Re: [PATCH RESEND v2 1/4] mm: hugetlb: free the 2nd vmemmap page associated with each HugeTLB page

[Barry Song <21cnbao@gmail.com>]

On Tue, Sep 21, 2021 at 10:23 PM Muchun Song <songmuchun@bytedance.com> wrote:
>
> On Sat, Sep 18, 2021 at 6:06 PM Muchun Song <songmuchun@bytedance.com> wrote:
> >
> > On Sat, Sep 18, 2021 at 12:39 PM Barry Song <21cnbao@gmail.com> wrote:
> > >
> > > On Sat, Sep 18, 2021 at 12:08 AM Muchun Song <songmuchun@bytedance.com> wrote:
> > > >
> > > > Currently, we only free 6 vmemmap pages associated with a 2MB HugeTLB
> > > > page. However, we can remap all tail vmemmap pages to the page frame
> > > > mapped to with the head vmemmap page. Finally, we can free 7 vmemmap
> > > > pages for a 2MB HugeTLB page. It is a fine gain (e.g. we can save
> > > > extra 2GB memory when there is 1TB HugeTLB pages in the system
> > > > compared with the current implementation).
> > > >
> > > > But the head vmemmap page is not freed to the buddy allocator and all
> > > > tail vmemmap pages are mapped to the head vmemmap page frame. So we
> > > > can see more than one struct page struct with PG_head (e.g. 8 per 2 MB
> > > > HugeTLB page) associated with each HugeTLB page. We should adjust
> > > > compound_head() to make it returns the real head struct page when the
> > > > parameter is the tail struct page but with PG_head flag.
> > > >
> > > > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > > > ---
> > > >  Documentation/admin-guide/kernel-parameters.txt |  2 +-
> > > >  include/linux/page-flags.h                      | 75 +++++++++++++++++++++++--
> > > >  mm/hugetlb_vmemmap.c                            | 60 +++++++++++---------
> > > >  mm/sparse-vmemmap.c                             | 21 +++++++
> > > >  4 files changed, 126 insertions(+), 32 deletions(-)
> > > >
> > > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > > > index bdb22006f713..a154a7b3b9a5 100644
> > > > --- a/Documentation/admin-guide/kernel-parameters.txt
> > > > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > > > @@ -1606,7 +1606,7 @@
> > > >                         [KNL] Reguires CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > > >                         enabled.
> > > >                         Allows heavy hugetlb users to free up some more
> > > > -                       memory (6 * PAGE_SIZE for each 2MB hugetlb page).
> > > > +                       memory (7 * PAGE_SIZE for each 2MB hugetlb page).
> > > >                         Format: { on | off (default) }
> > > >
> > > >                         on:  enable the feature
> > > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > > > index 8e1d97d8f3bd..7b1a918ebd43 100644
> > > > --- a/include/linux/page-flags.h
> > > > +++ b/include/linux/page-flags.h
> > > > @@ -184,13 +184,64 @@ enum pageflags {
> > > >
> > > >  #ifndef __GENERATING_BOUNDS_H
> > > >
> > > > +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > > > +extern bool hugetlb_free_vmemmap_enabled;
> > > > +
> > > > +/*
> > > > + * If the feature of freeing some vmemmap pages associated with each HugeTLB
> > > > + * page is enabled, the head vmemmap page frame is reused and all of the tail
> > > > + * vmemmap addresses map to the head vmemmap page frame (furture details can
> > > > + * refer to the figure at the head of the mm/hugetlb_vmemmap.c).  In other
> > > > + * word, there are more than one page struct with PG_head associated with each
> > > > + * HugeTLB page.  We __know__ that there is only one head page struct, the tail
> > > > + * page structs with PG_head are fake head page structs.  We need an approach
> > > > + * to distinguish between those two different types of page structs so that
> > > > + * compound_head() can return the real head page struct when the parameter is
> > > > + * the tail page struct but with PG_head.
> > > > + *
> > > > + * The page_head_if_fake() returns the real head page struct iff the @page may
> > > > + * be fake, otherwise, returns the @page if it cannot be a fake page struct.
> > > > + */
> > > > +static __always_inline const struct page *page_head_if_fake(const struct page *page)
> > > > +{
> > > > +       if (!hugetlb_free_vmemmap_enabled)
> > > > +               return page;
> > > > +
> > > > +       /*
> > > > +        * Only addresses aligned with PAGE_SIZE of struct page may be fake head
> > > > +        * struct page. The alignment check aims to avoid access the fields (
> > > > +        * e.g. compound_head) of the @page[1]. It can avoid touch a (possibly)
> > > > +        * cold cacheline in some cases.
> > > > +        */
> > > > +       if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) &&
> > > > +           test_bit(PG_head, &page->flags)) {
> > > > +               /*
> > > > +                * We can safely access the field of the @page[1] with PG_head
> > > > +                * because the @page is a compound page composed with at least
> > > > +                * two contiguous pages.
> > > > +                */
> > > > +               unsigned long head = READ_ONCE(page[1].compound_head);
> > > > +
> > > > +               if (likely(head & 1))
> > > > +                       return (const struct page *)(head - 1);
> > > > +       }
> > > > +
> > > > +       return page;
> > > > +}
> > > > +#else
> > > > +static __always_inline const struct page *page_head_if_fake(const struct page *page)
> > > > +{
> > > > +       return page;
> > > > +}
> > > > +#endif
> > > > +
> > > >  static inline unsigned long _compound_head(const struct page *page)
> > > >  {
> > > >         unsigned long head = READ_ONCE(page->compound_head);
> > > >
> > > >         if (unlikely(head & 1))
> > > >                 return head - 1;
> > > > -       return (unsigned long)page;
> > > > +       return (unsigned long)page_head_if_fake(page);
> > >
> > > hard to read. page_head_if_fake,  what is the other side of
> > > page_head_if_not_fake?
> >
> > 1) return itself if the @page is not a fake head page.
> > 2) return head page if @page is a fake head page.
> >
> > So I want to express that page_head_if_fake returns a
> > head page only and only if the parameter of @page is a
> > fake head page. Otherwise, it returns itself.
> >
> > > I would expect something like
> > > page_to_page_head()
> > > or
> > > get_page_head()
> > >
> >
> > Those names seem to be not appropriate as well, because
> > its functionality does not make sure it can return a head
> > page. If the parameter is a head page, it definitely
> > returns a head page, otherwise, it may return itself which
> > may be a tail page.
> >
> > From this point of view, I still prefer page_head_if_fake.
> >
> > > Anyway, I am not quite sure what is the best name. but page_head_if_fake(page)
> > > sounds odd to me. just like the things have two sides, but if_fake  presents
> > > one side only.
> >
> > If others have any ideas, comments are welcome.
> >
> > >
> > > >  }
> > > >
> > > >  #define compound_head(page)    ((typeof(page))_compound_head(page))
> > > > @@ -225,12 +276,14 @@ static inline unsigned long _compound_head(const struct page *page)
> > > >
> > > >  static __always_inline int PageTail(struct page *page)
> > > >  {
> > > > -       return READ_ONCE(page->compound_head) & 1;
> > > > +       return READ_ONCE(page->compound_head) & 1 ||
> > > > +              page_head_if_fake(page) != page;
> > >
> > > i would expect a wrapper like:
> > > page_is_fake_head()
> >
> > Good point. Will do.
> >
> > >
> > > and the above page_to_page_head() can leverage the wrapper.
> > > here too.
> > >
> > > >  }
> > > >
> > > >  static __always_inline int PageCompound(struct page *page)
> > > >  {
> > > > -       return test_bit(PG_head, &page->flags) || PageTail(page);
> > > > +       return test_bit(PG_head, &page->flags) ||
> > > > +              READ_ONCE(page->compound_head) & 1;
> > >
> > > hard to read. could it be something like the below?
> > > return PageHead(page) || PageTail(page);
> > >
> > > or do we really need to change this function? even a fake head still has
> > > the true test_bit(PG_head, &page->flags), though it is not a real head, it
> > > is still a pagecompound, right?
> >
> > Right. PageCompound() can not be changed.  It is odd but
> > efficient because calling page_head_if_fake is eliminated.
> > So I select performance not readability. I'm not sure if it's
> > worth it.
>
> In order to improve readability, I'll introduce 3 helpers as follows.
>
> 1) page_head_or_fake(), which returns true for the head page
>    or fake head page.
> 2) page_head_is_fake(), which returns true for fake head page.
> 3) page_tail_not_fake_head(), which returns true for the tail page
>    except the fake head page.
>
> In the end, PageHead(), PageTail() and PageCompound() become
> the following.
>
> static __always_inline int PageHead(struct page *page)
> {
>     return page_head_or_fake(page) && !page_head_is_fake(page);
> }
>
> static __always_inline int PageTail(struct page *page)
> {
>     return page_tail_not_fake_head(page) || page_head_is_fake(page);
> }
>
> static __always_inline int PageCompound(struct page *page)
> {
>     return page_head_or_fake(page) || page_tail_not_fake_head(page);
> }
>
> Do those look more readable?
>

