[PATCH v2] /proc/kpagecount: return 0 for special pages that are never mapped

[Anthony Yznaga]

Certain pages that are never mapped to userspace have a type
indicated in the page_type field of their struct pages (e.g. PG_buddy).
page_type overlaps with _mapcount so set the count to 0 and avoid
calling page_mapcount() for these pages.

Signed-off-by: Anthony Yznaga <anthony.yznaga@xxxxxxxxxx>
Acked-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
---
	v2 - incorporated feedback from Matthew Wilcox

 fs/proc/page.c             | 2 +-
 include/linux/page-flags.h | 6 ++++++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/proc/page.c b/fs/proc/page.c
index 6c517b11acf8..40b05e0d4274 100644
--- a/fs/proc/page.c
+++ b/fs/proc/page.c
@@ -46,7 +46,7 @@ static ssize_t kpagecount_read(struct file *file, char __user *buf,
 			ppage = pfn_to_page(pfn);
 		else
 			ppage = NULL;
-		if (!ppage || PageSlab(ppage))
+		if (!ppage || PageSlab(ppage) || page_has_type(ppage))
 			pcount = 0;
 		else
 			pcount = page_mapcount(ppage);
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 50ce1bddaf56..39b4494e29f1 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -669,6 +669,7 @@ static inline int TestClearPageDoubleMap(struct page *page)
 
 #define PAGE_TYPE_BASE	0xf0000000
 /* Reserve		0x0000007f to catch underflows of page_mapcount */
+#define PAGE_MAPCOUNT_RESERVE	-128
 #define PG_buddy	0x00000080
 #define PG_balloon	0x00000100
 #define PG_kmemcg	0x00000200
@@ -677,6 +678,11 @@ static inline int TestClearPageDoubleMap(struct page *page)
 #define PageType(page, flag)						\
 	((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
 
+static inline int page_has_type(struct page *page)
+{
+	return (int)page->page_type < PAGE_MAPCOUNT_RESERVE;
+}
+
 #define PAGE_TYPE_OPS(uname, lname)					\
 static __always_inline int Page##uname(struct page *page)		\
 {									\
-- 
1.8.3.1