[PATCH 0/2] Fix PageDoubleMap

[PATCH 0/2] Fix PageDoubleMap

[Matthew Wilcox (Oracle)]

This is a purely theoretical problem for now as none of the filesystems
which use PG_private_2 (ie PG_fscache) are being converted at this time,
but it's confusing to leave it like this.

Matthew Wilcox (Oracle) (2):
  mm: Move PageDoubleMap bit
  mm: Simplify PageDoubleMap with PF_SECOND policy

 include/linux/page-flags.h | 42 ++++++++++----------------------------
 1 file changed, 11 insertions(+), 31 deletions(-)

-- 
2.27.0

[PATCH 1/2] mm: Move PageDoubleMap bit

[Matthew Wilcox (Oracle)]

PG_private_2 is defined as being PF_ANY (applicable to tail pages
as well as regular & head pages).  That means that the first tail
page of a double-map page will appear to have Private2 set.  Use the
Workingset bit instead which is defined as PF_HEAD so any attempt to
access the Workingset bit on a tail page will redirect to the head page's
Workingset bit.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/page-flags.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 6be1aa559b1e..b4e6051aa311 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -164,7 +164,7 @@ enum pageflags {
 	PG_slob_free = PG_private,
 
 	/* Compound pages. Stored in first tail page's flags */
-	PG_double_map = PG_private_2,
+	PG_double_map = PG_workingset,
 
 	/* non-lru isolated movable page */
 	PG_isolated = PG_reclaim,
-- 
2.27.0

[PATCH 2/2] mm: Simplify PageDoubleMap with PF_SECOND policy

[Matthew Wilcox (Oracle)]

Introduce the new page policy of PF_SECOND which lets us use the
normal pageflags generation machinery to create the various DoubleMap
manipulation functions.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/page-flags.h | 40 ++++++++++----------------------------
 1 file changed, 10 insertions(+), 30 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index b4e6051aa311..7182103583d2 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -232,6 +232,9 @@ static inline void page_init_poison(struct page *page, size_t size)
  *
  * PF_NO_COMPOUND:
  *     the page flag is not relevant for compound pages.
+ *
+ * PF_SECOND:
+ *     the page flag is stored in the first tail page.
  */
 #define PF_POISONED_CHECK(page) ({					\
 		VM_BUG_ON_PGFLAGS(PagePoisoned(page), page);		\
@@ -247,6 +250,9 @@ static inline void page_init_poison(struct page *page, size_t size)
 #define PF_NO_COMPOUND(page, enforce) ({				\
 		VM_BUG_ON_PGFLAGS(enforce && PageCompound(page), page);	\
 		PF_POISONED_CHECK(page); })
+#define PF_SECOND(page, enforce) ({					\
+		VM_BUG_ON_PGFLAGS(!PageHead(page), page);		\
+		PF_POISONED_CHECK(&page[1]); })
 
 /*
  * Macros to create function definitions for page flags
@@ -685,42 +691,15 @@ static inline int PageTransTail(struct page *page)
  *
  * See also __split_huge_pmd_locked() and page_remove_anon_compound_rmap().
  */
-static inline int PageDoubleMap(struct page *page)
-{
-	return PageHead(page) && test_bit(PG_double_map, &page[1].flags);
-}
-
-static inline void SetPageDoubleMap(struct page *page)
-{
-	VM_BUG_ON_PAGE(!PageHead(page), page);
-	set_bit(PG_double_map, &page[1].flags);
-}
-
-static inline void ClearPageDoubleMap(struct page *page)
-{
-	VM_BUG_ON_PAGE(!PageHead(page), page);
-	clear_bit(PG_double_map, &page[1].flags);
-}
-static inline int TestSetPageDoubleMap(struct page *page)
-{
-	VM_BUG_ON_PAGE(!PageHead(page), page);
-	return test_and_set_bit(PG_double_map, &page[1].flags);
-}
-
-static inline int TestClearPageDoubleMap(struct page *page)
-{
-	VM_BUG_ON_PAGE(!PageHead(page), page);
-	return test_and_clear_bit(PG_double_map, &page[1].flags);
-}
-
+PAGEFLAG(DoubleMap, double_map, PF_SECOND)
+	TESTSCFLAG(DoubleMap, double_map, PF_SECOND)
 #else
 TESTPAGEFLAG_FALSE(TransHuge)
 TESTPAGEFLAG_FALSE(TransCompound)
 TESTPAGEFLAG_FALSE(TransCompoundMap)
 TESTPAGEFLAG_FALSE(TransTail)
 PAGEFLAG_FALSE(DoubleMap)
-	TESTSETFLAG_FALSE(DoubleMap)
-	TESTCLEARFLAG_FALSE(DoubleMap)
+	TESTSCFLAG_FALSE(DoubleMap)
 #endif
 
 /*
@@ -885,6 +864,7 @@ static inline int page_has_private(struct page *page)
 #undef PF_ONLY_HEAD
 #undef PF_NO_TAIL
 #undef PF_NO_COMPOUND
+#undef PF_SECOND
 #endif /* !__GENERATING_BOUNDS_H */
 
 #endif	/* PAGE_FLAGS_H */
-- 
2.27.0

Re: [PATCH 1/2] mm: Move PageDoubleMap bit

[Zi Yan]

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On 29 Jun 2020, at 11:19, Matthew Wilcox (Oracle) wrote:

> PG_private_2 is defined as being PF_ANY (applicable to tail pages
> as well as regular & head pages).  That means that the first tail
> page of a double-map page will appear to have Private2 set.  Use the
> Workingset bit instead which is defined as PF_HEAD so any attempt to
> access the Workingset bit on a tail page will redirect to the head page's
> Workingset bit.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Makes sense to me.

Reviewed-by: Zi Yan <ziy@nvidia.com>

—
Best Regards,
Yan Zi

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Re: [PATCH 2/2] mm: Simplify PageDoubleMap with PF_SECOND policy

[Zi Yan]

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On 29 Jun 2020, at 11:19, Matthew Wilcox (Oracle) wrote:

> Introduce the new page policy of PF_SECOND which lets us use the
> normal pageflags generation machinery to create the various DoubleMap
> manipulation functions.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  include/linux/page-flags.h | 40 ++++++++++----------------------------
>  1 file changed, 10 insertions(+), 30 deletions(-)
>
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index b4e6051aa311..7182103583d2 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -232,6 +232,9 @@ static inline void page_init_poison(struct page *page, size_t size)
>   *
>   * PF_NO_COMPOUND:
>   *     the page flag is not relevant for compound pages.
> + *
> + * PF_SECOND:
> + *     the page flag is stored in the first tail page.

Just wonder if PF_FIRST_TAIL_PAGE is more informative, although it has more characters to type. :)


—
Best Regards,
Yan Zi

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