[PATCH v4 00/16] Remove PG_private by using page/folio->private checks instead

[PATCH v4 00/16] Remove PG_private by using page/folio->private checks instead

[Zi Yan]

Hi all,

This patchset removes PG_private to make space for upcoming PG_folio for
identifying pages from a folio (more details in Note below). Instead of
checking PG_private, all code is changed to check page/folio->private !=
NULL instead.

MM people are cc'd on all patches and subsystem people are cc'd on the
cover letter and corresponding patches.

Patch 6 is picked up separately in f2fs tree, but since mm-new does not
have it yet, it is sent for MM testing.

Overview
===
Most code uses folio_attach/detach/change_private() functions, so folio
refcount is increased and decreased when folio->private is set and reset,
respectively. There is no need to change them.

Changes are needed for exceptional users:
1. zsmalloc uses PG_private to indicate first component zpdesc page and
   page->private is used to store zspage in zpdesc. To remove PG_private,
   is_first_zpdesc() is replaced by pointer comparison.

2. kernel/events/ring_buffer.c stores page order in page->private.
   Replacing PG_private with page->private != NULL works.

3. drivers/xen/grant-table.c stores xen_page_foreign in page->private,
   where on 32-bit, a pointer to xen_page_foreign is stored; on 64-bit,
   page->private is used as xen_page_foreign. PG_private check is replaced
   by page->private != NULL on 32-bit for xen_page_foreign deallocation.
   On 64-bit, page->private is cleared unconditionally since {domid=0,
   gref=0} (xen_page_foreign can be 0) is valid.

4. fs/crypto/crypto.c stores a folio pointer in page->private, PG_private
   checks are replaced by page->private != NULL.

5. fs/erofs has two different uses:

    5a. folio->private is used to form a reversed list of
    the outputs of readahead_folio(). readahead_folio_last() is added to
    output folios in reversed order, so that ->private is no longer needed.

    5b. folio->private is used as an in-flight I/O counter. Convert the
    code to use folio_attach/detach/get_private() and add bias==1 to the
    counter to avoid folio->private being zero.

6. fs/nfs/write.c: folio refcount maintenance is in a bigger scope than
   folio->private. So folio_attach/detach/get_private() is not used.
   Nothing to change.

7. fs/f2fs uses attach_page_private() to first reset folio->private then
   immediately sets PAGE_PRIVATE_NOT_POINTER bit on it. Change it to use
   attach_page_private() to set PAGE_PRIVATE_NOT_POINTER bit directly to
   avoid folio->private == NULL gap inside set_page_private_##name().

8. hugetlb uses folio_change_private(folio, NULL) without folio refcount
   maintenance. Change it to folio->private = NULL.

After the above changes, PG_private ops are converted to
page/folio->private ops.

folio_has_attached_private() is added to check filesystem-only private data
by excluding swapcache and hugetlb folios, because swapcache folios overlap
swp_entry_t swap with ->private and hugetlb sets its own flags in
->private.

Note
===
1. KPF_PRIVATE is removed after PG_private is removed.

2. Documentation/mm/hugetlbfs_reserv.rst is outdated, so I did not remove
   PG_private related text. It should be rewritten.

3. PG_folio is planned to be set on every page from a folio in
   page_rmappable_folio(), so folios with any order (currently
   PG_large_rmappable is used to identify >0 order folios, but not order-0
   folios) can be identified. Then vm_insert_*() can correctly reject all
   folios and rmap code will only see folios. Eventually, page_folio()
   will return NULL for non-folio pages by checking PG_folio, but before
   that all existing users that treat compound pages as folios will need
   to be converted.

Tests
===
1. allmodconfig build passed.

2. zsmalloc is tested using ext4 on a 1GB lz4 zram:
    2a. zram load + zsmalloc compaction;
    2b. concurrent zspage migration via memory compaction;
    2c. confirmed that multi-page zspages actually formed.

    Details: https://github.com/x-y-z/linux-dev/blob/b4/remove-pg_private/test_zsmalloc.md

3. erofs is tested on images created with -C4096 and lz4hc, lzma,
   deflate, and zstd algorithms:
   3a. cold read of all files, verify checksums match source;
   3b. readahead + reclaim/migration race.

   Details: https://github.com/x-y-z/linux-dev/blob/b4/remove-pg_private/test_erofs.md

4. fscrypt is tested on software-encrypted ext4 with writes to exercise
   bounce pages.

   Details: https://github.com/x-y-z/linux-dev/blob/b4/remove-pg_private/test_fscrypt.md

5. f2fs is tested on an image with inline_data,compress_algorithm=lz4:
    5a. INLINE_INODE — lots of tiny files;
    5b. REF_RESOURCE + general writeback — buffered write churn with fsync;
    5c. ONGOING_MIGRATION — force GC / page migration;
    5d. ATOMIC_WRITE — atomic-write ioctl path.

