On Sat, Sep 18, 2021 at 6:06 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
>
> On Sat, Sep 18, 2021 at 12:39 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
> >
> > On Sat, Sep 18, 2021 at 12:08 AM Muchun Song <songmuchun@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > > ---
> > > 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.
After some thinking, I figured out 2 names.
page_head_if_fake() always returns a head page if the parameter
of @page is not a compound page or its ->flags has PG_head set
(you can think the head page is itself if the page is not a compound
page). All the callers of it already guarantee this. It means it has to
return a head page unless the @page is a tail page (except fake
head page). So I propose two names as follows.
1) page_head_unless_tail
2) page_head_filter_fake
The former means it always returns a head page unless the
caller passes a tail page as a parameter. The latter means
it always returns a head page but filtering out the fake head
page. The former is inspired by get_page_unless_zero.
What do you think?
Thanks.
>
> > 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.
>
> Thanks.
