handle_mm_fault())

Do we really want that ? This might confuse some existing user of
handle_mm_fault() and i am not sure of the value of that information
to caller.

Note i do understand that you want to return retry if anything did
change from underneath and thus need to differentiate from when the
pte value are not the same.

Cheers,
Jérôme

Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

[Jerome Glisse]

On Tue, Mar 13, 2018 at 06:59:36PM +0100, Laurent Dufour 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.
> 
> Signed-off-by: Laurent Dufour 
> ---
>  include/linux/mm.h |  1 +
>  mm/memory.c| 29 +++--
>  2 files changed, 20 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 2f3e98edc94a..b6432a261e63 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1199,6 +1199,7 @@ 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 */
>  
>  #define VM_FAULT_ERROR   (VM_FAULT_OOM | VM_FAULT_SIGBUS | 
> VM_FAULT_SIGSEGV | \
>VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE | \
> diff --git a/mm/memory.c b/mm/memory.c
> index 21b1212a0892..4bc7b0bdcb40 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2309,21 +2309,29 @@ static bool pte_map_lock(struct vm_fault *vmf)
>   * parts, do_swap_page must check under lock before unmapping the pte and
>   * proceeding (but do_wp_page is only called after already making such a 
> check;
>   * and do_anonymous_page can safely check later on).
> + *
> + * pte_unmap_same() returns:
> + *   0   if the PTE are the same
> + *   VM_FAULT_PTNOTSAME  if the PTE are different
> + *   VM_FAULT_RETRY  if the VMA has changed in our back during
> + *   a speculative page fault handling.
>   */
> -static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
> - pte_t *page_table, pte_t orig_pte)
> +static inline int pte_unmap_same(struct vm_fault *vmf)
>  {
> - int same = 1;
> + int ret = 0;
> +
>  #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
>   if (sizeof(pte_t) > sizeof(unsigned long)) {
> - spinlock_t *ptl = pte_lockptr(mm, pmd);
> - spin_lock(ptl);
> - same = pte_same(*page_table, orig_pte);
> - spin_unlock(ptl);
> + if (pte_spinlock(vmf)) {
> + if (!pte_same(*vmf->pte, vmf->orig_pte))
> + ret = VM_FAULT_PTNOTSAME;
> + spin_unlock(vmf->ptl);
> + } else
> + ret = VM_FAULT_RETRY;
>   }
>  #endif
> - pte_unmap(page_table);
> - return same;
> + pte_unmap(vmf->pte);
> + return ret;
>  }
>  
>  static inline void cow_user_page(struct page *dst, struct page *src, 
> unsigned long va, struct vm_area_struct *vma)
> @@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
>   int exclusive = 0;
>   int ret = 0;
>  
> - if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
> + ret = pte_unmap_same(vmf);
> + if (ret)
>   goto out;
>  

This change what do_swap_page() returns ie before it was returning 0
when locked pte lookup was different from orig_pte. After this patch
it returns VM_FAULT_PTNOTSAME but this is a new return value for
handle_mm_fault() (the do_swap_page() return value is what ultimately
get return by handle_mm_fault())

Do we really want that ? This might confuse some existing user of
handle_mm_fault() and i am not sure of the value of that information
to caller.

Note i do understand that you want to return retry if anything did
change from underneath and thus need to differentiate from when the
pte value are not the same.

Cheers,
Jérôme

Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

[Laurent Dufour]

On 28/03/2018 12:20, David Rientjes wrote:
> On Wed, 28 Mar 2018, Laurent Dufour wrote:
> 
 @@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
int exclusive = 0;
int ret = 0;
>>>
>>> Initialization is now unneeded.
>>
>> I'm sorry, what "initialization" are you talking about here ?
>>
> 
> The initialization of the ret variable.
> 
> @@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
>   int exclusive = 0;
>   int ret = 0;
> 
> - if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
> + ret = pte_unmap_same(vmf);
> + if (ret)
>   goto out;
> 
>   entry = pte_to_swp_entry(vmf->orig_pte);
> 
> "ret" is immediately set to the return value of pte_unmap_same(), so there 
> is no need to initialize it to 0.

