[PATCH 0/5] Free the 2nd vmemmap page associated with each HugeTLB page

[PATCH 0/5] Free the 2nd vmemmap page associated with each HugeTLB page

[Muchun Song]

After the feature of "Free sonme vmemmap pages of HugeTLB page" is enabled,
the mapping of the vmemmap addresses associated with a 2MB HugeTLB page
becomes the figure below.

     HugeTLB                  struct pages(8 pages)         page frame(8 pages)
 +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+---> PG_head
 |           |                     |     0     | -------------> |     0     |
 |           |                     +-----------+                +-----------+
 |           |                     |     1     | -------------> |     1     |
 |           |                     +-----------+                +-----------+
 |           |                     |     2     | ----------------^ ^ ^ ^ ^ ^
 |           |                     +-----------+                   | | | | |
 |           |                     |     3     | ------------------+ | | | |
 |           |                     +-----------+                     | | | |
 |           |                     |     4     | --------------------+ | | |
 |    2MB    |                     +-----------+                       | | |
 |           |                     |     5     | ----------------------+ | |
 |           |                     +-----------+                         | |
 |           |                     |     6     | ------------------------+ |
 |           |                     +-----------+                           |
 |           |                     |     7     | --------------------------+
 |           |                     +-----------+
 |           |
 |           |
 |           |
 +-----------+

As we can see, the 2nd vmemmap page frame (indexed by 1) is reused and remaped.
However, the 2nd vmemmap page frame is also can be freed to the buddy allocator,
then we can change the mapping from the figure above to the figure below.

    HugeTLB                  struct pages(8 pages)         page frame(8 pages)
 +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+---> PG_head
 |           |                     |     0     | -------------> |     0     |
 |           |                     +-----------+                +-----------+
 |           |                     |     1     | ---------------^ ^ ^ ^ ^ ^ ^
 |           |                     +-----------+                  | | | | | |
 |           |                     |     2     | -----------------+ | | | | |
 |           |                     +-----------+                    | | | | |
 |           |                     |     3     | -------------------+ | | | |
 |           |                     +-----------+                      | | | |
 |           |                     |     4     | ---------------------+ | | |
 |    2MB    |                     +-----------+                        | | |
 |           |                     |     5     | -----------------------+ | |
 |           |                     +-----------+                          | |
 |           |                     |     6     | -------------------------+ |
 |           |                     +-----------+                            |
 |           |                     |     7     | ---------------------------+
 |           |                     +-----------+
 |           |
 |           |
 |           |
 +-----------+

After we do this, all tail vmemmap pages (1-7) are mapped to the head vmemmap
page frame (0). In other words, 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(), PageHead() and PageTail() can work properly
if the parameter is the tail page struct but with PG_head.

The following code snippet describes how to distinguish between real and fake
head page struct.

	if (test_bit(PG_head, &page->flags)) {
		unsigned long head = READ_ONCE(page[1].compound_head);

		if (head & 1) {
			if (head == (unsigned long)page + 1)
				==> head page struct
			else
				==> tail page struct
		} else
			==> head page struct
	}

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. The main
implementation is in the patch 3.

In our server, we can save extra 2GB memory with this patchset applied if there
are 1 TB HugeTLB (2 MB) pages. If the size of the HugeTLB page is 1 GB, it only
can save 4MB. For 2 MB HugeTLB page, it is a nice gain.

Muchun Song (5):
  mm: introduce PAGEFLAGS_MASK to replace ((1UL << NR_PAGEFLAGS) - 1)
  mm: introduce save_page_flags to cooperate with show_page_flags
  mm: hugetlb: free the 2nd vmemmap page associated with each HugeTLB
    page
  mm: hugetlb: replace hugetlb_free_vmemmap_enabled with a static_key
  mm: sparsemem: use page table lock to protect kernel pmd operations

 Documentation/admin-guide/kernel-parameters.txt |   2 +-
 include/linux/hugetlb.h                         |   6 +-
 include/linux/page-flags.h                      | 103 ++++++++++++++++++++++--
 include/trace/events/mmflags.h                  |   4 +
 include/trace/events/page_ref.h                 |   8 +-
 lib/test_printf.c                               |   2 +-
 lib/vsprintf.c                                  |   2 +-
 mm/hugetlb_vmemmap.c                            |  67 ++++++++-------
 mm/ptdump.c                                     |  16 +++-
 mm/sparse-vmemmap.c                             |  70 ++++++++++++----
 10 files changed, 218 insertions(+), 62 deletions(-)

-- 
2.11.0

[PATCH 1/5] mm: introduce PAGEFLAGS_MASK to replace ((1UL << NR_PAGEFLAGS) - 1)

[Muchun Song]

Instead of hard-coding ((1UL << NR_PAGEFLAGS) - 1) everywhere, introducing
PAGEFLAGS_MASK to make the code clear to get the page flags.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 include/linux/page-flags.h      | 4 +++-
 include/trace/events/page_ref.h | 4 ++--
 lib/test_printf.c               | 2 +-
 lib/vsprintf.c                  | 2 +-
 4 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 5922031ffab6..358d3f6fa976 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -178,6 +178,8 @@ enum pageflags {
 	PG_reported = PG_uptodate,
 };
 
+#define PAGEFLAGS_MASK		(~((1UL << NR_PAGEFLAGS) - 1))
+
 #ifndef __GENERATING_BOUNDS_H
 
 static inline unsigned long _compound_head(const struct page *page)
@@ -859,7 +861,7 @@ static inline void ClearPageSlabPfmemalloc(struct page *page)
  * alloc-free cycle to prevent from reusing the page.
  */
 #define PAGE_FLAGS_CHECK_AT_PREP	\
-	(((1UL << NR_PAGEFLAGS) - 1) & ~__PG_HWPOISON)
+	(~(PAGEFLAGS_MASK | __PG_HWPOISON))
 
