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Subject[PATCH v5 2/4] mm: Split page_type out from _mapcount
Date
From: Matthew Wilcox <mawilcox@microsoft.com>

We're already using a union of many fields here, so stop abusing the
_mapcount and make page_type its own field. That implies renaming some
of the machinery that creates PageBuddy, PageBalloon and PageKmemcg;
bring back the PG_buddy, PG_balloon and PG_kmemcg names.

As suggested by Kirill, make page_type a bitmask. Because it starts out
life as -1 (thanks to sharing the storage with _mapcount), setting a
page flag means clearing the appropriate bit. This gives us space for
probably twenty or so extra bits (depending how paranoid we want to be
about _mapcount underflow).

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
include/linux/mm_types.h | 13 ++++++++-----
include/linux/page-flags.h | 45 ++++++++++++++++++++++++++-------------------
kernel/crash_core.c | 1 +
mm/page_alloc.c | 13 +++++--------
scripts/tags.sh | 6 +++---
5 files changed, 43 insertions(+), 35 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index fd1af6b9591d..1c5dea402501 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -94,6 +94,14 @@ struct page {
};

union {
+ /*
+ * If the page is neither PageSlab nor PageAnon, the value
+ * stored here may help distinguish it from page cache pages.
+ * See page-flags.h for a list of page types which are
+ * currently stored here.
+ */
+ unsigned int page_type;
+
_slub_counter_t counters;
unsigned int active; /* SLAB */
struct { /* SLUB */
@@ -107,11 +115,6 @@ struct page {
/*
* Count of ptes mapped in mms, to show when
* page is mapped & limit reverse map searches.
- *
- * Extra information about page type may be
- * stored here for pages that are never mapped,
- * in which case the value MUST BE <= -2.
- * See page-flags.h for more details.
*/
atomic_t _mapcount;

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 50c2b8786831..be3cd45efeb6 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -630,49 +630,56 @@ PAGEFLAG_FALSE(DoubleMap)
#endif

/*
- * For pages that are never mapped to userspace, page->mapcount may be
- * used for storing extra information about page type. Any value used
- * for this purpose must be <= -2, but it's better start not too close
- * to -2 so that an underflow of the page_mapcount() won't be mistaken
- * for a special page.
+ * For pages that are never mapped to userspace (and aren't PageSlab),
+ * page_type may be used. Because it is initialised to -1, we invert the
+ * sense of the bit, so __SetPageFoo *clears* the bit used for PageFoo, and
+ * __ClearPageFoo *sets* the bit used for PageFoo. We reserve a few high and
+ * low bits so that an underflow or overflow of page_mapcount() won't be
+ * mistaken for a page type value.
*/
-#define PAGE_MAPCOUNT_OPS(uname, lname) \
+
+#define PAGE_TYPE_BASE 0xf0000000
+/* Reserve 0x0000007f to catch underflows of page_mapcount */
+#define PG_buddy 0x00000080
+#define PG_balloon 0x00000100
+#define PG_kmemcg 0x00000200
+
+#define PageType(page, flag) \
+ ((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
+
+#define PAGE_TYPE_OPS(uname, lname) \
static __always_inline int Page##uname(struct page *page) \
{ \
- return atomic_read(&page->_mapcount) == \
- PAGE_##lname##_MAPCOUNT_VALUE; \
+ return PageType(page, PG_##lname); \
} \
static __always_inline void __SetPage##uname(struct page *page) \
{ \
- VM_BUG_ON_PAGE(atomic_read(&page->_mapcount) != -1, page); \
- atomic_set(&page->_mapcount, PAGE_##lname##_MAPCOUNT_VALUE); \
+ VM_BUG_ON_PAGE(!PageType(page, 0), page); \
+ page->page_type &= ~PG_##lname; \
} \
static __always_inline void __ClearPage##uname(struct page *page) \
{ \
VM_BUG_ON_PAGE(!Page##uname(page), page); \
- atomic_set(&page->_mapcount, -1); \
+ page->page_type |= PG_##lname; \
}

/*
- * PageBuddy() indicate that the page is free and in the buddy system
+ * PageBuddy() indicates that the page is free and in the buddy system
* (see mm/page_alloc.c).
*/
-#define PAGE_BUDDY_MAPCOUNT_VALUE (-128)
-PAGE_MAPCOUNT_OPS(Buddy, BUDDY)
+PAGE_TYPE_OPS(Buddy, buddy)

/*
- * PageBalloon() is set on pages that are on the balloon page list
+ * PageBalloon() is true for pages that are on the balloon page list
* (see mm/balloon_compaction.c).
*/
-#define PAGE_BALLOON_MAPCOUNT_VALUE (-256)
-PAGE_MAPCOUNT_OPS(Balloon, BALLOON)
+PAGE_TYPE_OPS(Balloon, balloon)

