[PATCH] arm64: mm: Add pgd_page to support RCU fast_gup

[Jungseok Lee]

On Dec 22, 2014, at 9:30 PM, Catalin Marinas wrote:
> On Sun, Dec 21, 2014 at 10:56:33AM +0000, Will Deacon wrote:
>> On Sat, Dec 20, 2014 at 12:49:40AM +0000, Jungseok Lee wrote:
>>> This patch adds pgd_page definition in order to keep supporting
>>> HAVE_GENERIC_RCU_GUP configuration. In addition, it changes pud_page
>>> expression to align with pmd_page for readability.
>>> 
>>> An introduction of pgd_page resolves the following build breakage
>>> under 4KB + 4Level memory management combo.
>>> 
>>> mm/gup.c: In function 'gup_huge_pgd':
>>> mm/gup.c:889:2: error: implicit declaration of function 'pgd_page' [-Werror=implicit-function-declaration]
>>>  head = pgd_page(orig);
>>>  ^
>>> mm/gup.c:889:7: warning: assignment makes pointer from integer without a cast
>>>  head = pgd_page(orig);
>>> 
>>> Cc: Catalin Marinas <catalin.marinas at arm.com>
>>> Cc: Will Deacon <will.deacon at arm.com>
>>> Cc: Steve Capper <steve.capper at linaro.org>
>>> Signed-off-by: Jungseok Lee <jungseoklee85 at gmail.com>
>>> ---
>>> arch/arm64/include/asm/pgtable.h | 4 +++-
>>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>> 
>>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>>> index df22314..a1fe927 100644
>>> --- a/arch/arm64/include/asm/pgtable.h
>>> +++ b/arch/arm64/include/asm/pgtable.h
>>> @@ -401,7 +401,7 @@ static inline pmd_t *pmd_offset(pud_t *pud, unsigned long addr)
>>> 	return (pmd_t *)pud_page_vaddr(*pud) + pmd_index(addr);
>>> }
>>> 
>>> -#define pud_page(pud)           pmd_page(pud_pmd(pud))
>>> +#define pud_page(pud)		pfn_to_page(__phys_to_pfn(pud_val(pud) & PHYS_MASK))
>> 
>> It would be cleaner to use pud_pfn here, otherwise we end up passing a
>> physical address with the lower attributes present to __phys_to_pfn, which
>> "knows" to shift them away.
> 
> It looks like we did the same with pmd_page() but not with pte_page(),
> the latter using pte_pfn().
> 
> It's not a problem with lower attributes because they are within the
> first 12 bits, so the shifting gets rid of them. For pmd/pud/pgd, bits
> above 12 and up to the actual address must be 0.
> 
> I agree with changing all of them to use pte/pmd/pud/pgd_pfn but I'm
> inclined to merge this patch now to fix the kernel build and we'll look
> at the clean-up after Christmas.

Either way, I'm fine.

Best Regards
Jungseok Lee