Sorry, I missed that. I'll remove this initialization.

Thanks,
Laurent.

Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

[Laurent Dufour]

On 28/03/2018 12:20, David Rientjes wrote:
> On Wed, 28 Mar 2018, Laurent Dufour wrote:
> 
 @@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
int exclusive = 0;
int ret = 0;
>>>
>>> Initialization is now unneeded.
>>
>> I'm sorry, what "initialization" are you talking about here ?
>>
> 
> The initialization of the ret variable.
> 
> @@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
>   int exclusive = 0;
>   int ret = 0;
> 
> - if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
> + ret = pte_unmap_same(vmf);
> + if (ret)
>   goto out;
> 
>   entry = pte_to_swp_entry(vmf->orig_pte);
> 
> "ret" is immediately set to the return value of pte_unmap_same(), so there 
> is no need to initialize it to 0.

Sorry, I missed that. I'll remove this initialization.

Thanks,
Laurent.

Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

[David Rientjes]

On Wed, 28 Mar 2018, Laurent Dufour wrote:

> >> @@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
> >>int exclusive = 0;
> >>int ret = 0;
> > 
> > Initialization is now unneeded.
> 
> I'm sorry, what "initialization" are you talking about here ?
> 

The initialization of the ret variable.

@@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
int exclusive = 0;
int ret = 0;
 
-   if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
+   ret = pte_unmap_same(vmf);
+   if (ret)
goto out;
 
entry = pte_to_swp_entry(vmf->orig_pte);

"ret" is immediately set to the return value of pte_unmap_same(), so there 
is no need to initialize it to 0.

Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

[David Rientjes]

On Wed, 28 Mar 2018, Laurent Dufour wrote:

> >> @@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
> >>int exclusive = 0;
> >>int ret = 0;
> > 
> > Initialization is now unneeded.
> 
> I'm sorry, what "initialization" are you talking about here ?
> 

The initialization of the ret variable.

@@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
int exclusive = 0;
int ret = 0;
 
-   if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
+   ret = pte_unmap_same(vmf);
+   if (ret)
goto out;
 
entry = pte_to_swp_entry(vmf->orig_pte);

"ret" is immediately set to the return value of pte_unmap_same(), so there 
is no need to initialize it to 0.

Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

[Laurent Dufour]

On 27/03/2018 23:18, David Rientjes wrote:
> On Tue, 13 Mar 2018, Laurent Dufour wrote:
> 
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 2f3e98edc94a..b6432a261e63 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -1199,6 +1199,7 @@ 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 */
>>  
>>  #define VM_FAULT_ERROR  (VM_FAULT_OOM | VM_FAULT_SIGBUS | 
>> VM_FAULT_SIGSEGV | \
>>   VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE | \
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 21b1212a0892..4bc7b0bdcb40 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -2309,21 +2309,29 @@ static bool pte_map_lock(struct vm_fault *vmf)
>>   * parts, do_swap_page must check under lock before unmapping the pte and
>>   * proceeding (but do_wp_page is only called after already making such a 
>> check;
>>   * and do_anonymous_page can safely check later on).
>> + *
>> + * pte_unmap_same() returns:
>> + *  0   if the PTE are the same
>> + *  VM_FAULT_PTNOTSAME  if the PTE are different
>> + *  VM_FAULT_RETRY  if the VMA has changed in our back during
>> + *  a speculative page fault handling.
>>   */
>> -static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
>> -pte_t *page_table, pte_t orig_pte)
>> +static inline int pte_unmap_same(struct vm_fault *vmf)
>>  {
>> -int same = 1;
>> +int ret = 0;
>> +
>>  #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
>>  if (sizeof(pte_t) > sizeof(unsigned long)) {
>> -spinlock_t *ptl = pte_lockptr(mm, pmd);
>> -spin_lock(ptl);
>> -same = pte_same(*page_table, orig_pte);
>> -spin_unlock(ptl);
>> +if (pte_spinlock(vmf)) {
>> +if (!pte_same(*vmf->pte, vmf->orig_pte))
>> +ret = VM_FAULT_PTNOTSAME;
>> +spin_unlock(vmf->ptl);
>> +} else
>> +ret = VM_FAULT_RETRY;
>>  }
>>  #endif
>> -pte_unmap(page_table);
>> -return same;
>> +pte_unmap(vmf->pte);
>> +return ret;
>>  }
>>  
>>  static inline void cow_user_page(struct page *dst, struct page *src, 
>> unsigned long va, struct vm_area_struct *vma)
>> @@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
>>  int exclusive = 0;
>>  int ret = 0;
> 
> Initialization is now unneeded.

