[PATCH v3 00/14] Remove PG_private by using page/folio->private checks instead

[PATCH v3 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.

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_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, 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: Claude:claude-opus-4-8
Assisted-by: Codex:gpt-5
Signed-off-by: Zi Yan <ziy@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 (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                     | 43 +++++++++++++-----
 include/linux/pagemap.h                        | 60 +++++++++++++++++++++-----
 include/trace/events/mmflags.h                 |  3 +-
 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                                   |  6 +--
 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 -
 34 files changed, 172 insertions(+), 137 deletions(-)
---
base-commit: 5f4c0999b2bde1fbbba3c499208b415447ca4c7a
change-id: 20260728-remove-pg_private-cfe926c7f83c

Best regards,
--  
Yan, Zi

[PATCH v3 07/14] erofs: mm/pagemap: add readahead_folio_last() to avoid folio->private

[Zi Yan]

erofs needs to traverse readahead folios in reverse order to achieve
maximum performance by
1. reading all folios from readahead_folio();
2. storing the prior folio pointer in folio->private;
3. traverse from the last folio to the first one.

Add readahead_folio_last() to achieve the same function without using
folio->private. __readahead_advance() helper shares readahead_control
adjustment code among __readahead_folio(), readahead_folio_last(), and
__readahead_batch() by checking new private member, _forward, of
readahead_control.

It prepares for a future commit that replaces PG_private checks with
!folio->private checks. After switching the checks, erofs's use of
folio->private without bumping folio refcount can cause unexpected
outcomes, e.g., in filemap_release_folio(), try_to_free_buffers() becomes
reachable.

No functional change intended.

Assisted-by: Claude:claude-opus-4-8
Assisted-by: Codex:gpt-5
Signed-off-by: Zi Yan <ziy@nvidia.com>
To: Gao Xiang <xiang@kernel.org>
To: Chao Yu <chao@kernel.org>
To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
To: Jan Kara <jack@suse.cz>
Cc: Yue Hu <zbestahu@gmail.com>
Cc: Jeffle Xu <jefflexu@linux.alibaba.com>
Cc: Sandeep Dhavale <dhavale@google.com>
Cc: Hongbo Li <hongbohbli@tencent.com>
Cc: Chunhai Guo <guochunhai@vivo.com>
Cc: linux-erofs@lists.ozlabs.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org
---
 fs/erofs/zdata.c        | 13 +++---------
 include/linux/pagemap.h | 56 ++++++++++++++++++++++++++++++++++++++++++-------
 2 files changed, 51 insertions(+), 18 deletions(-)

diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
index e1e25ca0d1904..78fd7d980e957 100644
--- a/fs/erofs/zdata.c
+++ b/fs/erofs/zdata.c
@@ -1898,21 +1898,14 @@ static void z_erofs_readahead(struct readahead_control *rac)
 	struct inode *realinode = erofs_real_inode(sharedinode, &need_iput);
 	Z_EROFS_DEFINE_FRONTEND(f, realinode, sharedinode, readahead_pos(rac));
 	unsigned int nrpages = readahead_count(rac);
-	struct folio *head = NULL, *folio;
+	struct folio *folio;
 	int err;
 
 	trace_erofs_readahead(realinode, readahead_index(rac), nrpages, false);
 	z_erofs_pcluster_readmore(&f, rac, true);
-	while ((folio = readahead_folio(rac))) {
-		folio->private = head;
-		head = folio;
-	}
-
-	/* traverse in reverse order for best metadata I/O performance */
-	while (head) {
-		folio = head;
-		head = folio_get_private(folio);
 
+	/* traverse from last to first for best metadata I/O performance */
+	while ((folio = readahead_folio_last(rac))) {
 		err = z_erofs_scan_folio(&f, folio, true);
 		if (err && err != -EINTR)
 			erofs_err(realinode->i_sb, "readahead error at folio %lu @ nid %llu",
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 939f3a5e973f6..2257df004305e 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -1415,6 +1415,7 @@ struct readahead_control {
 	bool dropbehind;
 	bool _workingset;
 	unsigned long _pflags;
+	bool _forward;
 };
 
 #define DEFINE_READAHEAD(ractl, f, r, m, i)				\
@@ -1479,18 +1480,25 @@ void page_cache_async_readahead(struct address_space *mapping,
 	page_cache_async_ra(&ractl, folio, req_count);
 }
 
+static inline void __readahead_advance(struct readahead_control *rac)
+{
+	if (rac->_forward)
+		rac->_index += rac->_batch_count;
+
+	rac->_nr_pages -= rac->_batch_count;
+	rac->_batch_count = 0;
+}
+
 static inline struct folio *__readahead_folio(struct readahead_control *ractl)
 {
 	struct folio *folio;
 
 	BUG_ON(ractl->_batch_count > ractl->_nr_pages);
-	ractl->_nr_pages -= ractl->_batch_count;
-	ractl->_index += ractl->_batch_count;
+	__readahead_advance(ractl);
+	ractl->_forward = true;
 
-	if (!ractl->_nr_pages) {
-		ractl->_batch_count = 0;
+	if (!ractl->_nr_pages)
 		return NULL;
-	}
 
 	folio = xa_load(&ractl->mapping->i_pages, ractl->_index);
 	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
@@ -1516,6 +1524,39 @@ static inline struct folio *readahead_folio(struct readahead_control *ractl)
 	return folio;
 }
 
