Re: [PATCH RESEND v2 2/4] mm: hugetlb: replace hugetlb_free_vmemmap_enabled with a static_key

[Barry Song <21cnbao@gmail.com>]

On Sat, Sep 18, 2021 at 12:08 AM Muchun Song <songmuchun@bytedance.com> wrote:
>
> The page_head_if_fake() is used throughout memory management and the
> conditional check requires checking a global variable, although the
> overhead of this check may be small, it increases when the memory
> cache comes under pressure. Also, the global variable will not be
> modified after system boot, so it is very appropriate to use static
> key machanism.
>
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  include/linux/hugetlb.h    |  6 +++++-
>  include/linux/page-flags.h |  6 ++++--
>  mm/hugetlb_vmemmap.c       | 10 +++++-----
>  3 files changed, 14 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index f7ca1a3870ea..ee3ddf3d12cf 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -1057,7 +1057,11 @@ static inline void set_huge_swap_pte_at(struct mm_struct *mm, unsigned long addr
>  #endif /* CONFIG_HUGETLB_PAGE */
>
>  #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> -extern bool hugetlb_free_vmemmap_enabled;
> +DECLARE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
> +                        hugetlb_free_vmemmap_enabled_key);
> +#define hugetlb_free_vmemmap_enabled                                    \
> +       static_key_enabled(&hugetlb_free_vmemmap_enabled_key)
> +
>  #else
>  #define hugetlb_free_vmemmap_enabled   false
>  #endif
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 7b1a918ebd43..d68d2cf30d76 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -185,7 +185,8 @@ enum pageflags {
>  #ifndef __GENERATING_BOUNDS_H
>
>  #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> -extern bool hugetlb_free_vmemmap_enabled;
> +DECLARE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
> +                        hugetlb_free_vmemmap_enabled_key);
>
>  /*
>   * If the feature of freeing some vmemmap pages associated with each HugeTLB
> @@ -204,7 +205,8 @@ extern bool hugetlb_free_vmemmap_enabled;
>   */
>  static __always_inline const struct page *page_head_if_fake(const struct page *page)
>  {
> -       if (!hugetlb_free_vmemmap_enabled)
> +       if (!static_branch_maybe(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
> +                                &hugetlb_free_vmemmap_enabled_key))

A question bothering me is that we still have hugetlb_free_vmemmap_enabled
defined as static_key_enabled(&hugetlb_free_vmemmap_enabled_key).
but here you are using static_branch_maybe() with the CONFIG and refer the key
directly.
Do we only need one of them? Or something is wrong?

>                 return page;
>
>         /*
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 527bcaa44a48..5b80129c684c 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -188,9 +188,9 @@
>  #define RESERVE_VMEMMAP_NR             1U
>  #define RESERVE_VMEMMAP_SIZE           (RESERVE_VMEMMAP_NR << PAGE_SHIFT)
>
> -bool hugetlb_free_vmemmap_enabled __read_mostly =
> -       IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON);
> -EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled);
> +DEFINE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
> +                       hugetlb_free_vmemmap_enabled_key);
> +EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled_key);
>
>  static int __init early_hugetlb_free_vmemmap_param(char *buf)
>  {
> @@ -204,9 +204,9 @@ static int __init early_hugetlb_free_vmemmap_param(char *buf)
>                 return -EINVAL;
>
>         if (!strcmp(buf, "on"))
> -               hugetlb_free_vmemmap_enabled = true;
> +               static_branch_enable(&hugetlb_free_vmemmap_enabled_key);
>         else if (!strcmp(buf, "off"))
> -               hugetlb_free_vmemmap_enabled = false;
> +               static_branch_disable(&hugetlb_free_vmemmap_enabled_key);
>         else
>                 return -EINVAL;
>
> --
> 2.11.0
>

Thanks
barry