 #define PAGE_FLAGS_PRIVATE				\
 	(1UL << PG_private | 1UL << PG_private_2)
diff --git a/include/trace/events/page_ref.h b/include/trace/events/page_ref.h
index 5d2ea93956ce..643b1b4e9f27 100644
--- a/include/trace/events/page_ref.h
+++ b/include/trace/events/page_ref.h
@@ -38,7 +38,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_template,
 
 	TP_printk("pfn=0x%lx flags=%s count=%d mapcount=%d mapping=%p mt=%d val=%d",
 		__entry->pfn,
-		show_page_flags(__entry->flags & ((1UL << NR_PAGEFLAGS) - 1)),
+		show_page_flags(__entry->flags & ~PAGEFLAGS_MASK),
 		__entry->count,
 		__entry->mapcount, __entry->mapping, __entry->mt,
 		__entry->val)
@@ -88,7 +88,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_and_test_template,
 
 	TP_printk("pfn=0x%lx flags=%s count=%d mapcount=%d mapping=%p mt=%d val=%d ret=%d",
 		__entry->pfn,
-		show_page_flags(__entry->flags & ((1UL << NR_PAGEFLAGS) - 1)),
+		show_page_flags(__entry->flags & ~PAGEFLAGS_MASK),
 		__entry->count,
 		__entry->mapcount, __entry->mapping, __entry->mt,
 		__entry->val, __entry->ret)
diff --git a/lib/test_printf.c b/lib/test_printf.c
index 8ac71aee46af..eadcc6458b10 100644
--- a/lib/test_printf.c
+++ b/lib/test_printf.c
@@ -614,7 +614,7 @@ page_flags_test(int section, int node, int zone, int last_cpupid,
 	bool append = false;
 	int i;
 
-	flags &= BIT(NR_PAGEFLAGS) - 1;
+	flags &= ~PAGEFLAGS_MASK;
 	if (flags) {
 		page_flags |= flags;
 		snprintf(cmp_buf + size, BUF_SIZE - size, "%s", name);
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index dd006adfe853..ff7c180a2971 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -2019,7 +2019,7 @@ static const struct page_flags_fields pff[] = {
 static
 char *format_page_flags(char *buf, char *end, unsigned long flags)
 {
-	unsigned long main_flags = flags & (BIT(NR_PAGEFLAGS) - 1);
+	unsigned long main_flags = flags & ~PAGEFLAGS_MASK;
 	bool append = false;
 	int i;
 
-- 
2.11.0

Re: [PATCH 1/5] mm: introduce PAGEFLAGS_MASK to replace ((1UL << NR_PAGEFLAGS) - 1)

[Mike Kravetz]

On 7/14/21 2:17 AM, Muchun Song wrote:
> Instead of hard-coding ((1UL << NR_PAGEFLAGS) - 1) everywhere, introducing
> PAGEFLAGS_MASK to make the code clear to get the page flags.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  include/linux/page-flags.h      | 4 +++-
>  include/trace/events/page_ref.h | 4 ++--
>  lib/test_printf.c               | 2 +-
>  lib/vsprintf.c                  | 2 +-
>  4 files changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 5922031ffab6..358d3f6fa976 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -178,6 +178,8 @@ enum pageflags {
>  	PG_reported = PG_uptodate,
>  };
>  
> +#define PAGEFLAGS_MASK		(~((1UL << NR_PAGEFLAGS) - 1))

Can you explain why you chose this definition instead of

#define	PAGEFLAGS_MASK		((1UL << NR_PAGEFLAGS) - 1)

and mostly use ~PAGEFLAGS_MASK below?
-- 
Mike Kravetz

> +
>  #ifndef __GENERATING_BOUNDS_H
>  
>  static inline unsigned long _compound_head(const struct page *page)
> @@ -859,7 +861,7 @@ static inline void ClearPageSlabPfmemalloc(struct page *page)
>   * alloc-free cycle to prevent from reusing the page.
>   */
>  #define PAGE_FLAGS_CHECK_AT_PREP	\
> -	(((1UL << NR_PAGEFLAGS) - 1) & ~__PG_HWPOISON)
> +	(~(PAGEFLAGS_MASK | __PG_HWPOISON))
>  
>  #define PAGE_FLAGS_PRIVATE				\
>  	(1UL << PG_private | 1UL << PG_private_2)
> diff --git a/include/trace/events/page_ref.h b/include/trace/events/page_ref.h
> index 5d2ea93956ce..643b1b4e9f27 100644
> --- a/include/trace/events/page_ref.h
> +++ b/include/trace/events/page_ref.h
> @@ -38,7 +38,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_template,
>  
>  	TP_printk("pfn=0x%lx flags=%s count=%d mapcount=%d mapping=%p mt=%d val=%d",
>  		__entry->pfn,
> -		show_page_flags(__entry->flags & ((1UL << NR_PAGEFLAGS) - 1)),
> +		show_page_flags(__entry->flags & ~PAGEFLAGS_MASK),
>  		__entry->count,
>  		__entry->mapcount, __entry->mapping, __entry->mt,
>  		__entry->val)
> @@ -88,7 +88,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_and_test_template,
>  
>  	TP_printk("pfn=0x%lx flags=%s count=%d mapcount=%d mapping=%p mt=%d val=%d ret=%d",
>  		__entry->pfn,
> -		show_page_flags(__entry->flags & ((1UL << NR_PAGEFLAGS) - 1)),
> +		show_page_flags(__entry->flags & ~PAGEFLAGS_MASK),
>  		__entry->count,
>  		__entry->mapcount, __entry->mapping, __entry->mt,
>  		__entry->val, __entry->ret)
> diff --git a/lib/test_printf.c b/lib/test_printf.c
> index 8ac71aee46af..eadcc6458b10 100644
> --- a/lib/test_printf.c
> +++ b/lib/test_printf.c
> @@ -614,7 +614,7 @@ page_flags_test(int section, int node, int zone, int last_cpupid,
>  	bool append = false;
>  	int i;
>  
> -	flags &= BIT(NR_PAGEFLAGS) - 1;
> +	flags &= ~PAGEFLAGS_MASK;
>  	if (flags) {
>  		page_flags |= flags;
>  		snprintf(cmp_buf + size, BUF_SIZE - size, "%s", name);
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index dd006adfe853..ff7c180a2971 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -2019,7 +2019,7 @@ static const struct page_flags_fields pff[] = {
>  static
>  char *format_page_flags(char *buf, char *end, unsigned long flags)
>  {
> -	unsigned long main_flags = flags & (BIT(NR_PAGEFLAGS) - 1);
> +	unsigned long main_flags = flags & ~PAGEFLAGS_MASK;
>  	bool append = false;
>  	int i;
>  
> 

