/proc/kpagecount: return 0 for special pages that are never mapped

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.

[[email protected]: incorporate feedback from Matthew Wilcox]
  Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Anthony Yznaga <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Acked-by: Matthew Wilcox <[email protected]>
Reviewed-by: Naoya Horiguchi <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Alexander Duyck <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Miles Chen <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Anthony Yznaga

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);

include/linux/page-flags.h

@@ -669,6 +669,7 @@ PAGEFLAG_FALSE(DoubleMap)
 
 #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 @@ PAGEFLAG_FALSE(DoubleMap)
 #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)		\
 {									\