Re: [PATCH 4/6] powerpc/mm: Add devmap support for ppc64

[Oliver O'Halloran]

On Tue, May 23, 2017 at 8:40 PM, Balbir Singh  wrote:
> On Tue, May 23, 2017 at 2:05 PM, Oliver O'Halloran  wrote:
>> Add support for the devmap bit on PTEs and PMDs for PPC64 Book3S.  This
>> is used to differentiate device backed memory from transparent huge
>> pages since they are handled in more or less the same manner by the core
>> mm code.
>>
>> Cc: Aneesh Kumar K.V 
>> Signed-off-by: Oliver O'Halloran 
>> ---
>> v1 -> v2: Properly differentiate THP and PMD Devmap entries. The
>> mm core assumes that pmd_trans_huge() and pmd_devmap() are mutually
>> exclusive and v1 had pmd_trans_huge() being true on a devmap pmd.
>>
>> Aneesh, this has been fleshed out substantially since v1. Can you
>> re-review it? Also no explicit gup support is required in this patch
>> since devmap support was added generic GUP as a part of making x86 use
>> the generic version.
>> ---
>>  arch/powerpc/include/asm/book3s/64/hash-64k.h |  2 +-
>>  arch/powerpc/include/asm/book3s/64/pgtable.h  | 37 
>> ++-
>>  arch/powerpc/include/asm/book3s/64/radix.h|  2 +-
>>  arch/powerpc/mm/hugetlbpage.c |  2 +-
>>  arch/powerpc/mm/pgtable-book3s64.c|  4 +--
>>  arch/powerpc/mm/pgtable-hash64.c  |  4 ++-
>>  arch/powerpc/mm/pgtable-radix.c   |  3 ++-
>>  arch/powerpc/mm/pgtable_64.c  |  2 +-
>>  8 files changed, 47 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/powerpc/include/asm/book3s/64/hash-64k.h 
>> b/arch/powerpc/include/asm/book3s/64/hash-64k.h
>> index 9732837aaae8..eaaf613c5347 100644
>> --- a/arch/powerpc/include/asm/book3s/64/hash-64k.h
>> +++ b/arch/powerpc/include/asm/book3s/64/hash-64k.h
>> @@ -180,7 +180,7 @@ static inline void mark_hpte_slot_valid(unsigned char 
>> *hpte_slot_array,
>>   */
>>  static inline int hash__pmd_trans_huge(pmd_t pmd)
>>  {
>> -   return !!((pmd_val(pmd) & (_PAGE_PTE | H_PAGE_THP_HUGE)) ==
>> +   return !!((pmd_val(pmd) & (_PAGE_PTE | H_PAGE_THP_HUGE | 
>> _PAGE_DEVMAP)) ==
>>   (_PAGE_PTE | H_PAGE_THP_HUGE));
>>  }
>
> Like Aneesh suggested, I think we can probably skip this check here
>
>>
>> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h 
>> b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> index 85bc9875c3be..24634e92dd0b 100644
>> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
>> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> @@ -79,6 +79,9 @@
>>
>>  #define _PAGE_SOFT_DIRTY   _RPAGE_SW3 /* software: software dirty 
>> tracking */
>>  #define _PAGE_SPECIAL  _RPAGE_SW2 /* software: special page */
>> +#define _PAGE_DEVMAP   _RPAGE_SW1
>> +#define __HAVE_ARCH_PTE_DEVMAP
>> +
>>  /*
>>   * Drivers request for cache inhibited pte mapping using _PAGE_NO_CACHE
>>   * Instead of fixing all of them, add an alternate define which
>> @@ -599,6 +602,16 @@ static inline pte_t pte_mkhuge(pte_t pte)
>> return pte;
>>  }
>>
>> +static inline pte_t pte_mkdevmap(pte_t pte)
>> +{
>> +   return __pte(pte_val(pte) | _PAGE_SPECIAL|_PAGE_DEVMAP);
>> +}
>> +
>> +static inline int pte_devmap(pte_t pte)
>> +{
>> +   return !!(pte_raw(pte) & cpu_to_be64(_PAGE_DEVMAP));
>> +}
>> +
>>  static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
>>  {
>> /* FIXME!! check whether this need to be a conditional */
>> @@ -963,6 +976,9 @@ static inline pte_t *pmdp_ptep(pmd_t *pmd)
>>  #define pmd_mk_savedwrite(pmd) pte_pmd(pte_mk_savedwrite(pmd_pte(pmd)))
>>  #define pmd_clear_savedwrite(pmd)  
>> pte_pmd(pte_clear_savedwrite(pmd_pte(pmd)))
>>
>> +#define pud_pfn(...) (0)
>> +#define pgd_pfn(...) (0)
>> +
>
> Don't get these bits.. why are they zero?