    Details: https://github.com/x-y-z/linux-dev/blob/b4/remove-pg_private/test_f2fs.md
    (I did not run xfstests)

6. MM selftests passed.

LLM use
===
Claude was used to form a concrete plan on what code needs to be changed
and how to change them. The plan was reviewed by Codex until no issue was
spotted.

Plan is at: https://github.com/x-y-z/linux-dev/blob/b4/remove-pg_private/plan.md

I then followed the plan to make code changes. I did bounce ideas with
Claude how to change fs/erofs, since I did not like the original idea.
After each change, I asked Claude to review my code and git commit message.
I also asked Claude to give me test plans (see above).

At last, Codex was used to review all patches.

Comments and suggestions are welcome. Thanks.

Assisted-by: LLM
Signed-off-by: Zi Yan <ziy@nvidia.com>
---
Changes in v4:
1. dropped set_page_private(0) in balloon_retrieve(), since page->private
   is cleared at that point.
2. simplified the comment in add_hugetlb_folio().
3. additional cleanup for f2fs to remove fio->page uses and convert
   PAGE_PRIVATE_* flags and helper to folio-only.
4. added a comment for __readahead_advance().
5. added core-mm split/migration interaction information on newly added
   folio_attach/detach_private() for erofs.
6. renamed folio_test_fs_private() to folio_has_attached_private() and
   merged the commit introducing folio_test_fs_private() into its prior
   commit.
7. adjusted the patch subject: "treewide: remove folio_set/clear_private()
   *usage*"
8. split "treewide: replace PagePrivate() with page_private()" into three.
9. moved some comments in "treewide: remove PagePrivate() and PG_private
   from comments and docs" to prior patches along with code changes.
10. used PG_folio instead of __PG_folio to avoid additional
    code change in __def_pageflag_names().
- Link to v3: https://patch.msgid.link/20260907-remove-pg_private-v3-0-6ae22f9d9272@nvidia.com

Changes in v3:
1. changed folio_test_fs_private() to check PG_swapbacked instead of
   PG_swapcache for excluding swapcache folios. Because folio->private and
   PG_swapcache are not set as a whole, making folio_test_fs_private() give
   false positive, whereas PG_swapbacked is always set for swapcache
   folios.
2. added __DEF_PAGEFLAG_NAME() to show __PG_folio instead of open code.
3. f2fs change is picked up at
   https://git.kernel.org/jaegeuk/f2fs/c/5ad9409a9533, mm-new currently
   does not have it, so the patch is sent for MM testing purpose.
- Link to v2: https://patch.msgid.link/20260831-remove-pg_private-v2-0-3668159cd9e8@nvidia.com

Changes in v2:
1. removed is_first_zpdesc() in patch 1 and open coded the checks.
2. fixed wording in patch 2's commit message and clarified page_private()
   also works when ring buffer's AUX page order is 0.
3. removed the empty loop in 64-bit gnttab_pages_set_private().
4. clarified folio->private will be reset to NULL by
   fscrypt_free_bounce_page() in the commit message.
5. clarified why hugetlb needs to restore hugetlb_vmemmap_optimized.
6. renamed readahead_folio_reverse() readahead_folio_last() and
   reimplemented readahead_folio_last() by adding a new readahead_control
   private member, _forward, and a new helper __readahead_advance().
7. added a bias, 1, to erofs I/O counter, so that folio->private stays non
   NULL between folio_attach_private() and folio_detach_private().
8. converted more call sites to use folio_test_fs_private().
- Link to v1: https://lore.kernel.org/r/20260731-remove-pg_private-v1-0-142c97ba3562@nvidia.com

---
Zi Yan (16):
      mm/zsmalloc: replace PG_private with pointer comparison
      perf/ring_buffer: stop using PG_private as AUX page high-order marker
      xen/grant-table: stop setting PG_private on pages for grant mapping
      fscrypt: stop setting PG_private on bounce page
      mm/hugetlb: use direct assignment instead of folio_change_private()
      f2fs: stop using PG_private
      f2fs: convert the ->private flag helpers to folio-only
      erofs: mm/pagemap: add readahead_folio_last() to avoid folio->private
      erofs: use folio_attach/detach_private() instead of direct assignment
      mm/page-flags: check page/folio->private instead of PG_private
      treewide: remove folio_set/clear_private() usage
      ceph: replace PagePrivate() with page_private()
      md/md-bitmap: replace PagePrivate() with page_private()
      buffer: replace page_buffer() with page_private() and delete it
      treewide: remove PagePrivate() and PG_private from comments and docs
      mm/page-flags: remove PG_private

