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SubjectRe: [PATCH v14 8/8] mm: hugetlb: optimize the code with the help of the compiler
On Thu, Feb 04, 2021 at 11:50:43AM +0800, 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.

"When the "struct page size" crosses page boundaries we cannot
make use of this feature.
Let free_vmemmap_pages_per_hpage() return zero if that is the case,
most of the functions can be optimized away."

I think the above is more clear, but just a suggestion.


> Signed-off-by: Muchun Song <songmuchun@bytedance.com>

Reviewed-by: Oscar Salvador <osalvador@suse.de>

> ---
> 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;
>
> --
> 2.11.0
>
>

--
Oscar Salvador
SUSE L3

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