On Fri Jul 31, 2026 at 10:13 PM EDT, Zi Yan wrote:
> 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.
>
> folio_set/clear_private() and Set/ClearPagePrivate() become no-ops.
> PG_private is no longer checked at page free time.
>
> KPF_PRIVATE exposes PG_private to userspace. Change its code logic to check
> folio->private != NULL and exclude non-pagecache, swapcache, hugetlb, and
> anon folios. One minor semantic change, for orphaned pagecache folios
> (mapping == NULL) with fs-private data will no longer have KPF_PRIVATE.
>
> Assisted-by: Claude:claude-opus-4-8
> Assisted-by: Codex:gpt-5
> Signed-off-by: Zi Yan <[email protected]>
> To: Andrew Morton <[email protected]>
> To: David Hildenbrand <[email protected]>
> To: Steven Rostedt <[email protected]>
> To: Masami Hiramatsu <[email protected]>
> To: Lorenzo Stoakes <[email protected]>
> To: "Matthew Wilcox (Oracle)" <[email protected]>
> To: Jan Kara <[email protected]>
> To: Johannes Weiner <[email protected]>
> Cc: "Liam R. Howlett" <[email protected]>
> Cc: Vlastimil Babka <[email protected]>
> Cc: Mike Rapoport <[email protected]>
> Cc: Suren Baghdasaryan <[email protected]>
> Cc: Michal Hocko <[email protected]>
> Cc: Mathieu Desnoyers <[email protected]>
> Cc: Zi Yan <[email protected]>
> Cc: Baolin Wang <[email protected]>
> Cc: Nico Pache <[email protected]>
> Cc: Ryan Roberts <[email protected]>
> Cc: Dev Jain <[email protected]>
> Cc: Barry Song <[email protected]>
> Cc: Lance Yang <[email protected]>
> Cc: Usama Arif <[email protected]>
> Cc: Matthew Brost <[email protected]>
> Cc: Joshua Hahn <[email protected]>
> Cc: Rakie Kim <[email protected]>
> Cc: Byungchul Park <[email protected]>
> Cc: Gregory Price <[email protected]>
> Cc: Ying Huang <[email protected]>
> Cc: Alistair Popple <[email protected]>
> Cc: Qi Zheng <[email protected]>
> Cc: Shakeel Butt <[email protected]>
> Cc: Kairui Song <[email protected]>
> Cc: Axel Rasmussen <[email protected]>
> Cc: Yuanchu Xie <[email protected]>
> Cc: Wei Xu <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> ---
> fs/proc/page.c | 6 +++++-
> include/linux/mm.h | 11 ++++++-----
> include/linux/page-flags.h | 26 +++++++++++++++++++++-----
> include/trace/events/pagemap.h | 4 +++-
> mm/huge_memory.c | 4 +++-
> mm/migrate.c | 3 ++-
> mm/page-writeback.c | 5 ++++-
> mm/vmscan.c | 3 ++-
> 8 files changed, 46 insertions(+), 16 deletions(-)
>
> diff --git a/fs/proc/page.c b/fs/proc/page.c
> index 260772b20bd99..abfa6f7d890cc 100644
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -232,7 +232,11 @@ u64 stable_page_flags(const struct page *page)
>
> u |= kpf_copy_bit(k, KPF_RESERVED, PG_reserved);
> u |= kpf_copy_bit(k, KPF_OWNER_2, PG_owner_2);
> - u |= kpf_copy_bit(k, KPF_PRIVATE, PG_private);
> + /* preserve the original KPF_PRIVATE semantics by excluding non
> pagecache folios */
> + if (folio->mapping && !folio_test_anon(folio) &&
> + (folio_get_private(folio) && !folio_test_swapcache(folio) &&
> + !folio_test_hugetlb(folio)))
> + u |= BIT_ULL(KPF_PRIVATE);
Based on the discussion in THP cabal meeting, KPF_PRIVATE will be
removed in v2, since kernel page flags are not stable.
> u |= kpf_copy_bit(k, KPF_PRIVATE_2, PG_private_2);
> u |= kpf_copy_bit(k, KPF_OWNER_PRIVATE, PG_owner_priv_1);
> u |= kpf_copy_bit(k, KPF_ARCH, PG_arch_1);
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 7fabe6c66b4b7..ebc035ac26ccc 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2961,9 +2961,9 @@ 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),
> @@ -3003,8 +3003,9 @@ static inline int folio_expected_ref_count(const struct
> folio *folio)
> 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 from filesystem private data. */
> + ref_count += !!folio->private && !folio_test_hugetlb(folio) &&
> + !folio_test_swapcache(folio);
> }
Sashiko asked:
folio_expected_ref_count() can be called in lockless contexts, does
reading folio->private directly introduce a data race? And whether a
READ_ONCE() should be used for folio->private.
Answer:
Yes, we expect that and caller should recheck refcount after they hold
the folio lock. Use data_race() for folio->private reading. While at it,
add data_race() for folio->mapping too.
--
Best Regards,
Yan, Zi