 Documentation/admin-guide/kdump/vmcoreinfo.rst |  2 +-
 Documentation/filesystems/vfs.rst              |  6 +-
 arch/x86/events/intel/bts.c                    |  3 -
 arch/x86/events/intel/pt.c                     |  6 +-
 drivers/md/md-bitmap.c                         |  7 +-
 drivers/xen/grant-table.c                      | 11 ++-
 fs/ceph/addr.c                                 |  8 +--
 fs/crypto/crypto.c                             |  2 -
 fs/erofs/data.c                                | 16 +++--
 fs/erofs/zdata.c                               | 13 +---
 fs/f2fs/compress.c                             | 35 +++++----
 fs/f2fs/data.c                                 |  2 +-
 fs/f2fs/f2fs.h                                 | 99 ++++++++++----------------
 fs/f2fs/segment.c                              |  2 +-
 fs/nfs/file.c                                  |  4 +-
 fs/nfs/write.c                                 |  2 -
 fs/proc/page.c                                 |  1 -
 fs/ubifs/file.c                                |  8 +--
 include/linux/buffer_head.h                    |  6 --
 include/linux/kernel-page-flags.h              |  1 -
 include/linux/mm.h                             | 35 +++++----
 include/linux/mm_types.h                       |  4 +-
 include/linux/page-flags.h                     | 43 ++++++++---
 include/linux/pagemap.h                        | 64 ++++++++++++++---
 include/trace/events/mmflags.h                 |  2 +-
 include/trace/events/pagemap.h                 |  3 +-
 kernel/events/ring_buffer.c                    |  7 +-
 kernel/vmcore_info.c                           |  1 -
 mm/huge_memory.c                               |  3 +-
 mm/hugetlb.c                                   |  7 +-
 mm/migrate.c                                   |  3 +-
 mm/page-writeback.c                            |  3 +-
 mm/vmscan.c                                    |  2 +-
 mm/zpdesc.h                                    |  2 +-
 mm/zsmalloc.c                                  | 24 ++-----
 tools/mm/page-types.c                          |  2 -
 36 files changed, 228 insertions(+), 211 deletions(-)
---
base-commit: 3833e2f6aa6bf6af169f78a27843dfa2804be5a6
change-id: 20260728-remove-pg_private-cfe926c7f83c

Best regards,
--  
Yan, Zi

[PATCH v4 10/16] mm/page-flags: check page/folio->private instead of PG_private

[Zi Yan]

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_expected_ref_count() can be called without folio lock, so annotate
folio_test_private() with data_race() to avoid triggering race condition
checks. While at it, annotate folio->mapping too. Add data_race()
annotation for other lockless callers too.

folio_set/clear_private() and Set/ClearPagePrivate() become no-ops.
PG_private is no longer checked at page free time.

Remove KPF_PRIVATE since PG_private is no longer used.

Assisted-by: LLM
Signed-off-by: Zi Yan <ziy@nvidia.com>
To: Andrew Morton <akpm@linux-foundation.org>
To: David Hildenbrand <david@kernel.org>
To: Steven Rostedt <rostedt@goodmis.org>
To: Masami Hiramatsu <mhiramat@kernel.org>
To: Lorenzo Stoakes <ljs@kernel.org>
To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
To: Jan Kara <jack@suse.cz>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Liam R. Howlett" <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Nico Pache <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Qi Zheng <qi.zheng@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Kairui Song <kasong@tencent.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: Wei Xu <weixugc@google.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: linux-trace-kernel@vger.kernel.org
---
 fs/proc/page.c                    |  1 -
 include/linux/kernel-page-flags.h |  1 -
 include/linux/mm.h                | 20 +++++++++++------
 include/linux/page-flags.h        | 46 ++++++++++++++++++++++++++++++++++-----
 include/trace/events/pagemap.h    |  3 ++-
 mm/huge_memory.c                  |  3 ++-
 mm/migrate.c                      |  2 +-
 mm/page-writeback.c               |  3 ++-
 mm/vmscan.c                       |  2 +-
 tools/mm/page-types.c             |  2 --
 10 files changed, 62 insertions(+), 21 deletions(-)

diff --git a/fs/proc/page.c b/fs/proc/page.c
index 260772b20bd99..f90e1030825e9 100644
--- a/fs/proc/page.c
+++ b/fs/proc/page.c
@@ -232,7 +232,6 @@ 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);
 	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/kernel-page-flags.h b/include/linux/kernel-page-flags.h
index 196778a087c4d..fe5ab6e50bd70 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 969594074fd2d..c9aad2c39fd9c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3004,9 +3004,9 @@ 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),
@@ -3044,10 +3044,16 @@ 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.
+		 * Use data_race() since folio might not be locked.
+		 */
+		ref_count += data_race(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 462e89e055485..08988877331ba 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -577,7 +577,23 @@ FOLIO_FLAG(swapbacked, FOLIO_HEAD_PAGE)
  * for its own purposes.
  * - PG_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)
+{
+	return folio->private;
+}
+
+static __always_inline int PagePrivate(const struct page *page)
+{
+	return !!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 +1185,7 @@ 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 +1209,28 @@ 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. 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: true if folio->private is set and the folio is neither swapcache
+ * nor hugetlb.
+ */
+static inline bool folio_has_attached_private(const struct folio *folio)
+{
+	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 +1240,7 @@ 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
diff --git a/include/trace/events/pagemap.h b/include/trace/events/pagemap.h
index 36c3a90f0acca..304652d6d8f2a 100644
--- a/include/trace/events/pagemap.h
+++ b/include/trace/events/pagemap.h
@@ -22,7 +22,8 @@
 	(folio_test_swapcache(folio)	? PAGEMAP_SWAPCACHE  : 0) | \
 	(folio_test_swapbacked(folio)	? PAGEMAP_SWAPBACKED : 0) | \
 	(folio_test_mappedtodisk(folio)	? PAGEMAP_MAPPEDDISK : 0) | \
-	(folio_test_private(folio)	? PAGEMAP_BUFFERS    : 0) \
+	/* data_race() is used to read attached private locklessly */ \
+	(data_race(folio_has_attached_private(folio))	? PAGEMAP_BUFFERS    : 0) \
 	)
 