+/**
+ * readahead_folio_last - Get the next folio to read, from the tail.
+ * @ractl: The current readahead request.
+ *
+ * Like readahead_folio(), but walks the range back-to-front. The folio is
+ * returned locked with its refcount dropped; the caller unlocks it once I/O
+ * completes. Compound folios are returned once, at their head index.
+ *
+ * Context: The folio is locked.
+ * Return: A pointer to the next folio, or %NULL when done.
+ */
+static inline struct folio *readahead_folio_last(struct readahead_control *ractl)
+{
+	struct folio *folio;
+
+	/* Drop the previously returned batch from the remaining range. */
+	__readahead_advance(ractl);
+	ractl->_forward = false;
+
+	if (!ractl->_nr_pages)
+		return NULL;
+
+	/* xa_load() follows sibling entries, so a tail index returns the head */
+	folio = xa_load(&ractl->mapping->i_pages,
+			ractl->_index + ractl->_nr_pages - 1);
+	VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
+
+	ractl->_batch_count = folio_nr_pages(folio);
+
+	folio_put(folio);
+	return folio;
+}
+
 static inline unsigned int __readahead_batch(struct readahead_control *rac,
 		struct page **array, unsigned int array_sz)
 {
@@ -1524,9 +1565,8 @@ static inline unsigned int __readahead_batch(struct readahead_control *rac,
 	struct folio *folio;
 
 	BUG_ON(rac->_batch_count > rac->_nr_pages);
-	rac->_nr_pages -= rac->_batch_count;
-	rac->_index += rac->_batch_count;
-	rac->_batch_count = 0;
+	__readahead_advance(rac);
+	rac->_forward = true;
 
 	xas_set(&xas, rac->_index);
 	rcu_read_lock();

-- 
2.53.0

Re: [PATCH v3 07/14] erofs: mm/pagemap: add readahead_folio_last() to avoid folio->private

[David Hildenbrand (Arm)]

On 9/8/26 04:56, Zi Yan wrote:
> erofs needs to traverse readahead folios in reverse order to achieve
> maximum performance by
> 1. reading all folios from readahead_folio();
> 2. storing the prior folio pointer in folio->private;
> 3. traverse from the last folio to the first one.
> 
> Add readahead_folio_last() to achieve the same function without using
> folio->private. __readahead_advance() helper shares readahead_control
> adjustment code among __readahead_folio(), readahead_folio_last(), and
> __readahead_batch() by checking new private member, _forward, of
> readahead_control.
> 
> It prepares for a future commit that replaces PG_private checks with
> !folio->private checks. After switching the checks, erofs's use of
> folio->private without bumping folio refcount can cause unexpected
> outcomes, e.g., in filemap_release_folio(), try_to_free_buffers() becomes
> reachable.

Ah, I was just about to ask. So it's really about folios never using
folio->private manually (without the attach/detach).

> 
> No functional change intended.
> 
> Assisted-by: Claude:claude-opus-4-8
> Assisted-by: Codex:gpt-5
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> To: Gao Xiang <xiang@kernel.org>
> To: Chao Yu <chao@kernel.org>
> To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> To: Jan Kara <jack@suse.cz>
> Cc: Yue Hu <zbestahu@gmail.com>
> Cc: Jeffle Xu <jefflexu@linux.alibaba.com>
> Cc: Sandeep Dhavale <dhavale@google.com>
> Cc: Hongbo Li <hongbohbli@tencent.com>
> Cc: Chunhai Guo <guochunhai@vivo.com>
> Cc: linux-erofs@lists.ozlabs.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-mm@kvack.org
> ---
>  fs/erofs/zdata.c        | 13 +++---------
>  include/linux/pagemap.h | 56 ++++++++++++++++++++++++++++++++++++++++++-------
>  2 files changed, 51 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
> index e1e25ca0d1904..78fd7d980e957 100644
> --- a/fs/erofs/zdata.c
> +++ b/fs/erofs/zdata.c
> @@ -1898,21 +1898,14 @@ static void z_erofs_readahead(struct readahead_control *rac)
>  	struct inode *realinode = erofs_real_inode(sharedinode, &need_iput);
>  	Z_EROFS_DEFINE_FRONTEND(f, realinode, sharedinode, readahead_pos(rac));
>  	unsigned int nrpages = readahead_count(rac);
> -	struct folio *head = NULL, *folio;
> +	struct folio *folio;
>  	int err;
>  
>  	trace_erofs_readahead(realinode, readahead_index(rac), nrpages, false);
>  	z_erofs_pcluster_readmore(&f, rac, true);
> -	while ((folio = readahead_folio(rac))) {
> -		folio->private = head;
> -		head = folio;
> -	}
> -
> -	/* traverse in reverse order for best metadata I/O performance */
> -	while (head) {
> -		folio = head;
> -		head = folio_get_private(folio);
>  
> +	/* traverse from last to first for best metadata I/O performance */
> +	while ((folio = readahead_folio_last(rac))) {

Intuitively, this should be called readahead_folio_reverse /
readahead_folio_reversed, thinking of list_for_each_entry_reverse()?

	list_for_each_entry_reverse - iterate backwards over list of given type.

or maybe readahead_folio_backwards (which matches the forward below)

But I'm not a readahead expert :)

>  		err = z_erofs_scan_folio(&f, folio, true);
>  		if (err && err != -EINTR)
>  			erofs_err(realinode->i_sb, "readahead error at folio %lu @ nid %llu",
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 939f3a5e973f6..2257df004305e 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -1415,6 +1415,7 @@ struct readahead_control {
>  	bool dropbehind;
>  	bool _workingset;
>  	unsigned long _pflags;
> +	bool _forward;
>  };
>  
>  #define DEFINE_READAHEAD(ractl, f, r, m, i)				\
> @@ -1479,18 +1480,25 @@ void page_cache_async_readahead(struct address_space *mapping,
>  	page_cache_async_ra(&ractl, folio, req_count);
>  }
>  
> +static inline void __readahead_advance(struct readahead_control *rac)
> +{
> +	if (rac->_forward)
> +		rac->_index += rac->_batch_count;

No expert, but shouldn't we decrement the _index somewhere in the other case? Or
where is that done? A comment might help :)


-- 
Cheers,

David

Re: [PATCH v3 07/14] erofs: mm/pagemap: add readahead_folio_last() to avoid folio->private

[Zi Yan]