I'm sorry, what "initialization" are you talking about here ?

> 
> Otherwise:
> 
> Acked-by: David Rientjes 

Thanks,
Laurent.

Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

[Laurent Dufour]

On 27/03/2018 23:18, David Rientjes wrote:
> On Tue, 13 Mar 2018, Laurent Dufour wrote:
> 
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 2f3e98edc94a..b6432a261e63 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -1199,6 +1199,7 @@ 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 */
>>  
>>  #define VM_FAULT_ERROR  (VM_FAULT_OOM | VM_FAULT_SIGBUS | 
>> VM_FAULT_SIGSEGV | \
>>   VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE | \
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 21b1212a0892..4bc7b0bdcb40 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -2309,21 +2309,29 @@ static bool pte_map_lock(struct vm_fault *vmf)
>>   * parts, do_swap_page must check under lock before unmapping the pte and
>>   * proceeding (but do_wp_page is only called after already making such a 
>> check;
>>   * and do_anonymous_page can safely check later on).
>> + *
>> + * pte_unmap_same() returns:
>> + *  0   if the PTE are the same
>> + *  VM_FAULT_PTNOTSAME  if the PTE are different
>> + *  VM_FAULT_RETRY  if the VMA has changed in our back during
>> + *  a speculative page fault handling.
>>   */
>> -static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
>> -pte_t *page_table, pte_t orig_pte)
>> +static inline int pte_unmap_same(struct vm_fault *vmf)
>>  {
>> -int same = 1;
>> +int ret = 0;
>> +
>>  #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
>>  if (sizeof(pte_t) > sizeof(unsigned long)) {
>> -spinlock_t *ptl = pte_lockptr(mm, pmd);
>> -spin_lock(ptl);
>> -same = pte_same(*page_table, orig_pte);
>> -spin_unlock(ptl);
>> +if (pte_spinlock(vmf)) {
>> +if (!pte_same(*vmf->pte, vmf->orig_pte))
>> +ret = VM_FAULT_PTNOTSAME;
>> +spin_unlock(vmf->ptl);
>> +} else
>> +ret = VM_FAULT_RETRY;
>>  }
>>  #endif
>> -pte_unmap(page_table);
>> -return same;
>> +pte_unmap(vmf->pte);
>> +return ret;
>>  }
>>  
>>  static inline void cow_user_page(struct page *dst, struct page *src, 
>> unsigned long va, struct vm_area_struct *vma)
>> @@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
>>  int exclusive = 0;
>>  int ret = 0;
> 
> Initialization is now unneeded.

I'm sorry, what "initialization" are you talking about here ?

> 
> Otherwise:
> 
> Acked-by: David Rientjes 

Thanks,
Laurent.

Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

[David Rientjes]

On Tue, 13 Mar 2018, Laurent Dufour wrote:

