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

[Zi Yan <ziy@nvidia.com>]

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