On Tue Sep 8, 2026 at 12:04 PM EDT, David Hildenbrand (Arm) wrote:
> On 9/8/26 04:56, Zi Yan wrote:
>> erofs needs to traverse readahead folios in reverse order to achieve
>> maximum performance by
>> 1. reading all folios from readahead_folio();
>> 2. storing the prior folio pointer in folio->private;
>> 3. traverse from the last folio to the first one.
>> 
>> Add readahead_folio_last() to achieve the same function without using
>> folio->private. __readahead_advance() helper shares readahead_control
>> adjustment code among __readahead_folio(), readahead_folio_last(), and
>> __readahead_batch() by checking new private member, _forward, of
>> readahead_control.
>> 
>> It prepares for a future commit that replaces PG_private checks with
>> !folio->private checks. After switching the checks, erofs's use of
>> folio->private without bumping folio refcount can cause unexpected
>> outcomes, e.g., in filemap_release_folio(), try_to_free_buffers() becomes
>> reachable.
>
> Ah, I was just about to ask. So it's really about folios never using
> folio->private manually (without the attach/detach).
>
>> 
>> No functional change intended.
>> 
>> Assisted-by: Claude:claude-opus-4-8
>> Assisted-by: Codex:gpt-5
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> To: Gao Xiang <xiang@kernel.org>
>> To: Chao Yu <chao@kernel.org>
>> To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
>> To: Jan Kara <jack@suse.cz>
>> Cc: Yue Hu <zbestahu@gmail.com>
>> Cc: Jeffle Xu <jefflexu@linux.alibaba.com>
>> Cc: Sandeep Dhavale <dhavale@google.com>
>> Cc: Hongbo Li <hongbohbli@tencent.com>
>> Cc: Chunhai Guo <guochunhai@vivo.com>
>> Cc: linux-erofs@lists.ozlabs.org
>> Cc: linux-kernel@vger.kernel.org
>> Cc: linux-fsdevel@vger.kernel.org
>> Cc: linux-mm@kvack.org
>> ---
>>  fs/erofs/zdata.c        | 13 +++---------
>>  include/linux/pagemap.h | 56 ++++++++++++++++++++++++++++++++++++++++++-------
>>  2 files changed, 51 insertions(+), 18 deletions(-)
>> 
>> diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
>> index e1e25ca0d1904..78fd7d980e957 100644
>> --- a/fs/erofs/zdata.c
>> +++ b/fs/erofs/zdata.c
>> @@ -1898,21 +1898,14 @@ static void z_erofs_readahead(struct readahead_control *rac)
>>  	struct inode *realinode = erofs_real_inode(sharedinode, &need_iput);
>>  	Z_EROFS_DEFINE_FRONTEND(f, realinode, sharedinode, readahead_pos(rac));
>>  	unsigned int nrpages = readahead_count(rac);
>> -	struct folio *head = NULL, *folio;
>> +	struct folio *folio;
>>  	int err;
>>  
>>  	trace_erofs_readahead(realinode, readahead_index(rac), nrpages, false);
>>  	z_erofs_pcluster_readmore(&f, rac, true);
>> -	while ((folio = readahead_folio(rac))) {
>> -		folio->private = head;
>> -		head = folio;
>> -	}
>> -
>> -	/* traverse in reverse order for best metadata I/O performance */
>> -	while (head) {
>> -		folio = head;
>> -		head = folio_get_private(folio);
>>  
>> +	/* traverse from last to first for best metadata I/O performance */
>> +	while ((folio = readahead_folio_last(rac))) {
>
> Intuitively, this should be called readahead_folio_reverse /
> readahead_folio_reversed, thinking of list_for_each_entry_reverse()?
>
> 	list_for_each_entry_reverse - iterate backwards over list of given type.
>
> or maybe readahead_folio_backwards (which matches the forward below)
>
> But I'm not a readahead expert :)

Jan suggested the name[1]. It can be readahead_folio_reverse() if you
prefer it, like Jan said.

[1] https://lore.kernel.org/all/332rknj4vo3cfhvfhhlf6pvg37s3lbrnzbbnv4swa6gctsiu6a@ndotgokvnglc/

>
>>  		err = z_erofs_scan_folio(&f, folio, true);
>>  		if (err && err != -EINTR)
>>  			erofs_err(realinode->i_sb, "readahead error at folio %lu @ nid %llu",
>> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
>> index 939f3a5e973f6..2257df004305e 100644
>> --- a/include/linux/pagemap.h
>> +++ b/include/linux/pagemap.h
>> @@ -1415,6 +1415,7 @@ struct readahead_control {
>>  	bool dropbehind;
>>  	bool _workingset;
>>  	unsigned long _pflags;
>> +	bool _forward;
>>  };
>>  
>>  #define DEFINE_READAHEAD(ractl, f, r, m, i)				\
>> @@ -1479,18 +1480,25 @@ void page_cache_async_readahead(struct address_space *mapping,
>>  	page_cache_async_ra(&ractl, folio, req_count);
>>  }
>>  
>> +static inline void __readahead_advance(struct readahead_control *rac)
>> +{
>> +	if (rac->_forward)
>> +		rac->_index += rac->_batch_count;
>
> No expert, but shouldn't we decrement the _index somewhere in the other case? Or
> where is that done? A comment might help :)

A readahead folio comes from [_index, _index + _nr_pages), so for
last/reverse/backwards case, the code only needs to decrease _nr_pages.

Will add a comment about this.

