From e01f7ddcb18a32cd2fca3806a45b86751efc08ff Mon Sep 17 00:00:00 2001 From: Zi Yan Date: Sun, 20 Sep 2026 22:28:06 -0400 Subject: mm/page-flags: check page/folio->private instead of PG_private After the changes of the prior commits, page/folio->private != NULL is now equivalent to checking PG_private. Stop checking PG_private on pages and folios and use page/folio->private instead, except swapcache and hugetlb folios, because the former uses a field (swp_entry_t swap) overlapping with ->private and the latter sets its flags in ->private. Exclude swapcache and hugetlb when the code is meant to check PG_private only. PG_swapcache and folio->swap.val cannot be set/clear as a whole, so excluding swapcache with folio_test_swapcache() is not reliable. Instead, use folio_test_swapbacked(), since PG_swapbacked is stable when a folio is added to/removed from swapcache. Add a helper, folio_has_attached_private(), for this check. folio_test_private() and PagePrivate() now read folio/page->private plainly instead of an atomic read of PG_private bit, so KCSAN complains about possible data races. Annotate them with data_race(). folio_expected_ref_count() can be called without the folio lock, so annotate folio->mapping with data_race() while at it. folio_set/clear_private() and Set/ClearPagePrivate() become no-ops. PG_private is no longer checked at page free time. They will be removed in an upcoming commit. Remove KPF_PRIVATE since PG_private is no longer used. Link: https://lore.kernel.org/20260920-remove-pg_private-v5-10-bb68b6a21869@nvidia.com Signed-off-by: Zi Yan Signed-off-by: Andrew Morton Acked-by: David Hildenbrand (Arm) Assisted-by: LLM Cc: Steven Rostedt Cc: Masami Hiramatsu Cc: Lorenzo Stoakes Cc: "Matthew Wilcox (Oracle)" Cc: Jan Kara Cc: Johannes Weiner Cc: "Liam R. Howlett" Cc: Vlastimil Babka Cc: Mike Rapoport Cc: Suren Baghdasaryan Cc: Michal Hocko Cc: Mathieu Desnoyers Cc: Baolin Wang Cc: Nico Pache Cc: Ryan Roberts Cc: Dev Jain Cc: Barry Song Cc: Lance Yang Cc: Usama Arif Cc: Matthew Brost Cc: Joshua Hahn Cc: Rakie Kim Cc: Byungchul Park Cc: Gregory Price Cc: Ying Huang Cc: Alistair Popple Cc: Qi Zheng Cc: Shakeel Butt Cc: Kairui Song Cc: Axel Rasmussen Cc: Yuanchu Xie Cc: Wei Xu --- include/linux/kernel-page-flags.h | 1 - include/linux/mm.h | 19 ++++++++----- include/linux/page-flags.h | 57 ++++++++++++++++++++++++++++++++++----- 3 files changed, 63 insertions(+), 14 deletions(-) (limited to 'include/linux') diff --git a/include/linux/kernel-page-flags.h b/include/linux/kernel-page-flags.h index 196778a087c4..fe5ab6e50bd7 100644 --- a/include/linux/kernel-page-flags.h +++ b/include/linux/kernel-page-flags.h @@ -11,7 +11,6 @@ #define KPF_RESERVED 32 #define KPF_MLOCKED 33 #define KPF_OWNER_2 34 -#define KPF_PRIVATE 35 #define KPF_PRIVATE_2 36 #define KPF_OWNER_PRIVATE 37 #define KPF_ARCH 38 diff --git a/include/linux/mm.h b/include/linux/mm.h index 30a3365bca82..6c8df7715eb2 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -3005,9 +3005,9 @@ static inline bool folio_maybe_mapped_shared(struct folio *folio) * @folio: the folio * * Calculate the expected folio refcount, taking references from the pagecache, - * swapcache, PG_private and page table mappings into account. Useful in - * combination with folio_ref_count() to detect unexpected references (e.g., - * GUP or other temporary references). + * swapcache, private data (folio->private != NULL) and page table mappings into + * account. Useful in combination with folio_ref_count() to detect unexpected + * references (e.g., GUP or other temporary references). * * Does currently not consider references from the LRU cache. If the folio * was isolated from the LRU (which is the case during migration or split), @@ -3045,10 +3045,15 @@ static inline int folio_expected_ref_count(const struct folio *folio) ref_count += folio_test_swapcache(folio) << order; if (!folio_test_anon(folio)) { - /* One reference per page from the pagecache. */ - ref_count += !!folio->mapping << order; - /* One reference from PG_private. */ - ref_count += folio_test_private(folio); + /* + * One reference per page from the pagecache. + * Use data_race() since folio might not be locked. + */ + ref_count += !!data_race(folio->mapping) << order; + /* + * One reference from filesystem private data. + */ + ref_count += folio_has_attached_private(folio); } /* One reference per page table mapping. */ diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h index 7080a6a1a79e..6d839f50bdcb 100644 --- a/include/linux/page-flags.h +++ b/include/linux/page-flags.h @@ -575,9 +575,31 @@ FOLIO_FLAG(swapbacked, FOLIO_HEAD_PAGE) /* * Private page markings that may be used by the filesystem that owns the page * for its own purposes. - * - PG_private and PG_private_2 cause release_folio() and co to be invoked + * - folio->private and PG_private_2 cause release_folio() and co to be invoked */ -PAGEFLAG(Private, private, PF_ANY) + +static __always_inline bool folio_test_private(const struct folio *folio) +{ + /* + * data_race() is added for readers without holding the folio lock. + * Only the NULL/non-NULL answer is used and both are valid while + * private is being attached or detached, so the race is benign. + */ + return data_race(folio->private); +} + +static __always_inline int PagePrivate(const struct page *page) +{ + /* See folio_test_private() for data_race() use */ + return !!data_race(page->private); +} + +/* no-ops during transition */ +static __always_inline void folio_set_private(struct folio *folio) { } +static __always_inline void folio_clear_private(struct folio *folio) { } +static __always_inline void SetPagePrivate(struct page *page) { } +static __always_inline void ClearPagePrivate(struct page *page) { } + FOLIO_FLAG(private_2, FOLIO_HEAD_PAGE) /* owner_2 can be set on tail pages for anon memory */ @@ -1169,7 +1191,7 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page) */ #define PAGE_FLAGS_CHECK_AT_FREE \ (1UL << PG_lru | 1UL << PG_locked | \ - 1UL << PG_private | 1UL << PG_private_2 | \ + 1UL << PG_private_2 | \ 1UL << PG_writeback | 1UL << PG_reserved | \ 1UL << PG_active | \ 1UL << PG_unevictable | __PG_MLOCKED | LRU_GEN_MASK) @@ -1193,8 +1215,31 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page) (0xffUL /* order */ | 1UL << PG_has_hwpoisoned | \ 1UL << PG_large_rmappable | 1UL << PG_partially_mapped) -#define PAGE_FLAGS_PRIVATE \ - (1UL << PG_private | 1UL << PG_private_2) +/** + * folio_has_attached_private - check if the folio has private data attached + * @folio: The folio to check. + * + * Use this in code that may encounter swapcache or hugetlb folios but only + * wants to detect attached private data. + * + * Return: true if the folio has private data attached. + */ +static inline bool folio_has_attached_private(const struct folio *folio) +{ + /* + * Swapcache stores swp_entry_t in folio->swap, a union with + * folio->private, and hugetlb stores its own flags in folio->private; + * both are excluded. + * + * NOTE: For swapcache, folio->swap.val PG_swapcache are not set as + * a whole, so folio_test_swapcache() is not reliable to exclude + * swapcache. Use folio_test_swapbacked() instead, since it remains set + * when a folio is added to/removed from swapcache. + */ + + return folio_test_private(folio) && !folio_test_swapbacked(folio) && + !folio_test_hugetlb(folio); +} /** * folio_has_private - Determine if folio has private stuff * @folio: The folio to be checked @@ -1204,7 +1249,7 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page) */ static inline int folio_has_private(const struct folio *folio) { - return !!(folio->flags.f & PAGE_FLAGS_PRIVATE); + return folio_has_attached_private(folio) || folio_test_private_2(folio); } #undef PF_ANY -- cgit v1.2.3