Re: [PATCH 1/5] mm: introduce PAGEFLAGS_MASK to replace ((1UL << NR_PAGEFLAGS) - 1)

[Muchun Song]

On Tue, Jul 27, 2021 at 5:04 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 7/14/21 2:17 AM, Muchun Song wrote:
> > Instead of hard-coding ((1UL << NR_PAGEFLAGS) - 1) everywhere, introducing
> > PAGEFLAGS_MASK to make the code clear to get the page flags.
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  include/linux/page-flags.h      | 4 +++-
> >  include/trace/events/page_ref.h | 4 ++--
> >  lib/test_printf.c               | 2 +-
> >  lib/vsprintf.c                  | 2 +-
> >  4 files changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index 5922031ffab6..358d3f6fa976 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -178,6 +178,8 @@ enum pageflags {
> >       PG_reported = PG_uptodate,
> >  };
> >
> > +#define PAGEFLAGS_MASK               (~((1UL << NR_PAGEFLAGS) - 1))
>
> Can you explain why you chose this definition instead of
>
> #define PAGEFLAGS_MASK          ((1UL << NR_PAGEFLAGS) - 1)
>
> and mostly use ~PAGEFLAGS_MASK below?

Hi Mike,

Actually, I learned from PAGE_MASK. So I thought the macro
like xxx_MASK should be the format of 0x00...00ff...ff. I don't
know if this is an unwritten rule. Please correct me if I am
wrong.

Thanks.

> --
> Mike Kravetz

Re: [PATCH 1/5] mm: introduce PAGEFLAGS_MASK to replace ((1UL << NR_PAGEFLAGS) - 1)

[Muchun Song]

On Tue, Jul 27, 2021 at 2:27 PM Muchun Song <songmuchun@bytedance.com> wrote:
>
> On Tue, Jul 27, 2021 at 5:04 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
> >
> > On 7/14/21 2:17 AM, Muchun Song wrote:
> > > Instead of hard-coding ((1UL << NR_PAGEFLAGS) - 1) everywhere, introducing
> > > PAGEFLAGS_MASK to make the code clear to get the page flags.
> > >
> > > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > > ---
> > >  include/linux/page-flags.h      | 4 +++-
> > >  include/trace/events/page_ref.h | 4 ++--
> > >  lib/test_printf.c               | 2 +-
> > >  lib/vsprintf.c                  | 2 +-
> > >  4 files changed, 7 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > > index 5922031ffab6..358d3f6fa976 100644
> > > --- a/include/linux/page-flags.h
> > > +++ b/include/linux/page-flags.h
> > > @@ -178,6 +178,8 @@ enum pageflags {
> > >       PG_reported = PG_uptodate,
> > >  };
> > >
> > > +#define PAGEFLAGS_MASK               (~((1UL << NR_PAGEFLAGS) - 1))
> >
> > Can you explain why you chose this definition instead of
> >
> > #define PAGEFLAGS_MASK          ((1UL << NR_PAGEFLAGS) - 1)
> >
> > and mostly use ~PAGEFLAGS_MASK below?
>
> Hi Mike,
>
> Actually, I learned from PAGE_MASK. So I thought the macro
> like xxx_MASK should be the format of 0x00...00ff...ff. I don't
                                           ^^^
Sorry. I mean 0xff...ff00...00 here.

> know if this is an unwritten rule. Please correct me if I am
> wrong.
>
> Thanks.
>
> > --
> > Mike Kravetz

[PATCH 2/5] mm: introduce save_page_flags to cooperate with show_page_flags

[Muchun Song]

Introduce save_page_flags to return the page flags which can cooperate
with show_page_flags. If we want to hihe some page flags from users, it
will be useful to alter save_page_flags directly. This is a preparation
for the next patch to hide some page flags from users.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 include/trace/events/mmflags.h  | 3 +++
 include/trace/events/page_ref.h | 8 ++++----
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
index 390270e00a1d..69cb84b1257e 100644
--- a/include/trace/events/mmflags.h
+++ b/include/trace/events/mmflags.h
@@ -121,6 +121,9 @@ IF_HAVE_PG_IDLE(PG_idle,		"idle"		)		\
 IF_HAVE_PG_ARCH_2(PG_arch_2,		"arch_2"	)		\
 IF_HAVE_PG_SKIP_KASAN_POISON(PG_skip_kasan_poison, "skip_kasan_poison")
 
+#define save_page_flags(page)						\
+	(((page)->flags & ~PAGEFLAGS_MASK))
+
 #define show_page_flags(flags)						\
 	(flags) ? __print_flags(flags, "|",				\
 	__def_pageflag_names						\
diff --git a/include/trace/events/page_ref.h b/include/trace/events/page_ref.h
index 643b1b4e9f27..53d303048d27 100644
--- a/include/trace/events/page_ref.h
+++ b/include/trace/events/page_ref.h
@@ -28,7 +28,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_template,
 
 	TP_fast_assign(
 		__entry->pfn = page_to_pfn(page);
-		__entry->flags = page->flags;
+		__entry->flags = save_page_flags(page);
 		__entry->count = page_ref_count(page);
 		__entry->mapcount = page_mapcount(page);
 		__entry->mapping = page->mapping;
@@ -38,7 +38,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_template,
 
