[PATCH v2 mm-new] mm: check if folio has valid mapcount before folio_test_{anon,ksm}() when necessary

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

folio_test_anon() and folio_test_ksm() may return false positives when
the folio is a typed page (except hugetlb), because lower bits of
folio->mapping field can be set even if it doesn't mean
FOLIO_MAPPING_* flags.

To avoid false positives, folio_test_{anon,ksm}() should be called
only if !page_has_type(&folio->page) || folio_test_hugetlb(folio).
However, the check can be skipped if a folio is or will be mapped
to userspace because typed pages that are not hugetlb folios cannot
be mapped to userspace.

As folio_expected_ref_count() already does the check, introduce a helper
function folio_has_mapcount() and use it in folio_expected_ref_count()
and stable_page_flags().

Update the comment in FOLIO_MAPPING_* flags accordingly.

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

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>
---

No Cc: stable because incorrect kpageflags is not considered a serious bug.

v1: https://lore.kernel.org/linux-mm/20250609132723.13118-1-harry.yoo@oracle.com/

v1 -> v2:
  - Dropped tricks to fix folio_test_slab() false positives and instead
    adjusted Vlastimil's feedback: it sounds like a better approach to
    check folio_test_slab() before folio_test_{anon,ksm}().

    But a slight change from Vlastimil's suggestion: 
    check (!page_has_type() || PageHuge()) instead of folio_test_slab(),
    which should be more robust. (Borrowed from David's patch)

 fs/proc/page.c             | 19 +++++++++++--------
 include/linux/mm.h         |  2 +-
 include/linux/page-flags.h | 20 ++++++++++++++------
 3 files changed, 26 insertions(+), 15 deletions(-)

diff --git a/fs/proc/page.c b/fs/proc/page.c
index 0cdc78c0d23f..d6efddf6adf9 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 (folio_has_mapcount(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/mm.h b/include/linux/mm.h
index 805108d7bbc3..8bc6a38e0f9d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2167,7 +2167,7 @@ static inline int folio_expected_ref_count(const struct folio *folio)
 	const int order = folio_order(folio);
 	int ref_count = 0;
 
-	if (WARN_ON_ONCE(page_has_type(&folio->page) && !folio_test_hugetlb(folio)))
+	if (WARN_ON_ONCE(!folio_has_mapcount(folio)))
 		return 0;
 
 	if (folio_test_anon(folio)) {
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 970600d79dac..00ba2c8b221e 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 folio_has_mapcount() actually evaluates to true before calling
+ * the following functions (e.g., folio_test_anon).
+ *
+ * The folio_has_mapcount() check can be skipped if the folio is mapped
+ * to userspace, since a folio with !folio_has_mapcount() cannot be mapped
+ * to userspace at all.
  */
 #define FOLIO_MAPPING_ANON	0x1
 #define FOLIO_MAPPING_ANON_KSM	0x2
@@ -1092,6 +1095,11 @@ static inline bool PageHuge(const struct page *page)
 	return folio_test_hugetlb(page_folio(page));
 }
 
+static inline bool folio_has_mapcount(const struct folio *folio)
+{
+	return !page_has_type(&folio->page) || folio_test_hugetlb(folio);
+}
+
 /*
  * Check if a page is currently marked HWPoisoned. Note that this check is
  * best effort only and inherently racy: there is no way to synchronize with
-- 
2.43.0