I think that was just hacking stuff until it worked. pud_pfn() needs
to exist for the kernel to build when __HAVE_ARCH_PTE_DEVMAP is set,
but we don't need it to do anything (yet) since pud_pfn() is only used
for handing devmap PUD faults. We currently support those so we will
never hit that code path. pgd_pfn() can die though.

>>  #ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
>>  #define pmd_soft_dirty(pmd)pte_soft_dirty(pmd_pte(pmd))
>>  #define pmd_mksoft_dirty(pmd)  pte_pmd(pte_mksoft_dirty(pmd_pte(pmd)))
>> @@ -1137,7 +1153,6 @@ static inline int pmd_move_must_withdraw(struct 
>> spinlock *new_pmd_ptl,
>> return true;
>>  }
>>
>> -
>>  #define arch_needs_pgtable_deposit arch_needs_pgtable_deposit
>>  static inline bool arch_needs_pgtable_deposit(void)
>>  {
>> @@ -1146,6 +1161,26 @@ static inline bool arch_needs_pgtable_deposit(void)
>> return true;
>>  }
>>
>> +static inline pmd_t pmd_mkdevmap(pmd_t pmd)
>> +{
>> +   return pte_pmd(pte_mkdevmap(pmd_pte(pmd)));
>> +}
>> +
>> +static inline int pmd_devmap(pmd_t pmd)
>> +{
>> +   return pte_devmap(pmd_pte(pmd));
>> +}
>
> This should be defined only if #ifdef __HAVE_ARCH_PTE_DEVMAP

ok

>
> The rest looks OK
>
> Balbir 

Re: [PATCH 4/6] powerpc/mm: Add devmap support for ppc64

[Balbir Singh]

On Tue, May 23, 2017 at 2:05 PM, Oliver O'Halloran  wrote:
> Add support for the devmap bit on PTEs and PMDs for PPC64 Book3S.  This
> is used to differentiate device backed memory from transparent huge
> pages since they are handled in more or less the same manner by the core
> mm code.
>
> Cc: Aneesh Kumar K.V 
> Signed-off-by: Oliver O'Halloran 
> ---
> v1 -> v2: Properly differentiate THP and PMD Devmap entries. The
> mm core assumes that pmd_trans_huge() and pmd_devmap() are mutually
> exclusive and v1 had pmd_trans_huge() being true on a devmap pmd.
>
> Aneesh, this has been fleshed out substantially since v1. Can you
> re-review it? Also no explicit gup support is required in this patch
> since devmap support was added generic GUP as a part of making x86 use
> the generic version.
> ---
>  arch/powerpc/include/asm/book3s/64/hash-64k.h |  2 +-
>  arch/powerpc/include/asm/book3s/64/pgtable.h  | 37 
> ++-
>  arch/powerpc/include/asm/book3s/64/radix.h|  2 +-
>  arch/powerpc/mm/hugetlbpage.c |  2 +-
>  arch/powerpc/mm/pgtable-book3s64.c|  4 +--
>  arch/powerpc/mm/pgtable-hash64.c  |  4 ++-
>  arch/powerpc/mm/pgtable-radix.c   |  3 ++-
>  arch/powerpc/mm/pgtable_64.c  |  2 +-
>  8 files changed, 47 insertions(+), 9 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/book3s/64/hash-64k.h 
> b/arch/powerpc/include/asm/book3s/64/hash-64k.h
> index 9732837aaae8..eaaf613c5347 100644
> --- a/arch/powerpc/include/asm/book3s/64/hash-64k.h
> +++ b/arch/powerpc/include/asm/book3s/64/hash-64k.h
> @@ -180,7 +180,7 @@ static inline void mark_hpte_slot_valid(unsigned char 
> *hpte_slot_array,
>   */
>  static inline int hash__pmd_trans_huge(pmd_t pmd)
>  {
> -   return !!((pmd_val(pmd) & (_PAGE_PTE | H_PAGE_THP_HUGE)) ==
> +   return !!((pmd_val(pmd) & (_PAGE_PTE | H_PAGE_THP_HUGE | 
> _PAGE_DEVMAP)) ==
>   (_PAGE_PTE | H_PAGE_THP_HUGE));
>  }