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 2f3e98edc94a..b6432a261e63 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1199,6 +1199,7 @@ 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 */
>  
>  #define VM_FAULT_ERROR   (VM_FAULT_OOM | VM_FAULT_SIGBUS | 
> VM_FAULT_SIGSEGV | \
>VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE | \
> diff --git a/mm/memory.c b/mm/memory.c
> index 21b1212a0892..4bc7b0bdcb40 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2309,21 +2309,29 @@ static bool pte_map_lock(struct vm_fault *vmf)
>   * parts, do_swap_page must check under lock before unmapping the pte and
>   * proceeding (but do_wp_page is only called after already making such a 
> check;
>   * and do_anonymous_page can safely check later on).
> + *
> + * pte_unmap_same() returns:
> + *   0   if the PTE are the same
> + *   VM_FAULT_PTNOTSAME  if the PTE are different
> + *   VM_FAULT_RETRY  if the VMA has changed in our back during
> + *   a speculative page fault handling.
>   */
> -static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
> - pte_t *page_table, pte_t orig_pte)
> +static inline int pte_unmap_same(struct vm_fault *vmf)
>  {
> - int same = 1;
> + int ret = 0;
> +
>  #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
>   if (sizeof(pte_t) > sizeof(unsigned long)) {
> - spinlock_t *ptl = pte_lockptr(mm, pmd);
> - spin_lock(ptl);
> - same = pte_same(*page_table, orig_pte);
> - spin_unlock(ptl);
> + if (pte_spinlock(vmf)) {
> + if (!pte_same(*vmf->pte, vmf->orig_pte))
> + ret = VM_FAULT_PTNOTSAME;
> + spin_unlock(vmf->ptl);
> + } else
> + ret = VM_FAULT_RETRY;
>   }
>  #endif
> - pte_unmap(page_table);
> - return same;
> + pte_unmap(vmf->pte);
> + return ret;
>  }
>  
>  static inline void cow_user_page(struct page *dst, struct page *src, 
> unsigned long va, struct vm_area_struct *vma)
> @@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
>   int exclusive = 0;
>   int ret = 0;

Initialization is now unneeded.

Otherwise:

Acked-by: David Rientjes 

Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

[David Rientjes]

On Tue, 13 Mar 2018, Laurent Dufour wrote:

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 2f3e98edc94a..b6432a261e63 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1199,6 +1199,7 @@ 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 */
>  
>  #define VM_FAULT_ERROR   (VM_FAULT_OOM | VM_FAULT_SIGBUS | 
> VM_FAULT_SIGSEGV | \
>VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE | \
> diff --git a/mm/memory.c b/mm/memory.c
> index 21b1212a0892..4bc7b0bdcb40 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2309,21 +2309,29 @@ static bool pte_map_lock(struct vm_fault *vmf)
>   * parts, do_swap_page must check under lock before unmapping the pte and
>   * proceeding (but do_wp_page is only called after already making such a 
> check;
>   * and do_anonymous_page can safely check later on).
> + *
> + * pte_unmap_same() returns:
> + *   0   if the PTE are the same
> + *   VM_FAULT_PTNOTSAME  if the PTE are different
> + *   VM_FAULT_RETRY  if the VMA has changed in our back during
> + *   a speculative page fault handling.
>   */
> -static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
> - pte_t *page_table, pte_t orig_pte)
> +static inline int pte_unmap_same(struct vm_fault *vmf)
>  {
> - int same = 1;
> + int ret = 0;
> +
>  #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
>   if (sizeof(pte_t) > sizeof(unsigned long)) {
> - spinlock_t *ptl = pte_lockptr(mm, pmd);
> - spin_lock(ptl);
> - same = pte_same(*page_table, orig_pte);
> - spin_unlock(ptl);
> + if (pte_spinlock(vmf)) {
> + if (!pte_same(*vmf->pte, vmf->orig_pte))
> + ret = VM_FAULT_PTNOTSAME;
> + spin_unlock(vmf->ptl);
> + } else
> + ret = VM_FAULT_RETRY;
>   }
>  #endif
> - pte_unmap(page_table);
> - return same;
> + pte_unmap(vmf->pte);
> + return ret;
>  }
>  
>  static inline void cow_user_page(struct page *dst, struct page *src, 
> unsigned long va, struct vm_area_struct *vma)
> @@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
>   int exclusive = 0;
>   int ret = 0;

Initialization is now unneeded.

Otherwise:

Acked-by: David Rientjes 

[PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

[Laurent Dufour]

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.

