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

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

[Zi Yan]

Hi all,

This patchset removes PG_private to make space for upcoming PG_folio
(reserved as __PG_folio) for identifying pages from a folio (more details
in Note below). Instead of checking PG_private, all code is changed to
check page/folio->private != NULL instead.

MM people are cc'd on all patches and subsystem people are cc'd on the
cover letter and corresponding patches.

Overview
===
Most code uses folio_attach/detach/change_private() functions, so folio
refcount is increased and decreased when folio->private is set and reset,
respectively. There is no need to change them.

Changes are needed for exceptional users:
1. zsmalloc uses PG_private to indicate first component zpdesc page and
   page->private is used to store zspage in zpdesc. To remove PG_private,
   is_first_zpdesc() is changed to zpdesc->zspage->first_zpdesc == zpdesc
   instead of checking PG_private.

2. kernel/events/ring_buffer.c stores page order in page->private.
   Replacing PG_private with page->private != NULL works.

3. drivers/xen/grant-table.c stores xen_page_foreign in page->private,
   where on 32-bit, a pointer to xen_page_foreign is stored; on 64-bit,
   page->private is used as xen_page_foreign. PG_private check is replaced
   by page->private != NULL on 32-bit for xen_page_foreign deallocation.
   On 64-bit, page->private is cleared unconditionally since {domid=0,
   gref=0} (xen_page_foreign can be 0) is valid.

4. fs/crypto/crypto.c stores a folio pointer in page->private, PG_private
   checks are replaced by page->private != NULL.

5. fs/erofs has two different uses:

    5a. folio->private is used to form a reversed list of
    the outputs of readahead_folio(). readahead_folio_reverse() is added to
    output folios in reversed order, so that ->private is no longer needed.

    5b. folio->private is used as an in-flight I/O counter. Convert the
    code to use folio_attach/detach/get_private().

6. fs/nfs/write.c: folio refcount maintenance is in a bigger scope than
   folio->private. So folio_attach/detach/get_private() is not used.
   Nothing to change.

7. fs/f2fs uses attach_page_private() to first reset folio->private then
   immediately sets PAGE_PRIVATE_NOT_POINTER bit on it. Change it to use
   attach_page_private() to set PAGE_PRIVATE_NOT_POINTER bit directly to
   avoid folio->private == NULL gap inside set_page_private_##name().

8. hugetlb uses folio_change_private(folio, NULL) without folio refcount
   maintenance. Change it to folio->private = NULL.

After the above changes, PG_private ops are converted to
page/folio->private ops.

folio_test_fs_private() is added to check filesystem-only private data by
excluding swapcache and hugetlb folios, because swapcache folios overlap
swp_entry_t swap with ->private and hugetlb sets its own flags in
->private.

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

2. Documentation/mm/hugetlbfs_reserv.rst is outdated, so I did not remove
   PG_private related text. It should be rewritten.

3. PG_folio is planned to be set on every page from a folio in
   page_rmappable_folio(), so folios with any order (currently
   PG_large_rmappable is used to identify >0 order folios) can be
   identified, vm_insert_*() can correctly reject all folios, and rmap code
   can accept only folios. Eventually, page_folio() will return NULL for
   non-folio pages.

Tests
===
1. allmodconfig build passed.

2. zsmalloc is tested using ext4 on a 1GB lz4 zram:
    2a. zram load + zsmalloc compaction;
    2b. concurrent zspage migration via memory compaction;
    2c. confirmed that multi-page zspages actually formed.

    Details: https://github.com/x-y-z/linux-dev/blob/b4/remove-pg_private/test_zsmalloc.md

3. erofs is tested on images created with -C4096 and lz4hc, lzma,
   deflate, and zstd algorithms:
   3a. cold read of all files, verify checksums match source;
   3b. readahead + reclaim/migration race.

   Details: https://github.com/x-y-z/linux-dev/blob/b4/remove-pg_private/test_erofs.md

4. fscrypt is tested on software-encrypted ext4 with writes to exercise
   bounce pages.

