[PATCH RFC 00/14] Remove PG_private by using page/folio->private checks instead

[PATCH RFC 00/14] 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
(reserved as __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.

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 changed to zpdesc->zspage->first_zpdesc == zpdesc
   instead of checking PG_private.

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_reverse() 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().

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_test_fs_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 has a minor semantic change. Since PG_private will be
   removed, KPF_PRIVATE represents pagecache folios whose ->mapping is not
   NULL and with ->private set. Currently KPF_PRIVATE can be set for
   orphaned pagecache folios with ->mapping == NULL.

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) can be
   identified, vm_insert_*() can correctly reject all folios, and rmap code
   can accept only folios. Eventually, page_folio() will return NULL for
   non-folio pages.

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: Claude:claude-opus-4-8
Assisted-by: Codex:gpt-5
Signed-off-by: Zi Yan <ziy@nvidia.com>
---
Zi Yan (14):
      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
      fs/crypto: stop setting PG_private on bounce page
      mm/hugetlb: use direct assignment instead of folio_change_private()
      fs/f2fs: stop using PG_private
      fs/erofs: mm/pagemap: add readahead_folio_reverse() to avoid folio->private
      fs/erofs: use folio_attach/detach_private() instead of direct assignment
      mm/page-flags: check page/folio->private instead of PG_private
      mm/page-flags: introduce folio_test_fs_private()
      treewide: remove folio_set/clear_private()
      treewide: replace PagePrivate() with page_private()
      treewide: adjust comments on PagePrivate and PG_private
      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                         |  6 ++--
 drivers/xen/balloon.c                          |  5 ++++
 drivers/xen/grant-table.c                      |  7 ++---
 fs/ceph/addr.c                                 |  8 ++----
 fs/crypto/crypto.c                             |  2 --
 fs/erofs/data.c                                |  5 ++--
 fs/erofs/zdata.c                               | 11 ++------
 fs/f2fs/f2fs.h                                 |  8 +++---
 fs/nfs/file.c                                  |  4 +--
 fs/nfs/write.c                                 |  2 --
 fs/proc/page.c                                 |  5 +++-
 fs/ubifs/file.c                                |  6 ++--
 include/linux/buffer_head.h                    |  6 ----
 include/linux/mm.h                             | 16 +++++------
 include/linux/mm_types.h                       |  4 +--
 include/linux/page-flags.h                     | 38 ++++++++++++++++++--------
 include/linux/pagemap.h                        | 35 ++++++++++++++++++++++--
 include/trace/events/mmflags.h                 |  2 +-
 include/trace/events/pagemap.h                 |  2 +-
 kernel/events/ring_buffer.c                    |  7 ++---
 kernel/vmcore_info.c                           |  1 -
 mm/huge_memory.c                               |  2 +-
 mm/hugetlb.c                                   |  6 ++--
 mm/migrate.c                                   |  3 +-
 mm/page-writeback.c                            |  5 +++-
 mm/vmscan.c                                    |  3 +-
 mm/zpdesc.h                                    |  2 +-
 mm/zsmalloc.c                                  | 15 ++--------
 32 files changed, 127 insertions(+), 106 deletions(-)
---
base-commit: bcd5eb68a6a189497eb26c1b9f622538aa48895d
change-id: 20260728-remove-pg_private-cfe926c7f83c

Best regards,
-- 
Yan, Zi

[PATCH RFC 14/14] 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.

Also update files in Documentation. hugetlbfs_reserv.rst is outdated and
left unchanged. It should be rewritten.