Like Aneesh suggested, I think we can probably skip this check here

>
> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h 
> b/arch/powerpc/include/asm/book3s/64/pgtable.h
> index 85bc9875c3be..24634e92dd0b 100644
> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
> @@ -79,6 +79,9 @@
>
>  #define _PAGE_SOFT_DIRTY   _RPAGE_SW3 /* software: software dirty 
> tracking */
>  #define _PAGE_SPECIAL  _RPAGE_SW2 /* software: special page */
> +#define _PAGE_DEVMAP   _RPAGE_SW1
> +#define __HAVE_ARCH_PTE_DEVMAP
> +
>  /*
>   * Drivers request for cache inhibited pte mapping using _PAGE_NO_CACHE
>   * Instead of fixing all of them, add an alternate define which
> @@ -599,6 +602,16 @@ static inline pte_t pte_mkhuge(pte_t pte)
> return pte;
>  }
>
> +static inline pte_t pte_mkdevmap(pte_t pte)
> +{
> +   return __pte(pte_val(pte) | _PAGE_SPECIAL|_PAGE_DEVMAP);
> +}
> +
> +static inline int pte_devmap(pte_t pte)
> +{
> +   return !!(pte_raw(pte) & cpu_to_be64(_PAGE_DEVMAP));
> +}
> +
>  static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
>  {
> /* FIXME!! check whether this need to be a conditional */
> @@ -963,6 +976,9 @@ static inline pte_t *pmdp_ptep(pmd_t *pmd)
>  #define pmd_mk_savedwrite(pmd) pte_pmd(pte_mk_savedwrite(pmd_pte(pmd)))
>  #define pmd_clear_savedwrite(pmd)  
> pte_pmd(pte_clear_savedwrite(pmd_pte(pmd)))
>
> +#define pud_pfn(...) (0)
> +#define pgd_pfn(...) (0)
> +

Don't get these bits.. why are they zero?

>  #ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
>  #define pmd_soft_dirty(pmd)pte_soft_dirty(pmd_pte(pmd))
>  #define pmd_mksoft_dirty(pmd)  pte_pmd(pte_mksoft_dirty(pmd_pte(pmd)))
> @@ -1137,7 +1153,6 @@ static inline int pmd_move_must_withdraw(struct 
> spinlock *new_pmd_ptl,
> return true;
>  }
>
> -
>  #define arch_needs_pgtable_deposit arch_needs_pgtable_deposit
>  static inline bool arch_needs_pgtable_deposit(void)
>  {
> @@ -1146,6 +1161,26 @@ static inline bool arch_needs_pgtable_deposit(void)
> return true;
>  }
>
> +static inline pmd_t pmd_mkdevmap(pmd_t pmd)
> +{
> +   return pte_pmd(pte_mkdevmap(pmd_pte(pmd)));
> +}
> +
> +static inline int pmd_devmap(pmd_t pmd)
> +{
> +   return pte_devmap(pmd_pte(pmd));
> +}

This should be defined only if #ifdef __HAVE_ARCH_PTE_DEVMAP

The rest looks OK

Balbir Singh.