 	TP_printk("pfn=0x%lx flags=%s count=%d mapcount=%d mapping=%p mt=%d val=%d",
 		__entry->pfn,
-		show_page_flags(__entry->flags & ~PAGEFLAGS_MASK),
+		show_page_flags(__entry->flags),
 		__entry->count,
 		__entry->mapcount, __entry->mapping, __entry->mt,
 		__entry->val)
@@ -77,7 +77,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_and_test_template,
 
 	TP_fast_assign(
 		__entry->pfn = page_to_pfn(page);
-		__entry->flags = page->flags;
+		__entry->flags = save_page_flags(page);
 		__entry->count = page_ref_count(page);
 		__entry->mapcount = page_mapcount(page);
 		__entry->mapping = page->mapping;
@@ -88,7 +88,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_and_test_template,
 
 	TP_printk("pfn=0x%lx flags=%s count=%d mapcount=%d mapping=%p mt=%d val=%d ret=%d",
 		__entry->pfn,
-		show_page_flags(__entry->flags & ~PAGEFLAGS_MASK),
+		show_page_flags(__entry->flags),
 		__entry->count,
 		__entry->mapcount, __entry->mapping, __entry->mt,
 		__entry->val, __entry->ret)
-- 
2.11.0

Re: [PATCH 2/5] mm: introduce save_page_flags to cooperate with show_page_flags

[Mike Kravetz]

On 7/14/21 2:17 AM, Muchun Song wrote:
> Introduce save_page_flags to return the page flags which can cooperate
> with show_page_flags. If we want to hihe some page flags from users, it
> will be useful to alter save_page_flags directly. This is a preparation
> for the next patch to hide some page flags from users.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  include/trace/events/mmflags.h  | 3 +++
>  include/trace/events/page_ref.h | 8 ++++----
>  2 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> index 390270e00a1d..69cb84b1257e 100644
> --- a/include/trace/events/mmflags.h
> +++ b/include/trace/events/mmflags.h
> @@ -121,6 +121,9 @@ IF_HAVE_PG_IDLE(PG_idle,		"idle"		)		\
>  IF_HAVE_PG_ARCH_2(PG_arch_2,		"arch_2"	)		\
>  IF_HAVE_PG_SKIP_KASAN_POISON(PG_skip_kasan_poison, "skip_kasan_poison")
>  
> +#define save_page_flags(page)						\
> +	(((page)->flags & ~PAGEFLAGS_MASK))
> +

Looking ahead to the next patch, this is changed to hide the PG_head
flag for 'fake' head pages.

IIRC, all vmemmap pages except the first will be mapped read only.  So,
all vmemmap pages with 'fake' head pages will be read only.

It seems that all the modified trace events below are associated with
updates to page structs.  Therefore, it seems these events will never
experience a 'fake' head page.  Am I missing something?

-- 
Mike Kravetz


>  #define show_page_flags(flags)						\
>  	(flags) ? __print_flags(flags, "|",				\
>  	__def_pageflag_names						\
> diff --git a/include/trace/events/page_ref.h b/include/trace/events/page_ref.h
> index 643b1b4e9f27..53d303048d27 100644
> --- a/include/trace/events/page_ref.h
> +++ b/include/trace/events/page_ref.h
> @@ -28,7 +28,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_template,
>  
>  	TP_fast_assign(
>  		__entry->pfn = page_to_pfn(page);
> -		__entry->flags = page->flags;
> +		__entry->flags = save_page_flags(page);
>  		__entry->count = page_ref_count(page);
>  		__entry->mapcount = page_mapcount(page);
>  		__entry->mapping = page->mapping;
> @@ -38,7 +38,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_template,
>  
>  	TP_printk("pfn=0x%lx flags=%s count=%d mapcount=%d mapping=%p mt=%d val=%d",
>  		__entry->pfn,
> -		show_page_flags(__entry->flags & ~PAGEFLAGS_MASK),
> +		show_page_flags(__entry->flags),
>  		__entry->count,
>  		__entry->mapcount, __entry->mapping, __entry->mt,
>  		__entry->val)
> @@ -77,7 +77,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_and_test_template,
>  
>  	TP_fast_assign(
>  		__entry->pfn = page_to_pfn(page);
> -		__entry->flags = page->flags;
> +		__entry->flags = save_page_flags(page);
>  		__entry->count = page_ref_count(page);
>  		__entry->mapcount = page_mapcount(page);
>  		__entry->mapping = page->mapping;
> @@ -88,7 +88,7 @@ DECLARE_EVENT_CLASS(page_ref_mod_and_test_template,
>  
>  	TP_printk("pfn=0x%lx flags=%s count=%d mapcount=%d mapping=%p mt=%d val=%d ret=%d",
>  		__entry->pfn,
> -		show_page_flags(__entry->flags & ~PAGEFLAGS_MASK),
> +		show_page_flags(__entry->flags),
>  		__entry->count,
>  		__entry->mapcount, __entry->mapping, __entry->mt,
>  		__entry->val, __entry->ret)
> 

Re: [PATCH 2/5] mm: introduce save_page_flags to cooperate with show_page_flags

[Muchun Song]

On Tue, Jul 27, 2021 at 7:18 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 7/14/21 2:17 AM, Muchun Song wrote:
> > Introduce save_page_flags to return the page flags which can cooperate
> > with show_page_flags. If we want to hihe some page flags from users, it
> > will be useful to alter save_page_flags directly. This is a preparation
> > for the next patch to hide some page flags from users.
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  include/trace/events/mmflags.h  | 3 +++
> >  include/trace/events/page_ref.h | 8 ++++----
> >  2 files changed, 7 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> > index 390270e00a1d..69cb84b1257e 100644
> > --- a/include/trace/events/mmflags.h
> > +++ b/include/trace/events/mmflags.h
> > @@ -121,6 +121,9 @@ IF_HAVE_PG_IDLE(PG_idle,          "idle"          )               \
> >  IF_HAVE_PG_ARCH_2(PG_arch_2,         "arch_2"        )               \
> >  IF_HAVE_PG_SKIP_KASAN_POISON(PG_skip_kasan_poison, "skip_kasan_poison")
> >
> > +#define save_page_flags(page)                                                \
> > +     (((page)->flags & ~PAGEFLAGS_MASK))
> > +
>
> Looking ahead to the next patch, this is changed to hide the PG_head
> flag for 'fake' head pages.
>
> IIRC, all vmemmap pages except the first will be mapped read only.  So,
> all vmemmap pages with 'fake' head pages will be read only.
>
> It seems that all the modified trace events below are associated with
> updates to page structs.  Therefore, it seems these events will never
> experience a 'fake' head page.  Am I missing something?