-- 
Best Regards,
Yan, Zi

Re: [PATCH v3 07/14] erofs: mm/pagemap: add readahead_folio_last() to avoid folio->private

[David Hildenbrand (Arm)]

On 9/8/26 19:05, Zi Yan wrote:
> On Tue Sep 8, 2026 at 12:04 PM EDT, David Hildenbrand (Arm) wrote:
>> On 9/8/26 04:56, Zi Yan wrote:
>>> erofs needs to traverse readahead folios in reverse order to achieve
>>> maximum performance by
>>> 1. reading all folios from readahead_folio();
>>> 2. storing the prior folio pointer in folio->private;
>>> 3. traverse from the last folio to the first one.
>>>
>>> Add readahead_folio_last() to achieve the same function without using
>>> folio->private. __readahead_advance() helper shares readahead_control
>>> adjustment code among __readahead_folio(), readahead_folio_last(), and
>>> __readahead_batch() by checking new private member, _forward, of
>>> readahead_control.
>>>
>>> It prepares for a future commit that replaces PG_private checks with
>>> !folio->private checks. After switching the checks, erofs's use of
>>> folio->private without bumping folio refcount can cause unexpected
>>> outcomes, e.g., in filemap_release_folio(), try_to_free_buffers() becomes
>>> reachable.
>>
>> Ah, I was just about to ask. So it's really about folios never using
>> folio->private manually (without the attach/detach).
>>
>>>
>>> No functional change intended.
>>>
>>> Assisted-by: Claude:claude-opus-4-8
>>> Assisted-by: Codex:gpt-5
>>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>>> To: Gao Xiang <xiang@kernel.org>
>>> To: Chao Yu <chao@kernel.org>
>>> To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
>>> To: Jan Kara <jack@suse.cz>
>>> Cc: Yue Hu <zbestahu@gmail.com>
>>> Cc: Jeffle Xu <jefflexu@linux.alibaba.com>
>>> Cc: Sandeep Dhavale <dhavale@google.com>
>>> Cc: Hongbo Li <hongbohbli@tencent.com>
>>> Cc: Chunhai Guo <guochunhai@vivo.com>
>>> Cc: linux-erofs@lists.ozlabs.org
>>> Cc: linux-kernel@vger.kernel.org
>>> Cc: linux-fsdevel@vger.kernel.org
>>> Cc: linux-mm@kvack.org
>>> ---
>>>  fs/erofs/zdata.c        | 13 +++---------
>>>  include/linux/pagemap.h | 56 ++++++++++++++++++++++++++++++++++++++++++-------
>>>  2 files changed, 51 insertions(+), 18 deletions(-)
>>>
>>> diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
>>> index e1e25ca0d1904..78fd7d980e957 100644
>>> --- a/fs/erofs/zdata.c
>>> +++ b/fs/erofs/zdata.c
>>> @@ -1898,21 +1898,14 @@ static void z_erofs_readahead(struct readahead_control *rac)
>>>  	struct inode *realinode = erofs_real_inode(sharedinode, &need_iput);
>>>  	Z_EROFS_DEFINE_FRONTEND(f, realinode, sharedinode, readahead_pos(rac));
>>>  	unsigned int nrpages = readahead_count(rac);
>>> -	struct folio *head = NULL, *folio;
>>> +	struct folio *folio;
>>>  	int err;
>>>  
>>>  	trace_erofs_readahead(realinode, readahead_index(rac), nrpages, false);
>>>  	z_erofs_pcluster_readmore(&f, rac, true);
>>> -	while ((folio = readahead_folio(rac))) {
>>> -		folio->private = head;
>>> -		head = folio;
>>> -	}
>>> -
>>> -	/* traverse in reverse order for best metadata I/O performance */
>>> -	while (head) {
>>> -		folio = head;
>>> -		head = folio_get_private(folio);
>>>  
>>> +	/* traverse from last to first for best metadata I/O performance */
>>> +	while ((folio = readahead_folio_last(rac))) {
>>
>> Intuitively, this should be called readahead_folio_reverse /
>> readahead_folio_reversed, thinking of list_for_each_entry_reverse()?
>>
>> 	list_for_each_entry_reverse - iterate backwards over list of given type.
>>
>> or maybe readahead_folio_backwards (which matches the forward below)
>>
>> But I'm not a readahead expert :)
> 
> Jan suggested the name[1]. It can be readahead_folio_reverse() if you
> prefer it, like Jan said.
> 
> [1] https://lore.kernel.org/all/332rknj4vo3cfhvfhhlf6pvg37s3lbrnzbbnv4swa6gctsiu6a@ndotgokvnglc/

Heh, to me _reverse() is clearer; whatever people prefer.

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

-- 
Cheers,

David

[PATCH v3 09/14] 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.

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: 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: 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                | 22 +++++++++++++++-------
 include/linux/page-flags.h        | 26 +++++++++++++++++++++-----
 include/trace/events/pagemap.h    |  5 ++++-
 mm/huge_memory.c                  |  5 ++++-
 mm/migrate.c                      |  3 ++-
 mm/page-writeback.c               |  6 +++++-
 mm/vmscan.c                       |  3 ++-
 tools/mm/page-types.c             |  2 --
 10 files changed, 53 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 c49ef99b4413b..5eb8a62fafb56 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 +3044,18 @@ 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_test_private(folio)) &&
+			     !folio_test_hugetlb(folio) &&
+			     !folio_test_swapbacked(folio);
 	}
 
 	/* One reference per page table mapping. */
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 86dd0470da117..9cb4d64739798 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -578,7 +578,23 @@ 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 */
@@ -1170,7 +1186,7 @@ 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)
@@ -1194,8 +1210,6 @@ 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_private - Determine if folio has private stuff
  * @folio: The folio to be checked
@@ -1205,7 +1219,9 @@ 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->private && !folio_test_swapbacked(folio) &&
+		!folio_test_hugetlb(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..8193f91216822 100644
--- a/include/trace/events/pagemap.h
+++ b/include/trace/events/pagemap.h
@@ -22,7 +22,10 @@
 	(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 folio->private locklessly */ \
+	(data_race(folio_test_private(folio)) && \
+	 !folio_test_swapbacked(folio) && \
+	 !folio_test_hugetlb(folio)	? PAGEMAP_BUFFERS    : 0) \
 	)
 