still not good enough. After a second thought, page_head_if_fake seems
to have the best performance though this function returns an odd value.
i just made a little bit refine on your code in doc:

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 2c0d11e71e26..240c2fca13c7 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -197,8 +197,9 @@ extern bool hugetlb_free_vmemmap_enabled;
  * compound_head() can return the real head page struct when the parameter is
  * the tail page struct but with PG_head.
  *
- * The page_head_if_fake() returns the real head page struct iff the @page may
- * be fake, otherwise, returns the @page if it cannot be a fake page struct.
+ * The page_head_if_fake() returns the real head page struct if the @page is
+ * fake page_head, otherwise, returns @page which can either be a true page_
+ * head or tail.
  */
 static __always_inline const struct page *page_head_if_fake(const
struct page *page)
 {
@@ -226,6 +227,12 @@ static __always_inline const struct page
*page_head_if_fake(const struct page *p

        return page;
 }
+
+static __always_inline const struct page *page_is_fake_head(const
struct page *page)
+{
+       return page_head_if_fake(page) != page;
+}
+
 #else
 static __always_inline const struct page *page_head_if_fake(const
struct page *page)
 {
@@ -247,7 +254,7 @@ static inline unsigned long _compound_head(const
struct page *page)
 static __always_inline int PageTail(struct page *page)
 {
        return READ_ONCE(page->compound_head) & 1 ||
-              page_head_if_fake(page) != page;
+              page_is_fake_head(page);
 }

 static __always_inline int PageCompound(struct page *page)

> Thanks.

Thanks
barry