Totally right. I didn't realize this point before. We cannot see a fake page
struct in page refcount tracing. So this patch is definitely pointless. I'll
drop it in the next version. Thanks Mike.


>
> --
> Mike Kravetz

[PATCH 3/5] mm: hugetlb: free the 2nd vmemmap page associated with each HugeTLB page

[Muchun Song]

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                      | 97 ++++++++++++++++++++++++-
 include/trace/events/mmflags.h                  |  3 +-
 mm/hugetlb_vmemmap.c                            | 60 ++++++++-------
 mm/sparse-vmemmap.c                             | 21 ++++++
 5 files changed, 150 insertions(+), 33 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 358d3f6fa976..d29c16f7b176 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -182,25 +182,102 @@ 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. This is what page_head_if_fake()
+ * does.
+ *
+ * The page_head_if_fake() returns the real head page struct iff the @page may
+ * be fake, otherwise, returns NULL if the @page cannot be a fake page struct.
+ * The following figure describes how to distinguish between real and fake head
+ * page struct.
+ *
+ *	if (test_bit(PG_head, &page->flags)) {
+ *		unsigned long head = READ_ONCE(page[1].compound_head);
+ *
+ *		if (head & 1) {
+ *			if (head == (unsigned long)page + 1)
+ *				==> head page struct
+ *			else
+ *				==> tail page struct
+ *		} else
+ *			==> head page struct
+ *	} else
+ *		==> cannot be fake head page struct
+ *
+ * We can safely access the field of the @page[1] with PG_head because it means
+ * that the @page is a compound page composed with at least two contiguous
+ * pages.
+ */
+static __always_inline struct page *page_head_if_fake(const struct page *page)
+{
+	if (!hugetlb_free_vmemmap_enabled)
+		return NULL;
+
+	/*
+	 * 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)) {
+		unsigned long head = READ_ONCE(page[1].compound_head);
+
+		if (likely(head & 1))
+			return (struct page *)(head - 1);
+	}
+
+	return NULL;
+}
+#else
+static __always_inline struct page *page_head_if_fake(const struct page *page)
+{
+	return NULL;
+}
+#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;
+
+	head = (unsigned long)page_head_if_fake(page);
+
+	return head ? : (unsigned long)page;
 }
 
 #define compound_head(page)	((typeof(page))_compound_head(page))
 
 static __always_inline int PageTail(struct page *page)
 {
-	return READ_ONCE(page->compound_head) & 1;
+	struct page *head;
+
+	if (READ_ONCE(page->compound_head) & 1)
+		return 1;
+
+	head = page_head_if_fake(page);
+
+	return head ? head != page : 0;
 }
 
 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;
 }
 
 #define	PAGE_POISON_PATTERN	-1l
@@ -578,7 +655,19 @@ static inline void set_page_writeback_keepwrite(struct page *page)
 	test_set_page_writeback_keepwrite(page);
 }
 
-__PAGEFLAG(Head, head, PF_ANY) CLEARPAGEFLAG(Head, head, PF_ANY)
+static __always_inline int PageHead(struct page *page)
+{
+	struct page *head;
+
+	PF_POISONED_CHECK(page);
+	head = page_head_if_fake(page);
+
+	return head ? head == page : test_bit(PG_head, &page->flags);
+}
+
+__SETPAGEFLAG(Head, head, PF_ANY)
+__CLEARPAGEFLAG(Head, head, PF_ANY)
+CLEARPAGEFLAG(Head, head, PF_ANY)
 
 static __always_inline void set_compound_head(struct page *page, struct page *head)
 {
diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
index 69cb84b1257e..a0f0fd82e3e0 100644
--- a/include/trace/events/mmflags.h
+++ b/include/trace/events/mmflags.h
@@ -122,7 +122,8 @@ IF_HAVE_PG_ARCH_2(PG_arch_2,		"arch_2"	)		\
 IF_HAVE_PG_SKIP_KASAN_POISON(PG_skip_kasan_poison, "skip_kasan_poison")
 
 #define save_page_flags(page)						\
-	(((page)->flags & ~PAGEFLAGS_MASK))
+	(PageHead(page) ? ((page)->flags & ~PAGEFLAGS_MASK) :		\
+	 ((page)->flags & ~(PAGEFLAGS_MASK | PG_head_mask)))
 