 TRACE_EVENT(mm_lru_insertion,
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 30b7c63b0e359..8f4bcdccd8f35 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4845,8 +4845,9 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
 		 * For folios with private, split_huge_page_to_list_to_order()
 		 * will try to drop it before split and then check if the folio
 		 * can be split or not. So skip the check here.
+		 * data_race() is used to read attached private locklessly.
 		 */
-		if (!folio_test_private(folio) &&
+		if (!data_race(folio_has_attached_private(folio)) &&
 		    folio_expected_ref_count(folio) != folio_ref_count(folio))
 			goto next;
 
diff --git a/mm/migrate.c b/mm/migrate.c
index a369d0c95c386..b7b92925a28c3 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1327,7 +1327,7 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
 	 * free the metadata, so the page can be freed.
 	 */
 	if (!src->mapping) {
-		if (folio_test_private(src)) {
+		if (folio_has_attached_private(src)) {
 			try_to_free_buffers(src);
 			goto out;
 		}
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index eeab25d6ce364..0d754a678eac2 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2705,7 +2705,8 @@ bool filemap_dirty_folio(struct address_space *mapping, struct folio *folio)
 	if (folio_test_set_dirty(folio))
 		return false;
 
-	__folio_mark_dirty(folio, mapping, !folio_test_private(folio));
+	/* data_race() is used to read attached private locklessly */
+	__folio_mark_dirty(folio, mapping, !data_race(folio_has_attached_private(folio)));
 
 	if (mapping->host) {
 		/* !PageAnon && !swapper_space */
diff --git a/mm/vmscan.c b/mm/vmscan.c
index aaceed4759eeb..c2eb8fa9d5e50 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1029,7 +1029,7 @@ static void folio_check_dirty_writeback(struct folio *folio,
 	*writeback = folio_test_writeback(folio);
 
 	/* Verify dirty/writeback state if the filesystem supports it */
-	if (!folio_test_private(folio))
+	if (!folio_has_attached_private(folio))
 		return;
 
 	mapping = folio_mapping(folio);
diff --git a/tools/mm/page-types.c b/tools/mm/page-types.c
index 7fc5a8be5997f..47e4781c5fc38 100644
--- a/tools/mm/page-types.c
+++ b/tools/mm/page-types.c
@@ -73,7 +73,6 @@
 #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
@@ -131,7 +130,6 @@ static const char * const page_flag_names[] = {
 	[KPF_RESERVED]		= "r:reserved",
 	[KPF_MLOCKED]		= "m:mlocked",
 	[KPF_OWNER_2]		= "d:owner_2",
-	[KPF_PRIVATE]		= "P:private",
 	[KPF_PRIVATE_2]		= "p:private_2",
 	[KPF_OWNER_PRIVATE]	= "O:owner_private",
 	[KPF_ARCH]		= "h:arch",

-- 
2.53.0

[PATCH v4 16/16] mm/page-flags: remove PG_private

[Zi Yan]

folio->private != NULL indicates a folio carries private data, replacing
PG_private. All PG_private users are converted. Remove PG_private and
reserve the space as PG_folio for future use. Unused PG_private functions
are removed too.

Assisted-by: LLM
Signed-off-by: Zi Yan <ziy@nvidia.com>
To: Andrew Morton <akpm@linux-foundation.org>
To: Baoquan He <baoquan.he@linux.dev>
To: Mike Rapoport <rppt@kernel.org>
To: Pasha Tatashin <pasha.tatashin@soleen.com>
To: Pratyush Yadav <pratyush@kernel.org>
To: Jonathan Corbet <corbet@lwn.net>
To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
To: Jan Kara <jack@suse.cz>
To: David Hildenbrand <david@kernel.org>
To: Steven Rostedt <rostedt@goodmis.org>
To: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Dave Young <ruirui.yang@linux.dev>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: "Liam R. Howlett" <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: kexec@lists.infradead.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: linux-trace-kernel@vger.kernel.org
---
 include/linux/page-flags.h     | 19 ++-----------------
 include/trace/events/mmflags.h |  2 +-
 kernel/vmcore_info.c           |  1 -
 3 files changed, 3 insertions(+), 19 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 08988877331ba..10f4ad075ea48 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -44,10 +44,6 @@
  * Consequently, PG_reserved for a page mapped into user space can indicate
  * the zero page, the vDSO, MMIO pages or device memory.
  *
- * The PG_private bitflag is set on pagecache pages if they contain filesystem
- * specific data (which is normally at page->private). It can be used by
- * private allocations for its own usage.
- *
  * During initiation of disk I/O, PG_locked is set. This bit is set before I/O
  * and cleared when writeback _starts_ or when read _completes_. PG_writeback
  * is set before writeback starts and cleared when it finishes.
@@ -105,7 +101,7 @@ enum pageflags {
 	PG_owner_2,		/* Owner use. If pagecache, fs may use */
 	PG_arch_1,
 	PG_reserved,
-	PG_private,		/* If pagecache, has fs-private data */
+	PG_folio,		/* Do not use: reserved for folio identification */
 	PG_private_2,		/* If pagecache, has fs aux data */
 	PG_reclaim,		/* To be reclaimed asap */
 	PG_swapbacked,		/* Page is backed by RAM/swap */
@@ -575,7 +571,7 @@ 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
  */
 