Signed-off-by: Laurent Dufour 
---
 include/linux/mm.h |  1 +
 mm/memory.c| 29 +++--
 2 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 2f3e98edc94a..b6432a261e63 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1199,6 +1199,7 @@ 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 */
 
 #define VM_FAULT_ERROR (VM_FAULT_OOM | VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | \
 VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE | \
diff --git a/mm/memory.c b/mm/memory.c
index 21b1212a0892..4bc7b0bdcb40 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2309,21 +2309,29 @@ static bool pte_map_lock(struct vm_fault *vmf)
  * parts, do_swap_page must check under lock before unmapping the pte and
  * proceeding (but do_wp_page is only called after already making such a check;
  * and do_anonymous_page can safely check later on).
+ *
+ * pte_unmap_same() returns:
+ * 0   if the PTE are the same
+ * VM_FAULT_PTNOTSAME  if the PTE are different
+ * VM_FAULT_RETRY  if the VMA has changed in our back during
+ * a speculative page fault handling.
  */
-static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
-   pte_t *page_table, pte_t orig_pte)
+static inline int pte_unmap_same(struct vm_fault *vmf)
 {
-   int same = 1;
+   int ret = 0;
+
 #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
if (sizeof(pte_t) > sizeof(unsigned long)) {
-   spinlock_t *ptl = pte_lockptr(mm, pmd);
-   spin_lock(ptl);
-   same = pte_same(*page_table, orig_pte);
-   spin_unlock(ptl);
+   if (pte_spinlock(vmf)) {
+   if (!pte_same(*vmf->pte, vmf->orig_pte))
+   ret = VM_FAULT_PTNOTSAME;
+   spin_unlock(vmf->ptl);
+   } else
+   ret = VM_FAULT_RETRY;
}
 #endif
-   pte_unmap(page_table);
-   return same;
+   pte_unmap(vmf->pte);
+   return ret;
 }
 
 static inline void cow_user_page(struct page *dst, struct page *src, unsigned 
long va, struct vm_area_struct *vma)
@@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
int exclusive = 0;
int ret = 0;
 
-   if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
+   ret = pte_unmap_same(vmf);
+   if (ret)
goto out;
 
entry = pte_to_swp_entry(vmf->orig_pte);
-- 
2.7.4

[PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

[Laurent Dufour]

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.

Signed-off-by: Laurent Dufour 
---
 include/linux/mm.h |  1 +
 mm/memory.c| 29 +++--
 2 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 2f3e98edc94a..b6432a261e63 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1199,6 +1199,7 @@ 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 */
 
 #define VM_FAULT_ERROR (VM_FAULT_OOM | VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | \
 VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE | \
diff --git a/mm/memory.c b/mm/memory.c
index 21b1212a0892..4bc7b0bdcb40 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2309,21 +2309,29 @@ static bool pte_map_lock(struct vm_fault *vmf)
  * parts, do_swap_page must check under lock before unmapping the pte and
  * proceeding (but do_wp_page is only called after already making such a check;
  * and do_anonymous_page can safely check later on).
+ *
+ * pte_unmap_same() returns:
+ * 0   if the PTE are the same
+ * VM_FAULT_PTNOTSAME  if the PTE are different
+ * VM_FAULT_RETRY  if the VMA has changed in our back during
+ * a speculative page fault handling.
  */
-static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
-   pte_t *page_table, pte_t orig_pte)
+static inline int pte_unmap_same(struct vm_fault *vmf)
 {
-   int same = 1;
+   int ret = 0;
+
 #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
if (sizeof(pte_t) > sizeof(unsigned long)) {
-   spinlock_t *ptl = pte_lockptr(mm, pmd);
-   spin_lock(ptl);
-   same = pte_same(*page_table, orig_pte);
-   spin_unlock(ptl);
+   if (pte_spinlock(vmf)) {
+   if (!pte_same(*vmf->pte, vmf->orig_pte))
+   ret = VM_FAULT_PTNOTSAME;
+   spin_unlock(vmf->ptl);
+   } else
+   ret = VM_FAULT_RETRY;
}
 #endif
-   pte_unmap(page_table);
-   return same;
+   pte_unmap(vmf->pte);
+   return ret;
 }
 
 static inline void cow_user_page(struct page *dst, struct page *src, unsigned 
long va, struct vm_area_struct *vma)
@@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
int exclusive = 0;
int ret = 0;
 
-   if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
+   ret = pte_unmap_same(vmf);
+   if (ret)
goto out;
 
entry = pte_to_swp_entry(vmf->orig_pte);
-- 
2.7.4