Re: [PATCH v3 2/4] mm: hugetlb: replace hugetlb_free_vmemmap_enabled with a static_key

[Muchun Song]

On Fri, Sep 24, 2021 at 6:24 AM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> 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()?

Definitely. I'll update in the next version.

Thanks.

>
> > +}
> >  #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