 TRACE_EVENT(mm_lru_insertion,
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index d1ce061601bcd..9b2a9d0794a61 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4831,8 +4831,11 @@ 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 folio->private locklessly.
 		 */
-		if (!folio_test_private(folio) &&
+		if (!(data_race(folio_test_private(folio)) &&
+		      !folio_test_swapbacked(folio) &&
+		      !folio_test_hugetlb(folio)) &&
 		    folio_expected_ref_count(folio) != folio_ref_count(folio))
 			goto next;
 
diff --git a/mm/migrate.c b/mm/migrate.c
index a369d0c95c386..f6befd3ff1c46 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1327,7 +1327,8 @@ 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_test_private(src) && !folio_test_swapbacked(src) &&
+		    !folio_test_hugetlb(src)) {
 			try_to_free_buffers(src);
 			goto out;
 		}
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index eeab25d6ce364..02ad49b10be07 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2705,7 +2705,11 @@ 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 folio->private locklessly */
+	__folio_mark_dirty(folio, mapping,
+			   !(data_race(folio_test_private(folio)) &&
+			     !folio_test_swapbacked(folio) &&
+			     !folio_test_hugetlb(folio)));
 
 	if (mapping->host) {
 		/* !PageAnon && !swapper_space */
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 40d3f1b48a74c..9348ebf9de882 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -978,7 +978,8 @@ 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_test_private(folio) && !folio_test_swapbacked(folio) &&
+	      !folio_test_hugetlb(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

Re: [PATCH v3 09/14] mm/page-flags: check page/folio->private instead of PG_private

[David Hildenbrand (Arm)]

On 9/8/26 04:56, 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.
> 
> 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: 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: 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
> ---

[...]

>  		/* !PageAnon && !swapper_space */
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 40d3f1b48a74c..9348ebf9de882 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -978,7 +978,8 @@ 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_test_private(folio) && !folio_test_swapbacked(folio) &&
> +	      !folio_test_hugetlb(folio)))

Now that I read that check ... a fourth time? a fifth? I stopped counting :)

Should we have a (in light of folio_attach_private()):

static inline bool folio_has_attached_private(const struct folio *folio)
{
	/* Careful, we might get called on unlocked folios. */
	if (!data_race(folio->private))
		return false;
	/* On some folios ->private is used for different purposes. */
	return !folio_test_swapbacked(folio) && !folio_test_hugetlb(folio);
}

Of course, adding some nice documentation what having attached private means?

-- 
Cheers,

David

Re: [PATCH v3 09/14] mm/page-flags: check page/folio->private instead of PG_private

[Zi Yan]

On Tue Sep 8, 2026 at 12:56 PM EDT, David Hildenbrand (Arm) wrote:
> On 9/8/26 04:56, 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.
>> 
>> 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: 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: 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
>> ---
>
> [...]
>
>>  		/* !PageAnon && !swapper_space */
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 40d3f1b48a74c..9348ebf9de882 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -978,7 +978,8 @@ 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_test_private(folio) && !folio_test_swapbacked(folio) &&
>> +	      !folio_test_hugetlb(folio)))
>
> Now that I read that check ... a fourth time? a fifth? I stopped counting :)
>
> Should we have a (in light of folio_attach_private()):
>
> static inline bool folio_has_attached_private(const struct folio *folio)
> {
> 	/* Careful, we might get called on unlocked folios. */
> 	if (!data_race(folio->private))
> 		return false;
> 	/* On some folios ->private is used for different purposes. */
> 	return !folio_test_swapbacked(folio) && !folio_test_hugetlb(folio);
> }
>
> Of course, adding some nice documentation what having attached private means?

Will squash the next one into this one and use
folio_has_attached_private() instead of folio_test_fs_private(), like we
discussed in Patch 10.



-- 
Best Regards,
Yan, Zi

[PATCH v3 10/14] mm/page-flags: introduce folio_test_fs_private()

[Zi Yan]

folio_test_fs_private() wraps folio->private != NULL check and excludes
swapcache and hugetlb folios, since swapcache uses swp_entry_t overlapping
with folio->private and hugetlb sets its own flags in folio->private.
Replace open code with the helper, since core MM does this check
frequently.

No functional change intended.

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: David Hildenbrand <david@kernel.org>
To: Steven Rostedt <rostedt@goodmis.org>
To: Masami Hiramatsu <mhiramat@kernel.org>
To: Lorenzo Stoakes <ljs@kernel.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: 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/mm.h             |  4 +---
 include/linux/page-flags.h     | 26 +++++++++++++++++++++++---
 include/trace/events/pagemap.h |  4 +---
 mm/huge_memory.c               |  4 +---
 mm/migrate.c                   |  3 +--
 mm/page-writeback.c            |  5 +----
 mm/vmscan.c                    |  3 +--
 7 files changed, 29 insertions(+), 20 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 5eb8a62fafb56..c861525cda83f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3053,9 +3053,7 @@ static inline int folio_expected_ref_count(const struct folio *folio)
 		 * One reference from filesystem private data.
 		 * Use data_race() since folio might not be locked.
 		 */
-		ref_count += data_race(folio_test_private(folio)) &&
-			     !folio_test_hugetlb(folio) &&
-			     !folio_test_swapbacked(folio);
+		ref_count += data_race(folio_test_fs_private(folio));
 	}
 
 	/* One reference per page table mapping. */
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 9cb4d64739798..ce7fccd90367b 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -1210,6 +1210,28 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page)
 	(0xffUL /* order */		| 1UL << PG_has_hwpoisoned |	\
 	 1UL << PG_large_rmappable	| 1UL << PG_partially_mapped)
 