 static __always_inline bool folio_test_private(const struct folio *folio)
@@ -583,17 +579,6 @@ static __always_inline bool folio_test_private(const struct folio *folio)
 	return folio->private;
 }
 
-static __always_inline int PagePrivate(const struct page *page)
-{
-	return !!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 */
diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
index ef9aa388b84f7..3c153b3ad8450 100644
--- a/include/trace/events/mmflags.h
+++ b/include/trace/events/mmflags.h
@@ -144,7 +144,7 @@ TRACE_DEFINE_ENUM(___GFP_LAST_BIT);
 	DEF_PAGEFLAG_NAME(owner_2),					\
 	DEF_PAGEFLAG_NAME(arch_1),					\
 	DEF_PAGEFLAG_NAME(reserved),					\
-	DEF_PAGEFLAG_NAME(private),					\
+	DEF_PAGEFLAG_NAME(folio),					\
 	DEF_PAGEFLAG_NAME(private_2),					\
 	DEF_PAGEFLAG_NAME(writeback),					\
 	DEF_PAGEFLAG_NAME(head),					\
diff --git a/kernel/vmcore_info.c b/kernel/vmcore_info.c
index 8614430ca212a..5a417f8a922ab 100644
--- a/kernel/vmcore_info.c
+++ b/kernel/vmcore_info.c
@@ -216,7 +216,6 @@ static int __init crash_save_vmcoreinfo_init(void)
 	VMCOREINFO_LENGTH(free_area.free_list, MIGRATE_TYPES);
 	VMCOREINFO_NUMBER(NR_FREE_PAGES);
 	VMCOREINFO_NUMBER(PG_lru);
-	VMCOREINFO_NUMBER(PG_private);
 	VMCOREINFO_NUMBER(PG_swapcache);
 	VMCOREINFO_NUMBER(PG_swapbacked);
 #define PAGE_SLAB_MAPCOUNT_VALUE	(PGTY_slab << 24)

-- 
2.53.0

Re: [PATCH v4 16/16] mm/page-flags: remove PG_private

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Silent memory leak of folio private data due to missing check for leaked `folio->private` data when a head page is freed.
--

commit ebe4dd474a496c46897e3f790e03a43cc0508eaf
Author: Zi Yan <ziy@nvidia.com>

mm/page-flags: remove PG_private

folio->private != NULL indicates a folio carries private data, replacing
PG_private. All PG_private users are converted. Remove PG_private and
reserve the space as PG_folio for future use. Unused PG_private functions
are removed too.

> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 08988877331ba..10f4ad075ea48 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h

[ ... ]

> @@ -105,7 +101,7 @@ enum pageflags {
>  	PG_owner_2,		/* Owner use. If pagecache, fs may use */
>  	PG_arch_1,
>  	PG_reserved,
> -	PG_private,		/* If pagecache, has fs-private data */
> +	PG_folio,		/* Do not use: reserved for folio identification */

[Severity: High]
With the removal of PG_private, does the free path for head pages fail to
validate whether private data is leaked?

Previously, the memory management subsystem checked for PG_private during page
freeing (via PAGE_FLAGS_CHECK_AT_FREE) to catch filesystem leaks, and issued a
bad_page() warning if it was still set. 

While tail pages are explicitly checked for leaked private data in the free
path:

	if (tail_page->private) {
		bad_page(tail_page, "nonzero private");
		...

The head page's private field appears to be unconditionally zeroed without
validation in __free_pages_prepare() in mm/page_alloc.c:

	page_cpupid_reset_last(page);
	page->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;
	page->private = 0;
	reset_page_owner(page, order);

Can this silently mask bugs where filesystems leak private data (like an
un-detached buffer_head)? Should we add a check for page->private != NULL
for head pages before unconditionally zeroing it?

