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

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Date:   Wed, 10 Oct 2018 11:39:07 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, suzuki.poulose@....com,
        punit.agrawal@....com, will.deacon@....com, Steven.Price@....com,
        catalin.marinas@....com, mike.kravetz@...cle.com,
        n-horiguchi@...jp.nec.com
Subject: Re: [PATCH 1/4] mm/hugetlb: Enable PUD level huge page migration

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.

> 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.

-- 
Michal Hocko
SUSE Labs