[PATCH v3 mm-new] fs/proc/page: avoid anon folio checks on typed pages except hugetlb

[Harry Yoo <harry.yoo@oracle.com>]

folio_test_anon() and folio_test_ksm() may return false positives when
invoked on typed pages (except hugetlb folios), because lower bits of
folio->mapping may be set even when they do not indicate
FOLIO_MAPPING_* flags.

This leads to tools/mm/page-types reporting pages with
KPF_SLAB, KPF_ANON and KPF_KSM (with flags, page-counts, MB omitted):
  $ sudo ./page-types | grep slab
  _______S___________________________________   slab
  _______S____a________x_____________________   slab,anonymous,ksm

Currently, and going forward (The New York interpretation),
typed pages except hugetlb do not have FOLIO_MAPPING_* flags.
In the future, they won't even cast to folios.

For now, avoid checking FOLIO_MAPPING_* flags on pages if they are
typed pages that are not hugetlb folios.

Update the comment in FOLIO_MAPPING_* flags accordingly.

Fixes: 130d4df57390 ("mm/sl[au]b: rearrange struct slab fields to allow larger rcu_head")
Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
---

v2->v3: Did not introduce folio_has_mapcount() per David's suggestion
v2: https://lore.kernel.org/linux-mm/20250707120740.4413-1-harry.yoo@oracle.com

 fs/proc/page.c             | 19 +++++++++++--------
 include/linux/page-flags.h | 15 +++++++++------
 2 files changed, 20 insertions(+), 14 deletions(-)

diff --git a/fs/proc/page.c b/fs/proc/page.c
index 0cdc78c0d23f..07582dbdef45 100644
--- a/fs/proc/page.c
+++ b/fs/proc/page.c
@@ -148,18 +148,21 @@ u64 stable_page_flags(const struct page *page)
 	folio = page_folio(page);
 
 	k = folio->flags;
-	mapping = (unsigned long)folio->mapping;
-	is_anon = mapping & FOLIO_MAPPING_ANON;
 
 	/*
 	 * pseudo flags for the well known (anonymous) memory mapped pages
 	 */
-	if (page_mapped(page))
-		u |= 1 << KPF_MMAP;
-	if (is_anon) {
-		u |= 1 << KPF_ANON;
-		if (mapping & FOLIO_MAPPING_KSM)
-			u |= 1 << KPF_KSM;
+	if (!page_has_type(&folio->page) || folio_test_hugetlb(folio)) {
+		mapping = (unsigned long)folio->mapping;
+		is_anon = mapping & FOLIO_MAPPING_ANON;
+
+		if (page_mapped(page))
+			u |= 1 << KPF_MMAP;
+		if (is_anon) {
+			u |= 1 << KPF_ANON;
+			if (mapping & FOLIO_MAPPING_KSM)
+				u |= 1 << KPF_KSM;
+		}
 	}
 
 	/*
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 8e4d6eda8a8d..26ae4c7cf8dd 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -706,12 +706,15 @@ PAGEFLAG_FALSE(VmemmapSelfHosted, vmemmap_self_hosted)
  * address_space which maps the folio from disk; whereas "folio_mapped"
  * refers to user virtual address space into which the folio is mapped.
  *
- * For slab pages, since slab reuses the bits in struct page to store its
- * internal states, the folio->mapping does not exist as such, nor do
- * these flags below.  So in order to avoid testing non-existent bits,
- * please make sure that folio_test_slab(folio) actually evaluates to
- * false before calling the following functions (e.g., folio_test_anon).
- * See mm/slab.h.
+ * For certain typed pages like slabs, since they reuse bits in struct page
+ * to store internal states, folio->mapping does not point to a valid
+ * mapping, nor do these flags exist. To avoid testing non-existent bits,
+ * make sure !page_has_type(&folio->page) || folio_test_hugetlb(folio)
+ * actually evaluates to true before calling the following functions
+ * (e.g., folio_test_anon).
+ *
+ * This check can be skipped if the folio is mapped to userspace, since
+ * typed pages except hugetlb cannot be mapped to userspace at all.
  */
 #define FOLIO_MAPPING_ANON	0x1
 #define FOLIO_MAPPING_ANON_KSM	0x2
-- 
2.43.0