 #define show_page_flags(flags)						\
 	(flags) ? __print_flags(flags, "|",				\
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index c540c21e26f5..527bcaa44a48 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -124,9 +124,9 @@
  * page of page structs (page 0) associated with the HugeTLB page contains the 4
  * page structs necessary to describe the HugeTLB. The only use of the remaining
  * pages of page structs (page 1 to page 7) is to point to page->compound_head.
- * Therefore, we can remap pages 2 to 7 to page 1. Only 2 pages of page structs
+ * Therefore, we can remap pages 1 to 7 to page 0. Only 1 pages of page structs
  * will be used for each HugeTLB page. This will allow us to free the remaining
- * 6 pages to the buddy allocator.
+ * 7 pages to the buddy allocator.
  *
  * Here is how things look after remapping.
  *
@@ -134,30 +134,30 @@
  * +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
  * |           |                     |     0     | -------------> |     0     |
  * |           |                     +-----------+                +-----------+
- * |           |                     |     1     | -------------> |     1     |
- * |           |                     +-----------+                +-----------+
- * |           |                     |     2     | ----------------^ ^ ^ ^ ^ ^
- * |           |                     +-----------+                   | | | | |
- * |           |                     |     3     | ------------------+ | | | |
- * |           |                     +-----------+                     | | | |
- * |           |                     |     4     | --------------------+ | | |
- * |    PMD    |                     +-----------+                       | | |
- * |   level   |                     |     5     | ----------------------+ | |
- * |  mapping  |                     +-----------+                         | |
- * |           |                     |     6     | ------------------------+ |
- * |           |                     +-----------+                           |
- * |           |                     |     7     | --------------------------+
+ * |           |                     |     1     | ---------------^ ^ ^ ^ ^ ^ ^
+ * |           |                     +-----------+                  | | | | | |
+ * |           |                     |     2     | -----------------+ | | | | |
+ * |           |                     +-----------+                    | | | | |
+ * |           |                     |     3     | -------------------+ | | | |
+ * |           |                     +-----------+                      | | | |
+ * |           |                     |     4     | ---------------------+ | | |
+ * |    PMD    |                     +-----------+                        | | |
+ * |   level   |                     |     5     | -----------------------+ | |
+ * |  mapping  |                     +-----------+                          | |
+ * |           |                     |     6     | -------------------------+ |
+ * |           |                     +-----------+                            |
+ * |           |                     |     7     | ---------------------------+
  * |           |                     +-----------+
  * |           |
  * |           |
  * |           |
  * +-----------+
  *
- * When a HugeTLB is freed to the buddy system, we should allocate 6 pages for
+ * When a HugeTLB is freed to the buddy system, we should allocate 7 pages for
  * vmemmap pages and restore the previous mapping relationship.
  *
  * For the HugeTLB page of the pud level mapping. It is similar to the former.
- * We also can use this approach to free (PAGE_SIZE - 2) vmemmap pages.
+ * We also can use this approach to free (PAGE_SIZE - 1) vmemmap pages.
  *
  * Apart from the HugeTLB page of the pmd/pud level mapping, some architectures
  * (e.g. aarch64) provides a contiguous bit in the translation table entries
@@ -166,7 +166,13 @@
  *
  * The contiguous bit is used to increase the mapping size at the pmd and pte
  * (last) level. So this type of HugeTLB page can be optimized only when its
- * size of the struct page structs is greater than 2 pages.
+ * size of the struct page structs is greater than 1 pages.
+ *
+ * Notice: 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. The compound_head() can handle this
+ * correctly (more details refer to the comment above compound_head()).
  */
 #define pr_fmt(fmt)	"HugeTLB: " fmt
 
@@ -175,14 +181,16 @@
 /*
  * There are a lot of struct page structures associated with each HugeTLB page.
  * For tail pages, the value of compound_head is the same. So we can reuse first
- * page of tail page structures. We map the virtual addresses of the remaining
- * pages of tail page structures to the first tail page struct, and then free
- * these page frames. Therefore, we need to reserve two pages as vmemmap areas.
+ * page of head page structures. We map the virtual addresses of all the pages
+ * of tail page structures to the head page struct, and then free these page
+ * frames. Therefore, we need to reserve one pages as vmemmap areas.
  */
-#define RESERVE_VMEMMAP_NR		2U
+#define RESERVE_VMEMMAP_NR		1U
 #define RESERVE_VMEMMAP_SIZE		(RESERVE_VMEMMAP_NR << PAGE_SHIFT)
 
-bool hugetlb_free_vmemmap_enabled = IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON);
+bool hugetlb_free_vmemmap_enabled __read_mostly =
+	IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON);
+EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled);
 
 static int __init early_hugetlb_free_vmemmap_param(char *buf)
 {
@@ -236,7 +244,6 @@ int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
 	 */
 	ret = vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
 				  GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE);
-
 	if (!ret)
 		ClearHPageVmemmapOptimized(head);
 
@@ -282,9 +289,8 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
 
 	vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
 	/*
-	 * The head page and the first tail page are not to be freed to buddy
-	 * allocator, the other pages will map to the first tail page, so they
-	 * can be freed.
+	 * The head page is not to be freed to buddy allocator, the other tail
+	 * pages will map to the head page, so they can be freed.
 	 *
 	 * Could RESERVE_VMEMMAP_NR be greater than @vmemmap_pages? It is true
 	 * on some architectures (e.g. aarch64). See Documentation/arm64/
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index bdce883f9286..62e3d20648ce 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -53,6 +53,17 @@ struct vmemmap_remap_walk {
 	struct list_head *vmemmap_pages;
 };
 
+/*
+ * How many struct page structs need to be reset. When we reuse the head
+ * struct page, the special metadata (e.g. page->flags or page->mapping)
+ * cannot copy to the tail struct page structs. The invalid value will be
+ * checked in the free_tail_pages_check(). In order to avoid the message
+ * of "corrupted mapping in tail page". We need to reset at least 3 (one
+ * head struct page struct and two tail struct page structs) struct page
+ * structs.
+ */
+#define NR_RESET_STRUCT_PAGE		3
+
 static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start,
 				  struct vmemmap_remap_walk *walk)
 {
@@ -245,6 +256,15 @@ static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
 	set_pte_at(&init_mm, addr, pte, entry);
 }
 
+static inline void reset_struct_pages(struct page *start)
+{
+	int i;
+	struct page *from = start + NR_RESET_STRUCT_PAGE;
+
+	for (i = 0; i < NR_RESET_STRUCT_PAGE; i++)
+		memcpy(start + i, from, sizeof(*from));
+}
+
 static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
 				struct vmemmap_remap_walk *walk)
 {
@@ -258,6 +278,7 @@ static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
 	list_del(&page->lru);
 	to = page_to_virt(page);
 	copy_page(to, (void *)walk->reuse_addr);
+	reset_struct_pages(to);
 
 	set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
 }
-- 
2.11.0

Re: [PATCH 3/5] mm: hugetlb: free the 2nd vmemmap page associated with each HugeTLB page