/*
* If kmemcg is enabled, the buddy allocator will set PageKmemcg() on
* pages allocated with __GFP_ACCOUNT. It gets cleared on page free.
*/
-#define PAGE_KMEMCG_MAPCOUNT_VALUE (-512)
-PAGE_MAPCOUNT_OPS(Kmemcg, KMEMCG)
+PAGE_TYPE_OPS(Kmemcg, kmemcg)

extern bool is_free_buddy_page(struct page *page);

diff --git a/kernel/crash_core.c b/kernel/crash_core.c
index 4f63597c824d..b02340fb99ff 100644
--- a/kernel/crash_core.c
+++ b/kernel/crash_core.c
@@ -458,6 +458,7 @@ static int __init crash_save_vmcoreinfo_init(void)
VMCOREINFO_NUMBER(PG_hwpoison);
#endif
VMCOREINFO_NUMBER(PG_head_mask);
+#define PAGE_BUDDY_MAPCOUNT_VALUE (~PG_buddy)
VMCOREINFO_NUMBER(PAGE_BUDDY_MAPCOUNT_VALUE);
#ifdef CONFIG_HUGETLB_PAGE
VMCOREINFO_NUMBER(HUGETLB_PAGE_DTOR);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index cb416723538f..ac0b24603030 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -744,16 +744,14 @@ static inline void rmv_page_order(struct page *page)

/*
* This function checks whether a page is free && is the buddy
- * we can do coalesce a page and its buddy if
+ * we can coalesce a page and its buddy if
* (a) the buddy is not in a hole (check before calling!) &&
* (b) the buddy is in the buddy system &&
* (c) a page and its buddy have the same order &&
* (d) a page and its buddy are in the same zone.
*
- * For recording whether a page is in the buddy system, we set ->_mapcount
- * PAGE_BUDDY_MAPCOUNT_VALUE.
- * Setting, clearing, and testing _mapcount PAGE_BUDDY_MAPCOUNT_VALUE is
- * serialized by zone->lock.
+ * For recording whether a page is in the buddy system, we set PageBuddy.
+ * Setting, clearing, and testing PageBuddy is serialized by zone->lock.
*
* For recording page's order, we use page_private(page).
*/
@@ -798,9 +796,8 @@ static inline int page_is_buddy(struct page *page, struct page *buddy,
* as necessary, plus some accounting needed to play nicely with other
* parts of the VM system.
* At each level, we keep a list of pages, which are heads of continuous
- * free pages of length of (1 << order) and marked with _mapcount
- * PAGE_BUDDY_MAPCOUNT_VALUE. Page's order is recorded in page_private(page)
- * field.
+ * free pages of length of (1 << order) and marked with PageBuddy.
+ * Page's order is recorded in page_private(page) field.
* So when we are allocating or freeing one, we can derive the state of the
* other. That is, if we allocate a small block, and both were
* free, the remainder of the region must be split into blocks.
diff --git a/scripts/tags.sh b/scripts/tags.sh
index 78e546ff689c..8c3ae36d4ea8 100755
--- a/scripts/tags.sh
+++ b/scripts/tags.sh
@@ -188,9 +188,9 @@ regex_c=(
'/\<CLEARPAGEFLAG_NOOP(\([[:alnum:]_]*\).*/ClearPage\1/'
'/\<__CLEARPAGEFLAG_NOOP(\([[:alnum:]_]*\).*/__ClearPage\1/'
'/\<TESTCLEARFLAG_FALSE(\([[:alnum:]_]*\).*/TestClearPage\1/'
- '/^PAGE_MAPCOUNT_OPS(\([[:alnum:]_]*\).*/Page\1/'
- '/^PAGE_MAPCOUNT_OPS(\([[:alnum:]_]*\).*/__SetPage\1/'
- '/^PAGE_MAPCOUNT_OPS(\([[:alnum:]_]*\).*/__ClearPage\1/'
+ '/^PAGE_TYPE_OPS(\([[:alnum:]_]*\).*/Page\1/'
+ '/^PAGE_TYPE_OPS(\([[:alnum:]_]*\).*/__SetPage\1/'
+ '/^PAGE_TYPE_OPS(\([[:alnum:]_]*\).*/__ClearPage\1/'
'/^TASK_PFA_TEST([^,]*, *\([[:alnum:]_]*\))/task_\1/'
'/^TASK_PFA_SET([^,]*, *\([[:alnum:]_]*\))/task_set_\1/'
'/^TASK_PFA_CLEAR([^,]*, *\([[:alnum:]_]*\))/task_clear_\1/'
--
2.16.1
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