+/**
+ * folio_test_fs_private - check if the folio has filesystem private data
+ * @folio: The folio to check.
+ *
+ * Use this in code that may encounter swapcache or hugetlb folios but only
+ * wants to detect filesystem 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_test_fs_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
@@ -1219,9 +1241,7 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page)
  */
 static inline int folio_has_private(const struct folio *folio)
 {
-	return (!!folio->private && !folio_test_swapbacked(folio) &&
-		!folio_test_hugetlb(folio)) ||
-		folio_test_private_2(folio);
+	return folio_test_fs_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 8193f91216822..547a550b81ee1 100644
--- a/include/trace/events/pagemap.h
+++ b/include/trace/events/pagemap.h
@@ -23,9 +23,7 @@
 	(folio_test_swapbacked(folio)	? PAGEMAP_SWAPBACKED : 0) | \
 	(folio_test_mappedtodisk(folio)	? PAGEMAP_MAPPEDDISK : 0) | \
 	/* data_race() is used to read folio->private locklessly */ \
-	(data_race(folio_test_private(folio)) && \
-	 !folio_test_swapbacked(folio) && \
-	 !folio_test_hugetlb(folio)	? PAGEMAP_BUFFERS    : 0) \
+	(data_race(folio_test_fs_private(folio))	? PAGEMAP_BUFFERS    : 0) \
 	)
 
 TRACE_EVENT(mm_lru_insertion,
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 9b2a9d0794a61..4cb7d9bc47544 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4833,9 +4833,7 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
 		 * can be split or not. So skip the check here.
 		 * data_race() is used to read folio->private locklessly.
 		 */
-		if (!(data_race(folio_test_private(folio)) &&
-		      !folio_test_swapbacked(folio) &&
-		      !folio_test_hugetlb(folio)) &&
+		if (!data_race(folio_test_fs_private(folio)) &&
 		    folio_expected_ref_count(folio) != folio_ref_count(folio))
 			goto next;
 
diff --git a/mm/migrate.c b/mm/migrate.c
index f6befd3ff1c46..2e133d533c8da 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1327,8 +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) && !folio_test_swapbacked(src) &&
-		    !folio_test_hugetlb(src)) {
+		if (folio_test_fs_private(src)) {
 			try_to_free_buffers(src);
 			goto out;
 		}
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 02ad49b10be07..e7389020d2a56 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2706,10 +2706,7 @@ bool filemap_dirty_folio(struct address_space *mapping, struct folio *folio)
 		return false;
 
 	/* data_race() is used to read folio->private locklessly */
-	__folio_mark_dirty(folio, mapping,
-			   !(data_race(folio_test_private(folio)) &&
-			     !folio_test_swapbacked(folio) &&
-			     !folio_test_hugetlb(folio)));
+	__folio_mark_dirty(folio, mapping, !data_race(folio_test_fs_private(folio)));
 
 	if (mapping->host) {
 		/* !PageAnon && !swapper_space */
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9348ebf9de882..47352bafc5b6f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -978,8 +978,7 @@ 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) && !folio_test_swapbacked(folio) &&
-	      !folio_test_hugetlb(folio)))
+	if (!folio_test_fs_private(folio))
 		return;
 
 	mapping = folio_mapping(folio);

-- 
2.53.0

Re: [PATCH v3 10/14] mm/page-flags: introduce folio_test_fs_private()

[David Hildenbrand (Arm)]

On 9/8/26 04:56, Zi Yan wrote:
> folio_test_fs_private() wraps folio->private != NULL check and excludes
> swapcache and hugetlb folios, since swapcache uses swp_entry_t overlapping
> with folio->private and hugetlb sets its own flags in folio->private.
> Replace open code with the helper, since core MM does this check
> frequently.
> 
> No functional change intended.
> 
> 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: David Hildenbrand <david@kernel.org>
> To: Steven Rostedt <rostedt@goodmis.org>
> To: Masami Hiramatsu <mhiramat@kernel.org>
> To: Lorenzo Stoakes <ljs@kernel.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: 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/mm.h             |  4 +---
>  include/linux/page-flags.h     | 26 +++++++++++++++++++++++---
>  include/trace/events/pagemap.h |  4 +---
>  mm/huge_memory.c               |  4 +---
>  mm/migrate.c                   |  3 +--
>  mm/page-writeback.c            |  5 +----
>  mm/vmscan.c                    |  3 +--
>  7 files changed, 29 insertions(+), 20 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 5eb8a62fafb56..c861525cda83f 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3053,9 +3053,7 @@ static inline int folio_expected_ref_count(const struct folio *folio)
>  		 * One reference from filesystem private data.
>  		 * Use data_race() since folio might not be locked.
>  		 */
> -		ref_count += data_race(folio_test_private(folio)) &&
> -			     !folio_test_hugetlb(folio) &&
> -			     !folio_test_swapbacked(folio);
> +		ref_count += data_race(folio_test_fs_private(folio));
>  	}

Lol :) That's what I meant with folio_has_attached_private()

(I think I prefer that name as it matches folio_attach_private(), but I might be
biased ;) )


This really should be sqashed into the previous patch, OR, have a prep patch
that only replaces these instances and uses folio_test_private() until you
convert it to folio->private in the second patch.

In short: touch these call sites only once, not twice in a row.

-- 
Cheers,

David

Re: [PATCH v3 10/14] mm/page-flags: introduce folio_test_fs_private()

[Zi Yan]

On Tue Sep 8, 2026 at 12:59 PM EDT, David Hildenbrand (Arm) wrote:
> On 9/8/26 04:56, Zi Yan wrote:
>> folio_test_fs_private() wraps folio->private != NULL check and excludes
>> swapcache and hugetlb folios, since swapcache uses swp_entry_t overlapping
>> with folio->private and hugetlb sets its own flags in folio->private.
>> Replace open code with the helper, since core MM does this check
>> frequently.
>> 
>> No functional change intended.
>> 
>> 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: David Hildenbrand <david@kernel.org>
>> To: Steven Rostedt <rostedt@goodmis.org>
>> To: Masami Hiramatsu <mhiramat@kernel.org>
>> To: Lorenzo Stoakes <ljs@kernel.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: 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/mm.h             |  4 +---
>>  include/linux/page-flags.h     | 26 +++++++++++++++++++++++---
>>  include/trace/events/pagemap.h |  4 +---
>>  mm/huge_memory.c               |  4 +---
>>  mm/migrate.c                   |  3 +--
>>  mm/page-writeback.c            |  5 +----
>>  mm/vmscan.c                    |  3 +--
>>  7 files changed, 29 insertions(+), 20 deletions(-)
>> 
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 5eb8a62fafb56..c861525cda83f 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -3053,9 +3053,7 @@ static inline int folio_expected_ref_count(const struct folio *folio)
>>  		 * One reference from filesystem private data.
>>  		 * Use data_race() since folio might not be locked.
>>  		 */
>> -		ref_count += data_race(folio_test_private(folio)) &&
>> -			     !folio_test_hugetlb(folio) &&
>> -			     !folio_test_swapbacked(folio);
>> +		ref_count += data_race(folio_test_fs_private(folio));
>>  	}
>
> Lol :) That's what I meant with folio_has_attached_private()
>
> (I think I prefer that name as it matches folio_attach_private(), but I might be
> biased ;) )