Assisted-by: Claude:claude-opus-4-8
Assisted-by: Codex:gpt-5
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
---
 Documentation/admin-guide/kdump/vmcoreinfo.rst |  2 +-
 Documentation/filesystems/vfs.rst              |  6 +++---
 include/linux/page-flags.h                     | 19 ++-----------------
 include/trace/events/mmflags.h                 |  2 +-
 kernel/vmcore_info.c                           |  1 -
 5 files changed, 7 insertions(+), 23 deletions(-)

diff --git a/Documentation/admin-guide/kdump/vmcoreinfo.rst b/Documentation/admin-guide/kdump/vmcoreinfo.rst
index 7663c610fe901..5f1df6d080508 100644
--- a/Documentation/admin-guide/kdump/vmcoreinfo.rst
+++ b/Documentation/admin-guide/kdump/vmcoreinfo.rst
@@ -325,7 +325,7 @@ NR_FREE_PAGES
 On linux-2.6.21 or later, the number of free pages is in
 vm_stat[NR_FREE_PAGES]. Used to get the number of free pages.
 
-PG_lru|PG_private|PG_swapcache|PG_swapbacked|PG_hwpoison|PG_head_mask
+PG_lru|PG_swapcache|PG_swapbacked|PG_hwpoison|PG_head_mask
 --------------------------------------------------------------------------
 
 Page attributes. These flags are used to filter various unnecessary for
diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst
index e7677423a20f7..5cc07f82fe371 100644
--- a/Documentation/filesystems/vfs.rst
+++ b/Documentation/filesystems/vfs.rst
@@ -649,8 +649,8 @@ Writeback.
 
 The first can be used independently to the others.  The VM can try to
 release clean pages in order to reuse them.  To do this it can call
-->release_folio on clean folios with the private
-flag set.  Clean pages without PagePrivate and with no external references
+->release_folio on clean folios with folio->private set. Clean pages
+without folio->private set and with no external references
 will be released without notice being given to the address_space.
 
 To achieve this functionality, pages need to be placed on an LRU with
@@ -674,7 +674,7 @@ filemap_fdatawait_range, to wait for all writeback to complete.
 
 An address_space handler may attach extra information to a page,
 typically using the 'private' field in the 'struct page'.  If such
-information is attached, the PG_Private flag should be set.  This will
+information is attached, non-NULL 'private' field will
 cause various VM routines to make extra calls into the address_space
 handler to deal with that data.
 
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 0e3628ea080c4..eb2961ed61018 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 */
@@ -576,7 +572,7 @@ 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)
@@ -584,17 +580,6 @@ 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(page);
-}
-
-/* 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 935893e5ea53b..caf090cd6f85e 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),					\
+	{ 1UL << __PG_folio, "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 RFC 00/14] Remove PG_private by using page/folio->private checks instead

[Jürgen Groß]

[-- Attachment #1.1.1: Type: text/plain, Size: 637 bytes --]

On 01.08.26 04:13, Zi Yan wrote:
> Hi all,
> 
> This patchset removes PG_private to make space for upcoming PG_folio
> (reserved as __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.

I'm a little bit worried that page/folio->private is in a union, so today
it could (in theory) be != NULL while PG_private isn't set.

Is it really not possible to enter a path where PG_private is tested while
page/folio->private != NULL due to the union being used otherwise (PG_private
not set)?


Juergen

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[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 495 bytes --]

Re: [PATCH RFC 00/14] Remove PG_private by using page/folio->private checks instead

[Zi Yan]

On Mon Aug 3, 2026 at 5:07 AM EDT, Jürgen Groß wrote:
> On 01.08.26 04:13, Zi Yan wrote:
>> Hi all,
>> 
>> This patchset removes PG_private to make space for upcoming PG_folio
>> (reserved as __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.
>
> I'm a little bit worried that page/folio->private is in a union, so today
> it could (in theory) be != NULL while PG_private isn't set.
>
> Is it really not possible to enter a path where PG_private is tested while
> page/folio->private != NULL due to the union being used otherwise (PG_private
> not set)?

Yes, it is possible. See: #5 in the exceptional users: erofs uses
->private for reverse linked list and in-flight counters without setting
PG_private. I get rid of the first one and converted the second one to
use folio_attach/detach/get_private() to follow the general ->private
use pattern..

For non file system folios, anon swapcache puts swap_entry_t in
->private and hugetlb puts its flags in ->private. I added
folio_test_fs_private() to exclude them, but this helper is planned to
be used by core MM, since filesystem code should not encounter these
two.


-- 
Best Regards,
Yan, Zi