Re: [PATCH v8 12/12] mm/hugetlb: Optimize the code with the help of the compiler

Oscar Salvador
2020-12-10T11:40:00Z

On 2020-12-10 04:55, 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.

As I said earlier, I would squash this patch with patch#10 and
remove the !is_power_of_2 check in hugetlb_vmemmap_init and leave
only the check for the boot parameter.
That should be enough.

> 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));

Why? hugetlb_free_vmemmap_enabled can only become true
if the is_power_of_2 check succeeds in early_hugetlb_free_vmemmap_param.
The "is_power_of_2" check here can go.

> diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
> index 0a1c0d33a316..5f5e90c81cd2 100644
> --- a/mm/hugetlb_vmemmap.h
> +++ b/mm/hugetlb_vmemmap.h
> @@ -21,7 +21,7 @@ void free_huge_page_vmemmap(struct hstate *h, struct
> page *head);
> */
> static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate
> *h)
> {
> - return h->nr_free_vmemmap_pages;
> + return h->nr_free_vmemmap_pages && is_power_of_2(sizeof(struct
> page));

If hugetlb_free_vmemmap_enabled is false, hugetlb_vmemmap_init() leaves
h->nr_free_vmemmap_pages unset to 0, so no need for the is_power_of_2
check here.

--
Oscar Salvador
SUSE L3