>  	PG_private_2,		/* If pagecache, has fs aux data */
>  	PG_reclaim,		/* To be reclaimed asap */
>  	PG_swapbacked,		/* Page is backed by RAM/swap */

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260913-remove-pg_private-v4-0-848550f7574e@nvidia.com?part=16

Re: [PATCH v4 00/16] Remove PG_private by using page/folio->private checks instead

[Andrew Morton]

On Sun, 13 Sep 2026 22:23:58 -0400 Zi Yan <ziy@nvidia.com> wrote:

> This patchset removes PG_private to make space for upcoming PG_folio for
> identifying pages from a folio (more details in Note below). Instead of
> checking PG_private, all code is changed to check page/folio->private !=
> NULL instead.
> 
> MM people are cc'd on all patches and subsystem people are cc'd on the
> cover letter and corresponding patches.
> 
> Patch 6 is picked up separately in f2fs tree, but since mm-new does not
> have it yet, it is sent for MM testing.

AI review claims to have found a pre-existing critical level deadlock
in f2fs:

	https://sashiko.dev/#/patchset/20260913-remove-pg_private-v4-0-848550f7574e@nvidia.com

it also had a few things to say about this patchset and, as always,
hugetlb.c.


Thanks, The MM bits appear adequately reviewed and review of the non-MM
bits are, as usual:

  Great to have but I won't permit lack of other-than-MM review to
  block MM improvements.

So I'll queue it all up and shall push it into -next after a few days.

Acks from non-MM maintainers are appreciated.

If a non-MM patch appears in linux-next I'll autodrop the mm.git copy
of that patch.

Re: [PATCH v4 00/16] Remove PG_private by using page/folio->private checks instead

[Matthew Wilcox]

On Sun, Sep 13, 2026 at 10:23:58PM -0400, Zi Yan wrote:
>       md/md-bitmap: replace PagePrivate() with page_private()
>       buffer: replace page_buffer() with page_private() and delete it

Sorry, looks like I screwed up git send-email usage.  I'm proposing
replacing these two patches wth the three I sent.

Re: [PATCH v4 00/16] Remove PG_private by using page/folio->private checks instead

[David Hildenbrand (Arm)]

On 9/14/26 06:21, Matthew Wilcox wrote:
> On Sun, Sep 13, 2026 at 10:23:58PM -0400, Zi Yan wrote:
>>       md/md-bitmap: replace PagePrivate() with page_private()
>>       buffer: replace page_buffer() with page_private() and delete it
> 
> Sorry, looks like I screwed up git send-email usage.  I'm proposing
> replacing these two patches wth the three I sent.

For everybody CCed wondering "which patches":

https://lore.kernel.org/r/20260914041830.2072626-1-willy@infradead.org
https://lore.kernel.org/r/20260914041830.2072626-2-willy@infradead.org
https://lore.kernel.org/r/20260914041830.2072626-3-willy@infradead.org

-- 
Cheers,

David

Re: [PATCH v4 10/16] mm/page-flags: check page/folio->private instead of PG_private

[David Hildenbrand (Arm)]

On 9/14/26 04:24, 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. 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_expected_ref_count() can be called without folio lock, so annotate
> folio_test_private() with data_race() to avoid triggering race condition
> checks. While at it, annotate folio->mapping too. Add data_race()
> annotation for other lockless callers too.

Shouldn't we just move the data_race() into folio_has_attached_private() and
document there, that it can be called without the folio lock as well? Having the
data_race in multiple callers looks odd.

> 
> folio_set/clear_private() and Set/ClearPagePrivate() become no-ops.
> PG_private is no longer checked at page free time.

You can mention here that we will clean this part up next, to remove these
helpers entirely.

[...]

>  	}
>  
>  	/* One reference per page table mapping. */
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 462e89e055485..08988877331ba 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -577,7 +577,23 @@ FOLIO_FLAG(swapbacked, FOLIO_HEAD_PAGE)
>   * for its own purposes.
>   * - PG_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)
> +{
> +	return folio->private;
> +}
> +
> +static __always_inline int PagePrivate(const struct page *page)
> +{
> +	return !!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 +1185,7 @@ 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 +1209,28 @@ 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. 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.

This is an implementation detail that is irrelevant for callers. Move that above
the actual checks in the code below.

> + *
> + * 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.

Dito, a caller of this function doesn't care.

> + *
> + * Return: true if folio->private is set and the folio is neither swapcache
> + * nor hugetlb.

Can we remove these implementation details here instead and talk about "true if
the folio has private data attached" ?

> + */
> +static inline bool folio_has_attached_private(const struct folio *folio)
> +{
> +	return folio_test_private(folio) && !folio_test_swapbacked(folio) &&
> +	       !folio_test_hugetlb(folio);
> +}
Apart from that LGTM.