[Matthew Wilcox]

On Wed, Jul 14, 2021 at 05:17:58PM +0800, Muchun Song wrote:
> +#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. This is what page_head_if_fake()
> + * does.
> + *
> + * The page_head_if_fake() returns the real head page struct iff the @page may
> + * be fake, otherwise, returns NULL if the @page cannot be a fake page struct.
> + * The following figure describes how to distinguish between real and fake head
> + * page struct.
> + *
> + *	if (test_bit(PG_head, &page->flags)) {
> + *		unsigned long head = READ_ONCE(page[1].compound_head);
> + *
> + *		if (head & 1) {
> + *			if (head == (unsigned long)page + 1)
> + *				==> head page struct
> + *			else
> + *				==> tail page struct
> + *		} else
> + *			==> head page struct
> + *	} else
> + *		==> cannot be fake head page struct

I'm not sure we need the pseudocode when the code is right there ...

> + * We can safely access the field of the @page[1] with PG_head because it means
> + * that the @page is a compound page composed with at least two contiguous
> + * pages.
> + */
> +static __always_inline struct page *page_head_if_fake(const struct page *page)
> +{
> +	if (!hugetlb_free_vmemmap_enabled)
> +		return NULL;
> +
> +	/*
> +	 * 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)) {
> +		unsigned long head = READ_ONCE(page[1].compound_head);
> +
> +		if (likely(head & 1))
> +			return (struct page *)(head - 1);
> +	}
> +
> +	return NULL;
> +}

Why return 'NULL' instead of 'page'?

This is going to significantly increase the cost of calling
compound_page() (by whichever spelling it has).  That will make
the folio patchset more compelling ;-)

Re: [PATCH 3/5] mm: hugetlb: free the 2nd vmemmap page associated with each HugeTLB page

[Mike Kravetz]

On 7/26/21 2:16 PM, Matthew Wilcox wrote:
> On Wed, Jul 14, 2021 at 05:17:58PM +0800, Muchun Song wrote:
>> +static __always_inline struct page *page_head_if_fake(const struct page *page)
>> +{
>> +	if (!hugetlb_free_vmemmap_enabled)
>> +		return NULL;
>> +
>> +	/*
>> +	 * 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)) {
>> +		unsigned long head = READ_ONCE(page[1].compound_head);
>> +
>> +		if (likely(head & 1))
>> +			return (struct page *)(head - 1);
>> +	}
>> +
>> +	return NULL;
>> +}
> 
> Why return 'NULL' instead of 'page'?
> 
> This is going to significantly increase the cost of calling
> compound_page() (by whichever spelling it has).  That will make
> the folio patchset more compelling ;-)

Matthew, any suggestions for benchmarks/workloads to measure the
increased overhead?  Suspect you have some ideas based on folio work.

My concern is that we are introducing overhead for code paths not
associated with this feature.  The next patch even tries to minimize
this overhead a bit if hugetlb_free_vmemmap_enabled is not set.
-- 
Mike Kravetz

Re: [PATCH 3/5] mm: hugetlb: free the 2nd vmemmap page associated with each HugeTLB page

[Muchun Song]

On Tue, Jul 27, 2021 at 5:17 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Jul 14, 2021 at 05:17:58PM +0800, Muchun Song wrote:
> > +#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. This is what page_head_if_fake()
> > + * does.
> > + *
> > + * The page_head_if_fake() returns the real head page struct iff the @page may
> > + * be fake, otherwise, returns NULL if the @page cannot be a fake page struct.
> > + * The following figure describes how to distinguish between real and fake head
> > + * page struct.
> > + *
> > + *   if (test_bit(PG_head, &page->flags)) {
> > + *           unsigned long head = READ_ONCE(page[1].compound_head);
> > + *
> > + *           if (head & 1) {
> > + *                   if (head == (unsigned long)page + 1)
> > + *                           ==> head page struct
> > + *                   else
> > + *                           ==> tail page struct
> > + *           } else
> > + *                   ==> head page struct
> > + *   } else
> > + *           ==> cannot be fake head page struct
>
> I'm not sure we need the pseudocode when the code is right there ...

Maybe it is redundant. I'll remove this in the next version.

>
> > + * We can safely access the field of the @page[1] with PG_head because it means
> > + * that the @page is a compound page composed with at least two contiguous
> > + * pages.
> > + */
> > +static __always_inline struct page *page_head_if_fake(const struct page *page)
> > +{
> > +     if (!hugetlb_free_vmemmap_enabled)
> > +             return NULL;
> > +
> > +     /*
> > +      * 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)) {
> > +             unsigned long head = READ_ONCE(page[1].compound_head);
> > +
> > +             if (likely(head & 1))
> > +                     return (struct page *)(head - 1);
> > +     }
> > +
> > +     return NULL;
> > +}
>
> Why return 'NULL' instead of 'page'?

Returning @page is also fine. Will do in the next version.

>
> This is going to significantly increase the cost of calling
> compound_page() (by whichever spelling it has).  That will make
> the folio patchset more compelling ;-)

As Mike mentationed, do you have any recommended
benchmark (suspect you have a lot of experience on
this)?

Thanks.

[PATCH 4/5] mm: hugetlb: replace hugetlb_free_vmemmap_enabled with a static_key

[Muchun Song]

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@bytedance.com>
---
 include/linux/hugetlb.h    |  6 +++++-
 include/linux/page-flags.h |  6 ++++--
 mm/hugetlb_vmemmap.c       | 13 +++++++------
 3 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index f7ca1a3870ea..876a1af73bec 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -1057,7 +1057,11 @@ 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;
+DECLARE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
+			 hugetlb_free_vmemmap_enabled_key);
+#define hugetlb_free_vmemmap_enabled					 \
+	static_branch_maybe(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON, \
+			    &hugetlb_free_vmemmap_enabled_key)
 #else
 #define hugetlb_free_vmemmap_enabled	false
 #endif
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index d29c16f7b176..f05f806b98b4 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -183,7 +183,8 @@ 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
@@ -222,7 +223,8 @@ extern bool hugetlb_free_vmemmap_enabled;
  */
 static __always_inline struct page *page_head_if_fake(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 NULL;
 
 	/*
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 527bcaa44a48..a382e12f6ff0 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 +284,8 @@ 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 (!static_branch_maybe(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
+				 &hugetlb_free_vmemmap_enabled_key))
 		return;
 
 	vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
-- 
2.11.0

[PATCH 5/5] mm: sparsemem: use page table lock to protect kernel pmd operations

[Muchun Song]

The init_mm.page_table_lock is used to protect kernel page tables, we
can use it to serialize splitting vmemmap PMD mappings instead of mmap
write lock, which can increase the concurrency of vmemmap_remap_free().

