Subject: Re: [PATCH 02/10] mm/hugetlb: Add PGD based implementation awareness

From: Anshuman Khandual <[email-masked]>
Date: Mon, 11 Apr 2016 10:55:05 +0530

On 04/07/2016 02:34 PM, Balbir Singh wrote:
> 
> 
> On 07/04/16 15:37, Anshuman Khandual wrote:
>> Currently the config ARCH_WANT_GENERAL_HUGETLB enabled functions like
>> 'huge_pte_alloc' and 'huge_pte_offset' dont take into account HugeTLB
>> page implementation at the PGD level. This is also true for functions
>> like 'follow_page_mask' which is called from move_pages() system call.
>> This lack of PGD level huge page support prohibits some architectures
>> to use these generic HugeTLB functions.
>>
> 
> From what I know of move_pages(), it will always call follow_page_mask()
> with FOLL_GET (I could be wrong here) and the implementation below
> returns NULL for follow_huge_pgd().

You are right. This patch makes ARCH_WANT_GENERAL_HUGETLB functions aware
of PGD implementation so that we can do all transactions on 16GB pages
using these function instead of the present arch overrides. But that also
requires follow_page_mask() changes for every other access to the page
than the migrate_pages() usage.

But yes, we dont support migrate_pages() on PGD based pages yet, hence
it just returns NULL in that case. May be the commit message needs to
reflect this.

> 
>> This change adds the required PGD based implementation awareness and
>> with that, more architectures like POWER which implements 16GB pages
>> at the PGD level along with the 16MB pages at the PMD level can now
>> use ARCH_WANT_GENERAL_HUGETLB config option.
>>
>> Signed-off-by: Anshuman Khandual <[email-masked]>
>> ---
>>  include/linux/hugetlb.h |  3 +++
>>  mm/gup.c                |  6 ++++++
>>  mm/hugetlb.c            | 20 ++++++++++++++++++++
>>  3 files changed, 29 insertions(+)
>>
>> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
>> index 7d953c2..71832e1 100644
>> --- a/include/linux/hugetlb.h
>> +++ b/include/linux/hugetlb.h
>> @@ -115,6 +115,8 @@ struct page *follow_huge_pmd(struct mm_struct *mm, unsigned long address,
>>  				pmd_t *pmd, int flags);
>>  struct page *follow_huge_pud(struct mm_struct *mm, unsigned long address,
>>  				pud_t *pud, int flags);
>> +struct page *follow_huge_pgd(struct mm_struct *mm, unsigned long address,
>> +				pgd_t *pgd, int flags);
>>  int pmd_huge(pmd_t pmd);
>>  int pud_huge(pud_t pmd);
>>  unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
>> @@ -143,6 +145,7 @@ static inline void hugetlb_show_meminfo(void)
>>  }
>>  #define follow_huge_pmd(mm, addr, pmd, flags)	NULL
>>  #define follow_huge_pud(mm, addr, pud, flags)	NULL
>> +#define follow_huge_pgd(mm, addr, pgd, flags)	NULL
>>  #define prepare_hugepage_range(file, addr, len)	(-EINVAL)
>>  #define pmd_huge(x)	0
>>  #define pud_huge(x)	0
>> diff --git a/mm/gup.c b/mm/gup.c
>> index fb87aea..9bac78c 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -234,6 +234,12 @@ struct page *follow_page_mask(struct vm_area_struct *vma,
>>  	pgd = pgd_offset(mm, address);
>>  	if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd)))
>>  		return no_page_table(vma, flags);
>> +	if (pgd_huge(*pgd) && vma->vm_flags & VM_HUGETLB) {
>> +		page = follow_huge_pgd(mm, address, pgd, flags);
>> +		if (page)
>> +			return page;
>> +		return no_page_table(vma, flags);
> This will return NULL as well?

That right, no_page_table() returns NULL for FOLL_GET when we fall through
after failing on follow_huge_pgd().

>> +	}
>>  
>>  	pud = pud_offset(pgd, address);
>>  	if (pud_none(*pud))
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index 19d0d08..5ea3158 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -4250,6 +4250,11 @@ pte_t *huge_pte_alloc(struct mm_struct *mm,
>>  	pte_t *pte = NULL;
>>  
>>  	pgd = pgd_offset(mm, addr);
>> +	if (sz == PGDIR_SIZE) {
>> +		pte = (pte_t *)pgd;
>> +		goto huge_pgd;
>> +	}
>> +
> 
> No allocation for a pgd slot - right?

No, its already allocated for the mm during creation.