Re: [PATCH 4/6] powerpc/mm: Add devmap support for ppc64

[Oliver O'Halloran]

On Tue, May 23, 2017 at 2:23 PM, Aneesh Kumar K.V
 wrote:
> Oliver O'Halloran  writes:
>
>> Add support for the devmap bit on PTEs and PMDs for PPC64 Book3S.  This
>> is used to differentiate device backed memory from transparent huge
>> pages since they are handled in more or less the same manner by the core
>> mm code.
>>
>> Cc: Aneesh Kumar K.V 
>> Signed-off-by: Oliver O'Halloran 
>> ---
>> v1 -> v2: Properly differentiate THP and PMD Devmap entries. The
>> mm core assumes that pmd_trans_huge() and pmd_devmap() are mutually
>> exclusive and v1 had pmd_trans_huge() being true on a devmap pmd.
>>
>> Aneesh, this has been fleshed out substantially since v1. Can you
>> re-review it? Also no explicit gup support is required in this patch
>> since devmap support was added generic GUP as a part of making x86 use
>> the generic version.
>> ---
>>  arch/powerpc/include/asm/book3s/64/hash-64k.h |  2 +-
>>  arch/powerpc/include/asm/book3s/64/pgtable.h  | 37 
>> ++-
>>  arch/powerpc/include/asm/book3s/64/radix.h|  2 +-
>>  arch/powerpc/mm/hugetlbpage.c |  2 +-
>>  arch/powerpc/mm/pgtable-book3s64.c|  4 +--
>>  arch/powerpc/mm/pgtable-hash64.c  |  4 ++-
>>  arch/powerpc/mm/pgtable-radix.c   |  3 ++-
>>  arch/powerpc/mm/pgtable_64.c  |  2 +-
>>  8 files changed, 47 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/powerpc/include/asm/book3s/64/hash-64k.h 
>> b/arch/powerpc/include/asm/book3s/64/hash-64k.h
>> index 9732837aaae8..eaaf613c5347 100644
>> --- a/arch/powerpc/include/asm/book3s/64/hash-64k.h
>> +++ b/arch/powerpc/include/asm/book3s/64/hash-64k.h
>> @@ -180,7 +180,7 @@ static inline void mark_hpte_slot_valid(unsigned char 
>> *hpte_slot_array,
>>   */
>>  static inline int hash__pmd_trans_huge(pmd_t pmd)
>>  {
>> - return !!((pmd_val(pmd) & (_PAGE_PTE | H_PAGE_THP_HUGE)) ==
>> + return !!((pmd_val(pmd) & (_PAGE_PTE | H_PAGE_THP_HUGE | 
>> _PAGE_DEVMAP)) ==
>> (_PAGE_PTE | H_PAGE_THP_HUGE));
>>  }
>
> _PAGE_DEVMAP is not really needed here. We will set H_PAGE_THP_HUGE only
> for thp hugepage w.r.t hash. But putting it here also makes it clear
> that devmap entries are not considered trans huge.

Good point. I'll remove it.

