The patch titled Subject: mm: check if folio has valid mapcount before folio_test_{anon,ksm}() when necessary has been added to the -mm mm-new branch. Its filename is mm-check-if-folio-has-valid-mapcount-before-folio_test_anonksm-when-necessary.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-new branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Harry Yoo harry.yoo@oracle.com Subject: mm: check if folio has valid mapcount before folio_test_{anon,ksm}() when necessary Date: Mon, 7 Jul 2025 21:07:40 +0900

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

Link: https://lkml.kernel.org/r/20250707120740.4413-1-harry.yoo@oracle.com Fixes: 130d4df57390 ("mm/sl[au]b: rearrange struct slab fields to allow larger rcu_head") Signed-off-by: Harry Yoo harry.yoo@oracle.com Suggested-by: Vlastimil Babka vbabka@suse.cz Cc: Christoph Lameter (Ampere) cl@gentwo.org Cc: David Hildenbrand david@redhat.com Cc: David Rientjes rientjes@google.com Cc: Liam Howlett liam.howlett@oracle.com Cc: Lorenzo Stoakes lorenzo.stoakes@oracle.com Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Michal Hocko mhocko@suse.com Cc: Mike Rapoport rppt@kernel.org Cc: Suren Baghdasaryan surenb@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

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

--- a/fs/proc/page.c~mm-check-if-folio-has-valid-mapcount-before-folio_test_anonksm-when-necessary +++ a/fs/proc/page.c @@ -148,18 +148,21 @@ u64 stable_page_flags(const struct 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; + } }

/* --- a/include/linux/mm.h~mm-check-if-folio-has-valid-mapcount-before-folio_test_anonksm-when-necessary +++ a/include/linux/mm.h @@ -2167,7 +2167,7 @@ static inline int folio_expected_ref_cou 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)) { --- a/include/linux/page-flags.h~mm-check-if-folio-has-valid-mapcount-before-folio_test_anonksm-when-necessary +++ a/include/linux/page-flags.h @@ -706,12 +706,15 @@ PAGEFLAG_FALSE(VmemmapSelfHosted, vmemma * 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 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 _

Patches currently in -mm which might be from harry.yoo@oracle.com are

lib-alloc_tag-do-not-acquire-non-existent-lock-in-alloc_tag_top_users.patch lib-alloc_tag-do-not-acquire-non-existent-lock-in-alloc_tag_top_users-v3.patch mm-zsmalloc-do-not-pass-__gfp_movable-if-config_compaction=n.patch mm-check-if-folio-has-valid-mapcount-before-folio_test_anonksm-when-necessary.patch