-- 
Cheers,

David

Re: [PATCH v4 16/16] mm/page-flags: remove PG_private

[David Hildenbrand (Arm)]

On 9/14/26 04:47, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] Silent memory leak of folio private data due to missing check for leaked `folio->private` data when a head page is freed.
> --
> 
> commit ebe4dd474a496c46897e3f790e03a43cc0508eaf
> Author: Zi Yan <ziy@nvidia.com>
> 
> mm/page-flags: remove PG_private
> 
> folio->private != NULL indicates a folio carries private data, replacing
> PG_private. All PG_private users are converted. Remove PG_private and
> reserve the space as PG_folio for future use. Unused PG_private functions
> are removed too.
> 
>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>> index 08988877331ba..10f4ad075ea48 100644
>> --- a/include/linux/page-flags.h
>> +++ b/include/linux/page-flags.h
> 
> [ ... ]
> 
>> @@ -105,7 +101,7 @@ enum pageflags {
>>  	PG_owner_2,		/* Owner use. If pagecache, fs may use */
>>  	PG_arch_1,
>>  	PG_reserved,
>> -	PG_private,		/* If pagecache, has fs-private data */
>> +	PG_folio,		/* Do not use: reserved for folio identification */
> 
> [Severity: High]

"High" lol

-- 
Cheers,

David

Re: [PATCH v4 16/16] mm/page-flags: remove PG_private

[David Hildenbrand (Arm)]

On 9/14/26 04:24, Zi Yan wrote:
> folio->private != NULL indicates a folio carries private data, replacing
> PG_private. All PG_private users are converted. Remove PG_private and
> reserve the space as PG_folio for future use. Unused PG_private functions
> are removed too.
> 
> Assisted-by: LLM
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> To: Andrew Morton <akpm@linux-foundation.org>
> To: Baoquan He <baoquan.he@linux.dev>
> To: Mike Rapoport <rppt@kernel.org>
> To: Pasha Tatashin <pasha.tatashin@soleen.com>
> To: Pratyush Yadav <pratyush@kernel.org>
> To: Jonathan Corbet <corbet@lwn.net>
> To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> To: Jan Kara <jack@suse.cz>
> To: David Hildenbrand <david@kernel.org>
> To: Steven Rostedt <rostedt@goodmis.org>
> To: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Dave Young <ruirui.yang@linux.dev>
> Cc: Shuah Khan <skhan@linuxfoundation.org>
> Cc: Lorenzo Stoakes <ljs@kernel.org>
> Cc: "Liam R. Howlett" <liam@infradead.org>
> Cc: Vlastimil Babka <vbabka@kernel.org>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: kexec@lists.infradead.org
> Cc: linux-doc@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-mm@kvack.org
> Cc: linux-trace-kernel@vger.kernel.org
> ---

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David

Re: [PATCH v4 10/16] mm/page-flags: check page/folio->private instead of PG_private

[Zi Yan]

On 14 Sep 2026, at 9:26, David Hildenbrand (Arm) wrote:

> On 9/14/26 04:24, 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. 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_expected_ref_count() can be called without folio lock, so annotate
>> folio_test_private() with data_race() to avoid triggering race condition
>> checks. While at it, annotate folio->mapping too. Add data_race()
>> annotation for other lockless callers too.
>
> Shouldn't we just move the data_race() into folio_has_attached_private() and
> document there, that it can be called without the folio lock as well? Having the
> data_race in multiple callers looks odd.

Not all callers call it without holding a lock. Before the patch series,
it is fine for lockless reads because PagePrivate() is an atomic operation.
But after the patch series, lockless reads can cause data tearing of
folio->private. I think I need to add this to avoid the confusion.

>
>>
>> folio_set/clear_private() and Set/ClearPagePrivate() become no-ops.
>> PG_private is no longer checked at page free time.
>
> You can mention here that we will clean this part up next, to remove these
> helpers entirely.
>

Will do.

> [...]
>
>>  	}
>>
>>  	/* One reference per page table mapping. */
>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
>> index 462e89e055485..08988877331ba 100644
>> --- a/include/linux/page-flags.h
>> +++ b/include/linux/page-flags.h
>> @@ -577,7 +577,23 @@ FOLIO_FLAG(swapbacked, FOLIO_HEAD_PAGE)
>>   * for its own purposes.
>>   * - PG_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)
>> +{
>> +	return folio->private;
>> +}
>> +
>> +static __always_inline int PagePrivate(const struct page *page)
>> +{
>> +	return !!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 +1185,7 @@ 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 +1209,28 @@ 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. 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.
>
> This is an implementation detail that is irrelevant for callers. Move that above
> the actual checks in the code below.

OK.

>
>> + *
>> + * 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.
>
> Dito, a caller of this function doesn't care.

Got it.

>
>> + *
>> + * Return: true if folio->private is set and the folio is neither swapcache
>> + * nor hugetlb.
>
> Can we remove these implementation details here instead and talk about "true if
> the folio has private data attached" ?

OK.

>
>> + */
>> +static inline bool folio_has_attached_private(const struct folio *folio)
>> +{
>> +	return folio_test_private(folio) && !folio_test_swapbacked(folio) &&
>> +	       !folio_test_hugetlb(folio);
>> +}
> Apart from that LGTM.

Thanks.