>>
>> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h 
>> b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> index 85bc9875c3be..24634e92dd0b 100644
>> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
>> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> @@ -79,6 +79,9 @@
>>
>>  #define _PAGE_SOFT_DIRTY _RPAGE_SW3 /* software: software dirty 
>> tracking */
>>  #define _PAGE_SPECIAL_RPAGE_SW2 /* software: special page */
>> +#define _PAGE_DEVMAP _RPAGE_SW1
>> +#define __HAVE_ARCH_PTE_DEVMAP
>> +
>>  /*
>>   * Drivers request for cache inhibited pte mapping using _PAGE_NO_CACHE
>>   * Instead of fixing all of them, add an alternate define which
>> @@ -599,6 +602,16 @@ static inline pte_t pte_mkhuge(pte_t pte)
>>   return pte;
>>  }
>>
>> +static inline pte_t pte_mkdevmap(pte_t pte)
>> +{
>> + return __pte(pte_val(pte) | _PAGE_SPECIAL|_PAGE_DEVMAP);
>> +}
>> +
>> +static inline int pte_devmap(pte_t pte)
>> +{
>> + return !!(pte_raw(pte) & cpu_to_be64(_PAGE_DEVMAP));
>> +}
>> +
>>  static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
>>  {
>>   /* FIXME!! check whether this need to be a conditional */
>> @@ -963,6 +976,9 @@ static inline pte_t *pmdp_ptep(pmd_t *pmd)
>>  #define pmd_mk_savedwrite(pmd)   
>> pte_pmd(pte_mk_savedwrite(pmd_pte(pmd)))
>>  #define pmd_clear_savedwrite(pmd)
>> pte_pmd(pte_clear_savedwrite(pmd_pte(pmd)))
>>
>> +#define pud_pfn(...) (0)
>> +#define pgd_pfn(...) (0)
>> +
>>  #ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
>>  #define pmd_soft_dirty(pmd)pte_soft_dirty(pmd_pte(pmd))
>>  #define pmd_mksoft_dirty(pmd)  pte_pmd(pte_mksoft_dirty(pmd_pte(pmd)))
>> @@ -1137,7 +1153,6 @@ static inline int pmd_move_must_withdraw(struct 
>> spinlock *new_pmd_ptl,
>>   return true;
>>  }
>>
>> -
>>  #define arch_needs_pgtable_deposit arch_needs_pgtable_deposit
>>  static inline bool arch_needs_pgtable_deposit(void)
>>  {
>> @@ -1146,6 +1161,26 @@ static inline bool arch_needs_pgtable_deposit(void)
>>   return true;
>>  }
>>
>> +static inline pmd_t pmd_mkdevmap(pmd_t pmd)
>> +{
>> + return pte_pmd(pte_mkdevmap(pmd_pte(pmd)));
>> +}
>
>
> We avoided setting _PAGE_SPECIAL on pmd entries. This will set that, we
> may want to check if it is ok.  IIRC, we overloaded _PAGE_SPECIAL at some 
> point to indicate thp splitting. But good to double check.

I took a cursory look in arch/powerpc/ and mm/ for usages and didn't
see any usages of _PAGE_SPECIAL for pmds. There's no good reason to
set the flag though so I'll remove it.

>> +
>> +static inline int pmd_devmap(pmd_t pmd)
>> +{
>> + 

Re: [PATCH 4/6] powerpc/mm: Add devmap support for ppc64

[Aneesh Kumar K.V]

Oliver O'Halloran  writes:

> Add support for the devmap bit on PTEs and PMDs for PPC64 Book3S.  This
> is used to differentiate device backed memory from transparent huge
> pages since they are handled in more or less the same manner by the core
> mm code.
>
> Cc: Aneesh Kumar K.V 
> Signed-off-by: Oliver O'Halloran 
> ---
> v1 -> v2: Properly differentiate THP and PMD Devmap entries. The
> mm core assumes that pmd_trans_huge() and pmd_devmap() are mutually
> exclusive and v1 had pmd_trans_huge() being true on a devmap pmd.
>
> Aneesh, this has been fleshed out substantially since v1. Can you
> re-review it? Also no explicit gup support is required in this patch
> since devmap support was added generic GUP as a part of making x86 use
> the generic version.
> ---
>  arch/powerpc/include/asm/book3s/64/hash-64k.h |  2 +-
>  arch/powerpc/include/asm/book3s/64/pgtable.h  | 37 
> ++-
>  arch/powerpc/include/asm/book3s/64/radix.h|  2 +-
>  arch/powerpc/mm/hugetlbpage.c |  2 +-
>  arch/powerpc/mm/pgtable-book3s64.c|  4 +--
>  arch/powerpc/mm/pgtable-hash64.c  |  4 ++-
>  arch/powerpc/mm/pgtable-radix.c   |  3 ++-
>  arch/powerpc/mm/pgtable_64.c  |  2 +-
>  8 files changed, 47 insertions(+), 9 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/book3s/64/hash-64k.h 
> b/arch/powerpc/include/asm/book3s/64/hash-64k.h
> index 9732837aaae8..eaaf613c5347 100644
> --- a/arch/powerpc/include/asm/book3s/64/hash-64k.h
> +++ b/arch/powerpc/include/asm/book3s/64/hash-64k.h
> @@ -180,7 +180,7 @@ static inline void mark_hpte_slot_valid(unsigned char 
> *hpte_slot_array,
>   */
>  static inline int hash__pmd_trans_huge(pmd_t pmd)
>  {
> - return !!((pmd_val(pmd) & (_PAGE_PTE | H_PAGE_THP_HUGE)) ==
> + return !!((pmd_val(pmd) & (_PAGE_PTE | H_PAGE_THP_HUGE | _PAGE_DEVMAP)) 
> ==
> (_PAGE_PTE | H_PAGE_THP_HUGE));
>  }


_PAGE_DEVMAP is not really needed here. We will set H_PAGE_THP_HUGE only
for thp hugepage w.r.t hash. But putting it here also makes it clear
that devmap entries are not considered trans huge.

