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    SubjectRe: [PATCH v14 8/8] mm: hugetlb: optimize the code with the help of the compiler
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    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 <songmuchun@bytedance.com>

    I like it. Thanks.
    Reviewed-by: Miaohe Lin <linmiaohe@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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