Re: [PATCH 1/4] mm/hugetlb: Enable PUD level huge page migration

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 10/10/2018 03:09 PM, Michal Hocko wrote:
> On Wed 10-10-18 08:39:22, Anshuman Khandual wrote:
> [...]
>> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
>> index 9df1d59..4bcbf1e 100644
>> --- a/include/linux/hugetlb.h
>> +++ b/include/linux/hugetlb.h
>> @@ -504,6 +504,16 @@ static inline bool hugepage_migration_supported(struct hstate *h)
>>         return arch_hugetlb_migration_supported(h);
>>  }
>>  
>> +static inline bool hugepage_movable_supported(struct hstate *h)
>> +{
>> +       if (!hugepage_migration_supported(h)) --> calls arch override restricting the set
>> +               return false;
>> +
>> +       if (hstate_is_gigantic(h)	--------> restricts the set further
>> +               return false;
>> +       return true;
>> +}
>> +
>>  static inline spinlock_t *huge_pte_lockptr(struct hstate *h,
>>                                            struct mm_struct *mm, pte_t *pte)
>>  {
>> @@ -600,6 +610,11 @@ static inline bool hugepage_migration_supported(struct hstate *h)
>>         return false;
>>  }
>>  
>> +static inline bool hugepage_movable_supported(struct hstate *h)
>> +{
>> +       return false;
>> +}
>> +
>>  static inline spinlock_t *huge_pte_lockptr(struct hstate *h,
>>                                            struct mm_struct *mm, pte_t *pte)
>>  {
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index 3c21775..a5a111d 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -919,7 +919,7 @@ static struct page *dequeue_huge_page_nodemask(struct hstate *h, gfp_t gfp_mask,
>>  /* Movability of hugepages depends on migration support. */
>>  static inline gfp_t htlb_alloc_mask(struct hstate *h)
>>  {
>> -       if (hugepage_migration_supported(h))
>> +       if (hugepage_movable_supported(h))
>>                 return GFP_HIGHUSER_MOVABLE;
>>         else
>>                 return GFP_HIGHUSER;
> 
> Exactly what I've had in mind. It would be great to have a comment in
> hugepage_movable_supported to explain why we are not supporting giga
> pages even though they are migrateable and why we need that distinction.
sure, will do.

> 
>> The above patch is in addition to the following later patch in the series.
> [...]
>> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
>> index 9c1b77f..9df1d59 100644
>> --- a/include/linux/hugetlb.h
>> +++ b/include/linux/hugetlb.h
>> @@ -479,18 +479,29 @@ static inline pgoff_t basepage_index(struct page *page)
>>  extern int dissolve_free_huge_page(struct page *page);
>>  extern int dissolve_free_huge_pages(unsigned long start_pfn,
>>                                     unsigned long end_pfn);
>> -static inline bool hugepage_migration_supported(struct hstate *h)
>> -{
>> +
>>  #ifdef CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION
>> +#ifndef arch_hugetlb_migration_supported
>> +static inline bool arch_hugetlb_migration_supported(struct hstate *h)
>> +{
>>         if ((huge_page_shift(h) == PMD_SHIFT) ||
>>                 (huge_page_shift(h) == PUD_SHIFT) ||
>>                         (huge_page_shift(h) == PGDIR_SHIFT))
>>                 return true;
>>         else
>>                 return false;
>> +}
>> +#endif
>>  #else
>> +static inline bool arch_hugetlb_migration_supported(struct hstate *h)
>> +{
>>         return false;
>> +}
>>  #endif
>> +
>> +static inline bool hugepage_migration_supported(struct hstate *h)
>> +{
>> +       return arch_hugetlb_migration_supported(h);
>>  }
> 
> Yes making hugepage_migration_supported to have an arch override is
> definitely the right thing to do. Whether the above approach rather than
> a weak symbol is better is a matter of taste and I do not feel strongly
> about that.
Okay then, will carry this forward and re-spin the patch series. Thank you
for your detailed review till now.