Best Regards,
Yan, Zi

Re: [PATCH v4 10/16] mm/page-flags: check page/folio->private instead of PG_private

[David Hildenbrand (Arm)]

On 9/14/26 16:44, Zi Yan wrote:
> On 14 Sep 2026, at 9:26, David Hildenbrand (Arm) wrote:
> 
>> On 9/14/26 04:24, 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. 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_expected_ref_count() can be called without folio lock, so annotate
>>> folio_test_private() with data_race() to avoid triggering race condition
>>> checks. While at it, annotate folio->mapping too. Add data_race()
>>> annotation for other lockless callers too.
>>
>> Shouldn't we just move the data_race() into folio_has_attached_private() and
>> document there, that it can be called without the folio lock as well? Having the
>> data_race in multiple callers looks odd.
> 
> Not all callers call it without holding a lock. Before the patch series,
> it is fine for lockless reads because PagePrivate() is an atomic operation.
> But after the patch series, lockless reads can cause data tearing of
> folio->private. I think I need to add this to avoid the confusion.
Okay, but we should just keep that complexity internally.

E.g., just teach folio_test_private/PagePrivate about the data_race, because
that's exactly what worked before even with data races.

-- 
Cheers,

David

Re: [PATCH v4 10/16] mm/page-flags: check page/folio->private instead of PG_private

[Zi Yan]

On 14 Sep 2026, at 11:11, David Hildenbrand (Arm) wrote:

> On 9/14/26 16:44, Zi Yan wrote:
>> On 14 Sep 2026, at 9:26, David Hildenbrand (Arm) wrote:
>>
>>> On 9/14/26 04:24, 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. 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_expected_ref_count() can be called without folio lock, so annotate
>>>> folio_test_private() with data_race() to avoid triggering race condition
>>>> checks. While at it, annotate folio->mapping too. Add data_race()
>>>> annotation for other lockless callers too.
>>>
>>> Shouldn't we just move the data_race() into folio_has_attached_private() and
>>> document there, that it can be called without the folio lock as well? Having the
>>> data_race in multiple callers looks odd.
>>
>> Not all callers call it without holding a lock. Before the patch series,
>> it is fine for lockless reads because PagePrivate() is an atomic operation.
>> But after the patch series, lockless reads can cause data tearing of
>> folio->private. I think I need to add this to avoid the confusion.
> Okay, but we should just keep that complexity internally.
>
> E.g., just teach folio_test_private/PagePrivate about the data_race, because
> that's exactly what worked before even with data races.

Makes sense. Will update folio_test_private() and PagePrivate() and get rid
of callers’ data_race().

Best Regards,
Yan, Zi

Re: [f2fs-dev] [PATCH v4 00/16] Remove PG_private by using page/folio->private checks instead

[patchwork-bot+f2fs]

Hello:

This series was applied to jaegeuk/f2fs.git (dev)
by Jaegeuk Kim <jaegeuk@kernel.org>:

On Sun, 13 Sep 2026 22:23:58 -0400 you wrote:
> Hi all,
> 
> This patchset removes PG_private to make space for upcoming PG_folio for
> identifying pages from a folio (more details in Note below). Instead of
> checking PG_private, all code is changed to check page/folio->private !=
> NULL instead.
> 
> [...]

Here is the summary with links:
  - [f2fs-dev,v4,06/16] f2fs: stop using PG_private
    https://git.kernel.org/jaegeuk/f2fs/c/60105162524e
  - [f2fs-dev,v4,07/16] f2fs: convert the ->private flag helpers to folio-only
    (no matching commit)

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html

Re: [PATCH v4 00/16] Remove PG_private by using page/folio->private checks instead

[Zi Yan]

On 13 Sep 2026, at 23:39, Andrew Morton wrote:

> On Sun, 13 Sep 2026 22:23:58 -0400 Zi Yan <ziy@nvidia.com> wrote:
>
>> This patchset removes PG_private to make space for upcoming PG_folio for
>> identifying pages from a folio (more details in Note below). Instead of
>> checking PG_private, all code is changed to check page/folio->private !=
>> NULL instead.
>>
>> MM people are cc'd on all patches and subsystem people are cc'd on the
>> cover letter and corresponding patches.
>>
>> Patch 6 is picked up separately in f2fs tree, but since mm-new does not
>> have it yet, it is sent for MM testing.
>
> AI review claims to have found a pre-existing critical level deadlock
> in f2fs:
>
> 	https://sashiko.dev/#/patchset/20260913-remove-pg_private-v4-0-848550f7574e@nvidia.com
>
> it also had a few things to say about this patchset and, as always,
> hugetlb.c.

It seems that Sashiko overwrote my patch reviews with the reviews to
Matthew’s md patches. :/ I wonder if there is a way of recovering them.

>
>
> Thanks, The MM bits appear adequately reviewed and review of the non-MM
> bits are, as usual:
>
>   Great to have but I won't permit lack of other-than-MM review to
>   block MM improvements.
>
> So I'll queue it all up and shall push it into -next after a few days.
>
> Acks from non-MM maintainers are appreciated.
>
> If a non-MM patch appears in linux-next I'll autodrop the mm.git copy
> of that patch.

Thanks.

Best Regards,
Yan, Zi