[f2fs-dev] [PATCH v2 00/14] Remove PG_private by using page/folio->private checks instead

[f2fs-dev] [PATCH v2 00/14] Remove PG_private by using page/folio->private checks instead

[Zi Yan via Linux-f2fs-devel]

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 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_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 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) 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 by checking PG_folio, but before that all existing users
   that treat compound pages as folios will 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: Claude:claude-opus-4-8
Assisted-by: Codex:gpt-5
Signed-off-by: Zi Yan <ziy@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 (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
      fscrypt: stop setting PG_private on bounce page
      mm/hugetlb: use direct assignment instead of folio_change_private()
      f2fs: stop using PG_private
      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
      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                      | 11 +++--
 fs/ceph/addr.c                                 |  8 ++--
 fs/crypto/crypto.c                             |  2 -
 fs/erofs/data.c                                | 16 ++++---
 fs/erofs/zdata.c                               | 13 ++----
 fs/f2fs/f2fs.h                                 |  8 ++--
 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                     | 38 +++++++++++-----
 include/linux/pagemap.h                        | 60 +++++++++++++++++++++-----
 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                            |  2 +-
 mm/vmscan.c                                    |  2 +-
 mm/zpdesc.h                                    |  2 +-
 mm/zsmalloc.c                                  | 24 +++--------
 tools/mm/page-types.c                          |  2 -
 34 files changed, 163 insertions(+), 137 deletions(-)
---
base-commit: 443451c85ca8d6389d34b1299decada62128f1fe
change-id: 20260728-remove-pg_private-cfe926c7f83c

Best regards,
--  
Yan, Zi



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[f2fs-dev] [PATCH v2 06/14] f2fs: stop using PG_private

[Zi Yan via Linux-f2fs-devel]

f2fs sets its PAGE_PRIVATE_* flags in page->private and checking
page->private != NULL is equivalent to checking PG_private. Change
PagePrivate() to page_private(). Meanwhile, in set_page_private_##name(),
page->private is first set to 0/NULL before an PAGE_PRIVATE_* flag is set,
but it can cause confusion when PG_private is removed and
page->private != NULL is used instead. Change it to initialize
page->private to PAGE_PRIVATE_NOT_POINTER instead and retain the original
semantics.

It prepares for a future commit that removes PG_private.

No functional change intended.

Assisted-by: Claude:claude-opus-4-8
Assisted-by: Codex:gpt-5
To: Jaegeuk Kim <jaegeuk@kernel.org>
To: Chao Yu <chao@kernel.org>
Cc: linux-f2fs-devel@lists.sourceforge.net
Cc: linux-kernel@vger.kernel.org
Acked-by: Usama Arif <usama.arif@linux.dev>
Acked-by: Chao Yu <chao@kernel.org>
Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 fs/f2fs/f2fs.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index 9940a6cecf1a2..2f7ab5888b078 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -2691,7 +2691,7 @@ static inline bool folio_test_f2fs_##name(const struct folio *folio)	\
 }									\
 static inline bool page_private_##name(struct page *page) \
 { \
-	return PagePrivate(page) && \
+	return page_private(page) && \
 		test_bit(PAGE_PRIVATE_NOT_POINTER, &page_private(page)) && \
 		test_bit(PAGE_PRIVATE_##flagname, &page_private(page)); \
 }
@@ -2710,9 +2710,9 @@ static inline void folio_set_f2fs_##name(struct folio *folio)		\
 }									\
 static inline void set_page_private_##name(struct page *page) \
 { \
-	if (!PagePrivate(page)) \
-		attach_page_private(page, (void *)0); \
-	set_bit(PAGE_PRIVATE_NOT_POINTER, &page_private(page)); \
+	if (!page_private(page)) \
+		attach_page_private(page, \
+				(void *)BIT(PAGE_PRIVATE_NOT_POINTER)); \
 	set_bit(PAGE_PRIVATE_##flagname, &page_private(page)); \
 }
 

-- 
2.53.0



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Re: [f2fs-dev] [PATCH v2 06/14] f2fs: stop using PG_private

[Jaegeuk Kim via Linux-f2fs-devel]

Let me take this patch in f2fs tree, which looks like a clean up.

On 08/31, Zi Yan wrote:
> f2fs sets its PAGE_PRIVATE_* flags in page->private and checking
> page->private != NULL is equivalent to checking PG_private. Change
> PagePrivate() to page_private(). Meanwhile, in set_page_private_##name(),
> page->private is first set to 0/NULL before an PAGE_PRIVATE_* flag is set,
> but it can cause confusion when PG_private is removed and
> page->private != NULL is used instead. Change it to initialize
> page->private to PAGE_PRIVATE_NOT_POINTER instead and retain the original
> semantics.
> 
> It prepares for a future commit that removes PG_private.
> 
> No functional change intended.
> 
> Assisted-by: Claude:claude-opus-4-8
> Assisted-by: Codex:gpt-5
> To: Jaegeuk Kim <jaegeuk@kernel.org>
> To: Chao Yu <chao@kernel.org>
> Cc: linux-f2fs-devel@lists.sourceforge.net
> Cc: linux-kernel@vger.kernel.org
> Acked-by: Usama Arif <usama.arif@linux.dev>
> Acked-by: Chao Yu <chao@kernel.org>
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
>  fs/f2fs/f2fs.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index 9940a6cecf1a2..2f7ab5888b078 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -2691,7 +2691,7 @@ static inline bool folio_test_f2fs_##name(const struct folio *folio)	\
>  }									\
>  static inline bool page_private_##name(struct page *page) \
>  { \
> -	return PagePrivate(page) && \
> +	return page_private(page) && \
>  		test_bit(PAGE_PRIVATE_NOT_POINTER, &page_private(page)) && \
>  		test_bit(PAGE_PRIVATE_##flagname, &page_private(page)); \
>  }
> @@ -2710,9 +2710,9 @@ static inline void folio_set_f2fs_##name(struct folio *folio)		\
>  }									\
>  static inline void set_page_private_##name(struct page *page) \
>  { \
> -	if (!PagePrivate(page)) \
> -		attach_page_private(page, (void *)0); \
> -	set_bit(PAGE_PRIVATE_NOT_POINTER, &page_private(page)); \
> +	if (!page_private(page)) \
> +		attach_page_private(page, \
> +				(void *)BIT(PAGE_PRIVATE_NOT_POINTER)); \
>  	set_bit(PAGE_PRIVATE_##flagname, &page_private(page)); \
>  }
>  
> 
> -- 
> 2.53.0
> 


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Re: [f2fs-dev] [PATCH v2 00/14] Remove PG_private by using page/folio->private checks instead

[patchwork-bot+f2fs--- via Linux-f2fs-devel]

Hello:

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

On Mon, 31 Aug 2026 15:25:23 -0400 you 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.
> 
> [...]

Here is the summary with links:
  - [f2fs-dev,v2,06/14] f2fs: stop using PG_private
    https://git.kernel.org/jaegeuk/f2fs/c/5ad9409a9533

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
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