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