I can rename it, since it sounds more generic than "fs private".
>
>
> This really should be sqashed into the previous patch, OR, have a prep patch
> that only replaces these instances and uses folio_test_private() until you
> convert it to folio->private in the second patch.
>
> In short: touch these call sites only once, not twice in a row.

Will squash it to the prior patch.

-- 
Best Regards,
Yan, Zi

[PATCH v3 11/14] treewide: remove folio_set/clear_private()

[Zi Yan]

They are no-ops now. Remove them.

Assisted-by: Claude:claude-opus-4-8
Assisted-by: Codex:gpt-5
Signed-off-by: Zi Yan <ziy@nvidia.com>
To: Trond Myklebust <trondmy@kernel.org>
To: Anna Schumaker <anna@kernel.org>
To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
To: Jan Kara <jack@suse.cz>
To: Andrew Morton <akpm@linux-foundation.org>
To: David Hildenbrand <david@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
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: linux-nfs@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org
---
 fs/nfs/write.c          | 2 --
 include/linux/pagemap.h | 4 +---
 mm/migrate.c            | 1 -
 3 files changed, 1 insertion(+), 6 deletions(-)

diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index 623e7ef1f73d5..b6967b5286691 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -717,7 +717,6 @@ static void nfs_inode_add_request(struct nfs_page *req)
 	nfs_lock_request(req);
 	spin_lock(&mapping->i_private_lock);
 	set_bit(PG_MAPPED, &req->wb_flags);
-	folio_set_private(folio);
 	folio->private = req;
 	spin_unlock(&mapping->i_private_lock);
 	atomic_long_inc(&nfsi->nrequests);
@@ -745,7 +744,6 @@ static void nfs_inode_remove_request(struct nfs_page *req)
 
 			spin_lock(&mapping->i_private_lock);
 			folio->private = NULL;
-			folio_clear_private(folio);
 			clear_bit(PG_MAPPED, &req->wb_head->wb_flags);
 			spin_unlock(&mapping->i_private_lock);
 
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 2257df004305e..cc5c2cca814bc 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -594,7 +594,6 @@ static inline void folio_attach_private(struct folio *folio, void *data)
 {
 	folio_get(folio);
 	folio->private = data;
-	folio_set_private(folio);
 }
 
 /**
@@ -629,9 +628,8 @@ static inline void *folio_detach_private(struct folio *folio)
 {
 	void *data = folio_get_private(folio);
 
-	if (!folio_test_private(folio))
+	if (!data)
 		return NULL;
-	folio_clear_private(folio);
 	folio->private = NULL;
 	folio_put(folio);
 
diff --git a/mm/migrate.c b/mm/migrate.c
index 2e133d533c8da..77b89d0909f1f 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -835,7 +835,6 @@ void folio_migrate_flags(struct folio *newfolio, struct folio *folio)
 	 */
 	if (folio_test_swapcache(folio))
 		folio_clear_swapcache(folio);
-	folio_clear_private(folio);
 
 	/* page->private contains hugetlb specific flags */
 	if (!folio_test_hugetlb(folio))

-- 
2.53.0

Re: [PATCH v3 11/14] treewide: remove folio_set/clear_private()

[David Hildenbrand (Arm)]

On 9/8/26 04:56, Zi Yan wrote:
> They are no-ops now. Remove them.

Subject should likel y say that you remove folio_set/clear_private() *usage*.

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

-- 
Cheers,

David

Re: [PATCH v3 11/14] treewide: remove folio_set/clear_private()

[Zi Yan]

On Tue Sep 8, 2026 at 1:01 PM EDT, David Hildenbrand (Arm) wrote:
> On 9/8/26 04:56, Zi Yan wrote:
>> They are no-ops now. Remove them.
>
> Subject should likel y say that you remove folio_set/clear_private() *usage*.

Will adjust it.

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

Thanks.



-- 
Best Regards,
Yan, Zi

[PATCH v3 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.

__DEF_PAGEFLAG_NAME() is added to show __PG_folio.

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                 |  3 ++-
 kernel/vmcore_info.c                           |  1 -
 5 files changed, 8 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 d3a93eec3945f..dec7816303c6a 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 ce7fccd90367b..7b7783c0a5216 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;
-}
-
-/* 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..38a2d30230443 100644
--- a/include/trace/events/mmflags.h
+++ b/include/trace/events/mmflags.h
@@ -130,6 +130,7 @@ TRACE_DEFINE_ENUM(___GFP_LAST_BIT);
 #endif
 
 #define DEF_PAGEFLAG_NAME(_name) { 1UL <<  PG_##_name, __stringify(_name) }
+#define __DEF_PAGEFLAG_NAME(_name) { 1UL <<  __PG_##_name, __stringify(_name) }
 
 #define __def_pageflag_names						\
 	DEF_PAGEFLAG_NAME(locked),					\
@@ -144,7 +145,7 @@ 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 v3 14/14] mm/page-flags: remove PG_private

[David Hildenbrand (Arm)]

On 9/8/26 04:56, 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.
> 
> __DEF_PAGEFLAG_NAME() is added to show __PG_folio.
> 
> 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                 |  3 ++-
>  kernel/vmcore_info.c                           |  1 -
>  5 files changed, 8 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 d3a93eec3945f..dec7816303c6a 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.

