On Wed, Sep 22, 2021 at 10:27 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
>
> The page_head_if_fake() is used throughout memory management and the
> conditional check requires checking a global variable, although the
> overhead of this check may be small, it increases when the memory
> cache comes under pressure. Also, the global variable will not be
> modified after system boot, so it is very appropriate to use static
> key machanism.
>
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> ---
> include/linux/hugetlb.h | 10 ++++++++--
> include/linux/page-flags.h | 6 ++++--
> mm/hugetlb_vmemmap.c | 12 ++++++------
> mm/memory_hotplug.c | 2 +-
> 4 files changed, 19 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 1faebe1cd0ed..4cc647a5dbf8 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -1066,9 +1066,15 @@ static inline void set_huge_swap_pte_at(struct mm_struct *mm, unsigned long addr
> #endif /* CONFIG_HUGETLB_PAGE */
>
> #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> -extern bool hugetlb_free_vmemmap_enabled;
> +static inline bool hugetlb_free_vmemmap_enabled(void)
> +{
> + return static_key_enabled(&hugetlb_free_vmemmap_enabled_key);
could it be
if (static_branch_maybe(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
&hugetlb_free_vmemmap_enabled_key))
then we are able to remove the duplication in page_fixed_fake_head()?
> +}
> #else
> -#define hugetlb_free_vmemmap_enabled false
> +static inline bool hugetlb_free_vmemmap_enabled(void)
> +{
> + return false;
> +}
> #endif
>
> static inline spinlock_t *huge_pte_lock(struct hstate *h,
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index b47a7f51d2c3..54e119e44496 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -185,7 +185,8 @@ enum pageflags {
> #ifndef __GENERATING_BOUNDS_H
>
> #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> -extern bool hugetlb_free_vmemmap_enabled;
> +DECLARE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
> + hugetlb_free_vmemmap_enabled_key);
>
> /*
> * If the feature of freeing some vmemmap pages associated with each HugeTLB
> @@ -205,7 +206,8 @@ extern bool hugetlb_free_vmemmap_enabled;
> */
> static __always_inline const struct page *page_fixed_fake_head(const struct page *page)
> {
> - if (!hugetlb_free_vmemmap_enabled)
> + if (!static_branch_maybe(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
> + &hugetlb_free_vmemmap_enabled_key))
> return page;
> /*
> * Only addresses aligned with PAGE_SIZE of struct page may be fake head
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 527bcaa44a48..47517e878ed5 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -188,9 +188,9 @@
> #define RESERVE_VMEMMAP_NR 1U
> #define RESERVE_VMEMMAP_SIZE (RESERVE_VMEMMAP_NR << PAGE_SHIFT)
>
> -bool hugetlb_free_vmemmap_enabled __read_mostly =
> - IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON);
> -EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled);
> +DEFINE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
> + hugetlb_free_vmemmap_enabled_key);
> +EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled_key);
>
> static int __init early_hugetlb_free_vmemmap_param(char *buf)
> {
> @@ -204,9 +204,9 @@ static int __init early_hugetlb_free_vmemmap_param(char *buf)
> return -EINVAL;
>
> if (!strcmp(buf, "on"))
> - hugetlb_free_vmemmap_enabled = true;
> + static_branch_enable(&hugetlb_free_vmemmap_enabled_key);
> else if (!strcmp(buf, "off"))
> - hugetlb_free_vmemmap_enabled = false;
> + static_branch_disable(&hugetlb_free_vmemmap_enabled_key);
> else
> return -EINVAL;
>
> @@ -284,7 +284,7 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
> BUILD_BUG_ON(__NR_USED_SUBPAGE >=
> RESERVE_VMEMMAP_SIZE / sizeof(struct page));
>
> - if (!hugetlb_free_vmemmap_enabled)
> + if (!hugetlb_free_vmemmap_enabled())
> return;
>
> vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 0488eed3327c..89c1fde02162 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1341,7 +1341,7 @@ bool mhp_supports_memmap_on_memory(unsigned long size)
> * populate a single PMD.
> */
> return memmap_on_memory &&
> - !hugetlb_free_vmemmap_enabled &&
> + !hugetlb_free_vmemmap_enabled() &&
> IS_ENABLED(CONFIG_MHP_MEMMAP_ON_MEMORY) &&
> size == memory_block_size_bytes() &&
> IS_ALIGNED(vmemmap_size, PMD_SIZE) &&
> --
> 2.11.0
>
Thanks
barry
