Date:   Wed, 22 Sep 2021 10:38:25 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Barry Song <21cnbao@...il.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Michal Hocko <mhocko@...e.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        David Hildenbrand <david@...hat.com>,
        Chen Huang <chenhuang5@...wei.com>,
        "Bodeddula, Balasubramaniam" <bodeddub@...zon.com>,
        Jonathan Corbet <corbet@....net>,
        Matthew Wilcox <willy@...radead.org>,
        Xiongchun duan <duanxiongchun@...edance.com>,
        fam.zheng@...edance.com, Muchun Song <smuchun@...il.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH RESEND v2 1/4] mm: hugetlb: free the 2nd vmemmap page
 associated with each HugeTLB page

On Wed, Sep 22, 2021 at 4:43 AM Barry Song <21cnbao@...il.com> wrote:
>
> On Wed, Sep 22, 2021 at 1:46 AM Muchun Song <songmuchun@...edance.com> wrote:
> >
> > On Tue, Sep 21, 2021 at 8:11 PM Barry Song <21cnbao@...il.com> wrote:
> > >
> > > On Tue, Sep 21, 2021 at 10:23 PM Muchun Song <songmuchun@...edance.com> wrote:
> > > >
> > > > On Sat, Sep 18, 2021 at 6:06 PM Muchun Song <songmuchun@...edance.com> wrote:
> > > > >
> > > > > On Sat, Sep 18, 2021 at 12:39 PM Barry Song <21cnbao@...il.com> wrote:
> > > > > >
> > > > > > On Sat, Sep 18, 2021 at 12:08 AM Muchun Song <songmuchun@...edance.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@...edance.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:
> >
> > Right. page_head_if_fake is the choice for performance.
> >
> > >
> > > 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.
> > >   */
> >
> > Good annotation.
> >
> > >  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);
> > >  }
> >
> > Yeah, this makes PageTail more readable. In your previous thread,
> > you proposed that why not use PageTail in PageCompound directly
> > to improve code readability. So I want to introduce 2 more helpers
> > besides page_is_fake_head().
> >
> > static __always_inline int page_tail_not_fake_head(struct page *page)
> > {
> >     return READ_ONCE(page->compound_head) & 1;
> > }
> >
> > static __always_inline int page_head_or_fake(struct page *page)
> > {
> >     return test_bit(PG_head, &page->flags);
> > }
> >
> > Then PageTail() and PageCompound() change to the following.
> >
> > static __always_inline int PageTail(struct page *page)
> > {
> >     return page_tail_not_fake_head(page) || page_is_fake_head(page);
> > }
> >
> > static __always_inline int PageCompound(struct page *page)
> > {
> >     return page_head_or_fake(page) || page_tail_not_fake_head(page);
> > }
> >
> > From the point of names of helpers, they act as self-annotation.
> > So I think PageTail and PageCompound become readable
> > as well. But you said "still not good enough". Is it because of
> > the names of helpers or introducing more complexity?
>
> I really don't think it is worth this complexity. If there is anything to make

Got it.

> the code more readable, I would rename page_head_if_fake() to
> page_fixed_dup_head().

Here means page_fixed_up_head, right? Is it a typo?

Thanks.

>
> this function fixes up the page:
> 1. if the page is a fake head, we need to return its true head (things
> get fixed.)
> 2. if the page is not a fake head, in other words, it is either true
> head or tail, no need to fix anything.
>
> >
> > Thanks.
>
> Thanks
> barry

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