On 2021/2/4 11:50, Muchun Song wrote:
> We cannot optimize if a "struct page" crosses page boundaries. If
> it is true, we can optimize the code with the help of a compiler.
> When free_vmemmap_pages_per_hpage() returns zero, most functions are
> optimized by the compiler.
> 
> Signed-off-by: Muchun Song <songmuc...@bytedance.com>
I like it. Thanks.
Reviewed-by: Miaohe Lin <linmia...@huawei.com>

> ---
>  include/linux/hugetlb.h |  3 ++-
>  mm/hugetlb_vmemmap.c    | 13 +++++++++++++
>  2 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 822ab2f5542a..7bfb06e16298 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -878,7 +878,8 @@ extern bool hugetlb_free_vmemmap_enabled;
>  
>  static inline bool is_hugetlb_free_vmemmap_enabled(void)
>  {
> -     return hugetlb_free_vmemmap_enabled;
> +     return hugetlb_free_vmemmap_enabled &&
> +            is_power_of_2(sizeof(struct page));
>  }
>  #else
>  static inline bool is_hugetlb_free_vmemmap_enabled(void)
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 8efad9978821..068d0e0cebc8 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -211,6 +211,12 @@ early_param("hugetlb_free_vmemmap", 
> early_hugetlb_free_vmemmap_param);
>   */
>  static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
>  {
> +     /*
> +      * This check aims to let the compiler help us optimize the code as
> +      * much as possible.
> +      */
> +     if (!is_power_of_2(sizeof(struct page)))
> +             return 0;
>       return h->nr_free_vmemmap_pages;
>  }
>  
> @@ -280,6 +286,13 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
>       BUILD_BUG_ON(NR_USED_SUBPAGE >=
>                    RESERVE_VMEMMAP_SIZE / sizeof(struct page));
>  
> +     /*
> +      * The compiler can help us to optimize this function to null
> +      * when the size of the struct page is not power of 2.
> +      */
> +     if (!is_power_of_2(sizeof(struct page)))
> +             return;
> +
>       if (!hugetlb_free_vmemmap_enabled)
>               return;
>  
> 

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