[PATCH 4/9] mm: Support page_mapcount() on page_has_type() pages

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

Return 0 for pages which can't be mapped.  This matches how page_mapped()
works.  It is more convenient for users to not have to filter out
these pages.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 fs/proc/page.c             | 7 ++-----
 include/linux/mm.h         | 8 +++++---
 include/linux/page-flags.h | 4 ++--
 3 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/fs/proc/page.c b/fs/proc/page.c
index 195b077c0fac..9223856c934b 100644
--- a/fs/proc/page.c
+++ b/fs/proc/page.c
@@ -67,7 +67,7 @@ static ssize_t kpagecount_read(struct file *file, char __user *buf,
 		 */
 		ppage = pfn_to_online_page(pfn);
 
-		if (!ppage || PageSlab(ppage) || page_has_type(ppage))
+		if (!ppage)
 			pcount = 0;
 		else
 			pcount = page_mapcount(ppage);
@@ -124,11 +124,8 @@ u64 stable_page_flags(struct page *page)
 
 	/*
 	 * pseudo flags for the well known (anonymous) memory mapped pages
-	 *
-	 * Note that page->_mapcount is overloaded in SLAB, so the
-	 * simple test in page_mapped() is not enough.
 	 */
-	if (!PageSlab(page) && page_mapped(page))
+	if (page_mapped(page))
 		u |= 1 << KPF_MMAP;
 	if (PageAnon(page))
 		u |= 1 << KPF_ANON;
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 0436b919f1c7..5ff3d687bc6c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1223,14 +1223,16 @@ static inline void page_mapcount_reset(struct page *page)
  * a large folio, it includes the number of times this page is mapped
  * as part of that folio.
  *
- * The result is undefined for pages which cannot be mapped into userspace.
- * For example SLAB or special types of pages. See function page_has_type().
- * They use this field in struct page differently.
+ * Will report 0 for pages which cannot be mapped into userspace, eg
+ * slab, page tables and similar.
  */
 static inline int page_mapcount(struct page *page)
 {
 	int mapcount = atomic_read(&page->_mapcount) + 1;
 
+	/* Handle page_has_type() pages */
+	if (mapcount < 0)
+		mapcount = 0;
 	if (unlikely(PageCompound(page)))
 		mapcount += folio_entire_mapcount(page_folio(page));
 
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 8d0e6ce25ca2..5852f967c640 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -971,12 +971,12 @@ static inline bool is_page_hwpoison(struct page *page)
  * 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
+ * low bits so that an underflow or overflow of _mapcount won't be
  * mistaken for a page type value.
  */
 
 #define PAGE_TYPE_BASE	0xf0000000
-/* Reserve		0x0000007f to catch underflows of page_mapcount */
+/* Reserve		0x0000007f to catch underflows of _mapcount */
 #define PAGE_MAPCOUNT_RESERVE	-128
 #define PG_buddy	0x00000080
 #define PG_offline	0x00000100
-- 
2.43.0