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SubjectRe: [External] Re: [PATCH v14 8/8] mm: hugetlb: optimize the code with the help of the compiler
On Fri, Feb 5, 2021 at 5:09 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> 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.

OK. I will use it. Thanks for your suggestions.

>
>
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>

Thanks.

>
> > ---
> > 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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