>
> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h 
> b/arch/powerpc/include/asm/book3s/64/pgtable.h
> index 85bc9875c3be..24634e92dd0b 100644
> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
> @@ -79,6 +79,9 @@
>
>  #define _PAGE_SOFT_DIRTY _RPAGE_SW3 /* software: software dirty tracking 
> */
>  #define _PAGE_SPECIAL_RPAGE_SW2 /* software: special page */
> +#define _PAGE_DEVMAP _RPAGE_SW1
> +#define __HAVE_ARCH_PTE_DEVMAP
> +
>  /*
>   * Drivers request for cache inhibited pte mapping using _PAGE_NO_CACHE
>   * Instead of fixing all of them, add an alternate define which
> @@ -599,6 +602,16 @@ static inline pte_t pte_mkhuge(pte_t pte)
>   return pte;
>  }
>
> +static inline pte_t pte_mkdevmap(pte_t pte)
> +{
> + return __pte(pte_val(pte) | _PAGE_SPECIAL|_PAGE_DEVMAP);
> +}
> +
> +static inline int pte_devmap(pte_t pte)
> +{
> + return !!(pte_raw(pte) & cpu_to_be64(_PAGE_DEVMAP));
> +}
> +
>  static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
>  {
>   /* FIXME!! check whether this need to be a conditional */
> @@ -963,6 +976,9 @@ static inline pte_t *pmdp_ptep(pmd_t *pmd)
>  #define pmd_mk_savedwrite(pmd)   pte_pmd(pte_mk_savedwrite(pmd_pte(pmd)))
>  #define pmd_clear_savedwrite(pmd)
> pte_pmd(pte_clear_savedwrite(pmd_pte(pmd)))
>
> +#define pud_pfn(...) (0)
> +#define pgd_pfn(...) (0)
> +
>  #ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
>  #define pmd_soft_dirty(pmd)pte_soft_dirty(pmd_pte(pmd))
>  #define pmd_mksoft_dirty(pmd)  pte_pmd(pte_mksoft_dirty(pmd_pte(pmd)))
> @@ -1137,7 +1153,6 @@ static inline int pmd_move_must_withdraw(struct 
> spinlock *new_pmd_ptl,
>   return true;
>  }
>
> -
>  #define arch_needs_pgtable_deposit arch_needs_pgtable_deposit
>  static inline bool arch_needs_pgtable_deposit(void)
>  {
> @@ -1146,6 +1161,26 @@ static inline bool arch_needs_pgtable_deposit(void)
>   return true;
>  }
>
> +static inline pmd_t pmd_mkdevmap(pmd_t pmd)
> +{
> + return pte_pmd(pte_mkdevmap(pmd_pte(pmd)));
> +}


We avoided setting _PAGE_SPECIAL on pmd entries. This will set that, we
may want to check if it is ok.  IIRC, we overloaded _PAGE_SPECIAL at
some point to indicate thp splitting. But good to double check. 

> +
> +static inline int pmd_devmap(pmd_t pmd)
> +{
> + return pte_devmap(pmd_pte(pmd));
> +}
> +
> +static inline int pud_devmap(pud_t pud)
> +{
> + return 0;
> +}
> +
> +static inline int pgd_devmap(pgd_t pgd)
> +{
> + return 0;
> +}
> +
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>  #endif /* __ASSEMBLY__ */
>  #endif /* _ASM_POWERPC_BOOK3S_64_PGTABLE_H_ */
> diff --git a/arch/powerpc/include/asm/book3s/64/radix.h 
> b/arch/powerpc/include/asm/book3s/64/radix.h
> index