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/ptdump.c         | 16 ++++++++++++----
 mm/sparse-vmemmap.c | 49 ++++++++++++++++++++++++++++++++++---------------
 2 files changed, 46 insertions(+), 19 deletions(-)

diff --git a/mm/ptdump.c b/mm/ptdump.c
index da751448d0e4..eea3d28d173c 100644
--- a/mm/ptdump.c
+++ b/mm/ptdump.c
@@ -40,8 +40,10 @@ static int ptdump_pgd_entry(pgd_t *pgd, unsigned long addr,
 	if (st->effective_prot)
 		st->effective_prot(st, 0, pgd_val(val));
 
-	if (pgd_leaf(val))
+	if (pgd_leaf(val)) {
 		st->note_page(st, addr, 0, pgd_val(val));
+		walk->action = ACTION_CONTINUE;
+	}
 
 	return 0;
 }
@@ -61,8 +63,10 @@ static int ptdump_p4d_entry(p4d_t *p4d, unsigned long addr,
 	if (st->effective_prot)
 		st->effective_prot(st, 1, p4d_val(val));
 
-	if (p4d_leaf(val))
+	if (p4d_leaf(val)) {
 		st->note_page(st, addr, 1, p4d_val(val));
+		walk->action = ACTION_CONTINUE;
+	}
 
 	return 0;
 }
@@ -82,8 +86,10 @@ static int ptdump_pud_entry(pud_t *pud, unsigned long addr,
 	if (st->effective_prot)
 		st->effective_prot(st, 2, pud_val(val));
 
-	if (pud_leaf(val))
+	if (pud_leaf(val)) {
 		st->note_page(st, addr, 2, pud_val(val));
+		walk->action = ACTION_CONTINUE;
+	}
 
 	return 0;
 }
@@ -101,8 +107,10 @@ static int ptdump_pmd_entry(pmd_t *pmd, unsigned long addr,
 
 	if (st->effective_prot)
 		st->effective_prot(st, 3, pmd_val(val));
-	if (pmd_leaf(val))
+	if (pmd_leaf(val)) {
 		st->note_page(st, addr, 3, pmd_val(val));
+		walk->action = ACTION_CONTINUE;
+	}
 
 	return 0;
 }
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index 62e3d20648ce..e636943ccfc4 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -64,8 +64,8 @@ struct vmemmap_remap_walk {
  */
 #define NR_RESET_STRUCT_PAGE		3
 
-static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start,
-				  struct vmemmap_remap_walk *walk)
+static int __split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start,
+				    struct vmemmap_remap_walk *walk)
 {
 	pmd_t __pmd;
 	int i;
@@ -87,15 +87,37 @@ static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start,
 		set_pte_at(&init_mm, addr, pte, entry);
 	}
 
-	/* Make pte visible before pmd. See comment in __pte_alloc(). */
-	smp_wmb();
-	pmd_populate_kernel(&init_mm, pmd, pgtable);
+	spin_lock(&init_mm.page_table_lock);
+	if (likely(pmd_leaf(*pmd))) {
+		/* Make pte visible before pmd. See comment in __pte_alloc(). */
+		smp_wmb();
+		pmd_populate_kernel(&init_mm, pmd, pgtable);
+		flush_tlb_kernel_range(start, start + PMD_SIZE);
+		spin_unlock(&init_mm.page_table_lock);
 
-	flush_tlb_kernel_range(start, start + PMD_SIZE);
+		return 0;
+	}
+	spin_unlock(&init_mm.page_table_lock);
+	pte_free_kernel(&init_mm, pgtable);
 
 	return 0;
 }
 
+static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start,
+				  struct vmemmap_remap_walk *walk)
+{
+	int ret;
+
+	spin_lock(&init_mm.page_table_lock);
+	ret = pmd_leaf(*pmd);
+	spin_unlock(&init_mm.page_table_lock);
+
+	if (ret)
+		ret = __split_vmemmap_huge_pmd(pmd, start, walk);
+
+	return ret;
+}
+
 static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
 			      unsigned long end,
 			      struct vmemmap_remap_walk *walk)
@@ -132,13 +154,12 @@ static int vmemmap_pmd_range(pud_t *pud, unsigned long addr,
 
 	pmd = pmd_offset(pud, addr);
 	do {
-		if (pmd_leaf(*pmd)) {
-			int ret;
+		int ret;
+
+		ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK, walk);
+		if (ret)
+			return ret;
 
-			ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK, walk);
-			if (ret)
-				return ret;
-		}
 		next = pmd_addr_end(addr, end);
 		vmemmap_pte_range(pmd, addr, next, walk);
 	} while (pmd++, addr = next, addr != end);
@@ -321,10 +342,8 @@ int vmemmap_remap_free(unsigned long start, unsigned long end,
 	 */
 	BUG_ON(start - reuse != PAGE_SIZE);
 
-	mmap_write_lock(&init_mm);
+	mmap_read_lock(&init_mm);
 	ret = vmemmap_remap_range(reuse, end, &walk);
-	mmap_write_downgrade(&init_mm);
-
 	if (ret && walk.nr_walked) {
 		end = reuse + walk.nr_walked * PAGE_SIZE;
 		/*
-- 
2.11.0