Subject: Re: [PATCH V2 1/2] mm: Update generic gup implementation to handle hugepage directory

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Thu, 23 Oct 2014 13:38:30 +0530

Andrew Morton <[email-masked]> writes:

> On Fri, 17 Oct 2014 10:08:06 +0530 "Aneesh Kumar K.V" <[email-masked]> wrote:
>
>> Update generic gup implementation with powerpc specific details.
>> On powerpc at pmd level we can have hugepte, normal pmd pointer
>> or a pointer to the hugepage directory.
>> 
>> ...
>>
>> --- a/arch/arm/include/asm/pgtable.h
>> +++ b/arch/arm/include/asm/pgtable.h
>> @@ -181,6 +181,8 @@ extern pgd_t swapper_pg_dir[PTRS_PER_PGD];
>>  /* to find an entry in a kernel page-table-directory */
>>  #define pgd_offset_k(addr)	pgd_offset(&init_mm, addr)
>>  
>> +#define pgd_huge(pgd)		(0)
>> +
>>  #define pmd_none(pmd)		(!pmd_val(pmd))
>>  #define pmd_present(pmd)	(pmd_val(pmd))
>>  
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index cefd3e825612..ed8f42497ac4 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -464,6 +464,8 @@ static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot)
>>  extern pgd_t swapper_pg_dir[PTRS_PER_PGD];
>>  extern pgd_t idmap_pg_dir[PTRS_PER_PGD];
>>  
>> +#define pgd_huge(pgd)		(0)
>> +
>
> So only arm, arm64 and powerpc implement CONFIG_HAVE_GENERIC_RCU_GUP
> and only powerpc impements pgd_huge().
>
> Could we get a bit of documentation in place for pgd_huge() so that
> people who aren't familiar with powerpc can understand what's going
> on?

I ended up moving that to include/linux/hugetlb.h with the below
comments added. Let me know if the below is ok 

/*
 * hugepages at page global directory. If arch support
 * hugepages at pgd level, they need to define this.
 */
#ifndef pgd_huge
#define pgd_huge(x)	0
#endif

#ifndef is_hugepd
/*
 * Some architectures requires a hugepage directory format that is
 * required to support multiple hugepage sizes. For example
 * a4fe3ce7699bfe1bd88f816b55d42d8fe1dac655 introduced the same
 * on powerpc. This allows for a more flexible hugepage pagetable
 * layout.
 */
typedef struct { unsigned long pd; } hugepd_t;
#define is_hugepd(hugepd) (0)
#define __hugepd(x) ((hugepd_t) { (x) })
static inline int gup_huge_pd(hugepd_t hugepd, unsigned long addr,
			      unsigned pdshift, unsigned long end,
			      int write, struct page **pages, int *nr)
{
	return 0;
}
#else
extern int gup_huge_pd(hugepd_t hugepd, unsigned long addr,
		       unsigned pdshift, unsigned long end,
		       int write, struct page **pages, int *nr);
#endif

-aneesh