mm: support page_mapcount() on page_has_type() pages

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.

Link: https://lkml.kernel.org/r/20240321142448.1645400-5-willy@infradead.org
Fixes: 9c5ccf2db04b ("mm: remove HUGETLB_PAGE_DTOR")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/fs/proc/page.c b/fs/proc/page.c
index 195b077..9223856 100644 (file)
--- 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 7b0ee64..b6bdaa1 100644 (file)
--- 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 dc1607f..35a0087 100644 (file)
--- 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