This reads like it would belong into patch #13?

>  
>  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

Same here?

Likely this could have been restructured to cause less head scratches. I'd
expect any documentation that refers to PG_private to get removed before finally
removing the bit.

Not the end of the world, just a bit confusing while reviewing.

>  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 ce7fccd90367b..7b7783c0a5216 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 */

Do we really have to annotate it with __PG_folio ? I'd just keep it simple and
have the comment. That also avoids __DEF_PAGEFLAG_NAME just for this use case.

(sorry if this was discussed in previous review rounds)



-- 
Cheers,

David

Re: [PATCH v3 14/14] mm/page-flags: remove PG_private

[Zi Yan]

On 9 Sep 2026, at 10:31, David Hildenbrand (Arm) wrote:

> On 9/8/26 04:56, 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.
>>
>> __DEF_PAGEFLAG_NAME() is added to show __PG_folio.
>>
>> 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                 |  3 ++-
>>  kernel/vmcore_info.c                           |  1 -
>>  5 files changed, 8 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 d3a93eec3945f..dec7816303c6a 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.
>
> This reads like it would belong into patch #13?
>
>>
>>  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
>
> Same here?
>
> Likely this could have been restructured to cause less head scratches. I'd
> expect any documentation that refers to PG_private to get removed before finally
> removing the bit.
>
> Not the end of the world, just a bit confusing while reviewing.

Yeah, I will fold the document changes into the corresponding code change patches.

>
>>  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 ce7fccd90367b..7b7783c0a5216 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 */
>
> Do we really have to annotate it with __PG_folio ? I'd just keep it simple and
> have the comment. That also avoids __DEF_PAGEFLAG_NAME just for this use case.
>
> (sorry if this was discussed in previous review rounds)

No one complained about this yet. :)

I do this because I do not want to change PG_* values after PG_private after
PG_private is removed. And they will be changed back to their original values
when I add PG_folio. That PG_* value churn might be a headache for kdump?
If I add PG_folio the last page flag, it can be a one-time change though.

Best Regards,
Yan, Zi

Re: [PATCH v3 14/14] mm/page-flags: remove PG_private

[David Hildenbrand (Arm)]

On 9/9/26 16:48, Zi Yan wrote:
> On 9 Sep 2026, at 10:31, David Hildenbrand (Arm) wrote:
> 
>> On 9/8/26 04:56, 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.
>>>
>>> __DEF_PAGEFLAG_NAME() is added to show __PG_folio.
>>>
>>> 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                 |  3 ++-
>>>  kernel/vmcore_info.c                           |  1 -
>>>  5 files changed, 8 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 d3a93eec3945f..dec7816303c6a 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.
>>
>> This reads like it would belong into patch #13?
>>
>>>
>>>  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
>>
>> Same here?
>>
>> Likely this could have been restructured to cause less head scratches. I'd
>> expect any documentation that refers to PG_private to get removed before finally
>> removing the bit.
>>
>> Not the end of the world, just a bit confusing while reviewing.
> 
> Yeah, I will fold the document changes into the corresponding code change patches.
> 
>>
>>>  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 ce7fccd90367b..7b7783c0a5216 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 */
>>
>> Do we really have to annotate it with __PG_folio ? I'd just keep it simple and
>> have the comment. That also avoids __DEF_PAGEFLAG_NAME just for this use case.
>>
>> (sorry if this was discussed in previous review rounds)
> 
> No one complained about this yet. :)
> 
> I do this because I do not want to change PG_* values after PG_private after
> PG_private is removed. And they will be changed back to their original values
> when I add PG_folio. That PG_* value churn might be a headache for kdump?
> If I add PG_folio the last page flag, it can be a one-time change though.

Sorry, I meant that you just use

PG_folio,		/* Do not use: reserved for folio identification */

Without any further churn. Or is there a problem with this?

-- 
Cheers,

David

Re: [PATCH v3 14/14] mm/page-flags: remove PG_private

[Zi Yan]

On 9 Sep 2026, at 10:50, David Hildenbrand (Arm) wrote:

> On 9/9/26 16:48, Zi Yan wrote:
>> On 9 Sep 2026, at 10:31, David Hildenbrand (Arm) wrote:
>>
>>> On 9/8/26 04:56, 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.
>>>>
>>>> __DEF_PAGEFLAG_NAME() is added to show __PG_folio.
>>>>
>>>> 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                 |  3 ++-
>>>>  kernel/vmcore_info.c                           |  1 -
>>>>  5 files changed, 8 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 d3a93eec3945f..dec7816303c6a 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.
>>>
>>> This reads like it would belong into patch #13?
>>>
>>>>
>>>>  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
>>>
>>> Same here?
>>>
>>> Likely this could have been restructured to cause less head scratches. I'd
>>> expect any documentation that refers to PG_private to get removed before finally
>>> removing the bit.
>>>
>>> Not the end of the world, just a bit confusing while reviewing.
>>
>> Yeah, I will fold the document changes into the corresponding code change patches.
>>
>>>
>>>>  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 ce7fccd90367b..7b7783c0a5216 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 */
>>>
>>> Do we really have to annotate it with __PG_folio ? I'd just keep it simple and
>>> have the comment. That also avoids __DEF_PAGEFLAG_NAME just for this use case.
>>>
>>> (sorry if this was discussed in previous review rounds)
>>
>> No one complained about this yet. :)
>>
>> I do this because I do not want to change PG_* values after PG_private after
>> PG_private is removed. And they will be changed back to their original values
>> when I add PG_folio. That PG_* value churn might be a headache for kdump?
>> If I add PG_folio the last page flag, it can be a one-time change though.
>
> Sorry, I meant that you just use
>
> PG_folio,		/* Do not use: reserved for folio identification */
>
> Without any further churn. Or is there a problem with this?

Probably not. Let me do that.

Best Regards,
Yan, Zi