   Details: https://github.com/x-y-z/linux-dev/blob/b4/remove-pg_private/test_fscrypt.md

5. f2fs is tested on an image with inline_data,compress_algorithm=lz4:
    5a. INLINE_INODE — lots of tiny files;
    5b. REF_RESOURCE + general writeback — buffered write churn with fsync;
    5c. ONGOING_MIGRATION — force GC / page migration;
    5d. ATOMIC_WRITE — atomic-write ioctl path.

    Details: https://github.com/x-y-z/linux-dev/blob/b4/remove-pg_private/test_f2fs.md
    (I did not run xfstests)

6. MM selftests passed.

LLM use
===
Claude was used to form a concrete plan on what code needs to be changed
and how to change them. The plan was reviewed by Codex until no issue was
spotted.

Plan is at: https://github.com/x-y-z/linux-dev/blob/b4/remove-pg_private/plan.md

I then followed the plan to make code changes. I did bounce ideas with
Claude how to change fs/erofs, since I did not like the original idea.
After each change, I asked Claude to review my code and git commit message.
I also asked Claude to give me test plans (see above).

At last, Codex was used to review all patches.

Comments and suggestions are welcome. Thanks.

Assisted-by: Claude:claude-opus-4-8
Assisted-by: Codex:gpt-5
Signed-off-by: Zi Yan <ziy@nvidia.com>
---
Zi Yan (14):
      mm/zsmalloc: replace PG_private with pointer comparison
      perf/ring_buffer: stop using PG_private as AUX page high-order marker
      xen/grant-table: stop setting PG_private on pages for grant mapping
      fs/crypto: stop setting PG_private on bounce page
      mm/hugetlb: use direct assignment instead of folio_change_private()
      fs/f2fs: stop using PG_private
      fs/erofs: mm/pagemap: add readahead_folio_reverse() to avoid folio->private
      fs/erofs: use folio_attach/detach_private() instead of direct assignment
      mm/page-flags: check page/folio->private instead of PG_private
      mm/page-flags: introduce folio_test_fs_private()
      treewide: remove folio_set/clear_private()
      treewide: replace PagePrivate() with page_private()
      treewide: adjust comments on PagePrivate and PG_private
      mm/page-flags: remove PG_private

 Documentation/admin-guide/kdump/vmcoreinfo.rst |  2 +-
 Documentation/filesystems/vfs.rst              |  6 ++--
 arch/x86/events/intel/bts.c                    |  3 --
 arch/x86/events/intel/pt.c                     |  6 ++--
 drivers/md/md-bitmap.c                         |  6 ++--
 drivers/xen/balloon.c                          |  5 ++++
 drivers/xen/grant-table.c                      |  7 ++---
 fs/ceph/addr.c                                 |  8 ++----
 fs/crypto/crypto.c                             |  2 --
 fs/erofs/data.c                                |  5 ++--
 fs/erofs/zdata.c                               | 11 ++------
 fs/f2fs/f2fs.h                                 |  8 +++---
 fs/nfs/file.c                                  |  4 +--
 fs/nfs/write.c                                 |  2 --
 fs/proc/page.c                                 |  5 +++-
 fs/ubifs/file.c                                |  6 ++--
 include/linux/buffer_head.h                    |  6 ----
 include/linux/mm.h                             | 16 +++++------
 include/linux/mm_types.h                       |  4 +--
 include/linux/page-flags.h                     | 38 ++++++++++++++++++--------
 include/linux/pagemap.h                        | 35 ++++++++++++++++++++++--
 include/trace/events/mmflags.h                 |  2 +-
 include/trace/events/pagemap.h                 |  2 +-
 kernel/events/ring_buffer.c                    |  7 ++---
 kernel/vmcore_info.c                           |  1 -
 mm/huge_memory.c                               |  2 +-
 mm/hugetlb.c                                   |  6 ++--
 mm/migrate.c                                   |  3 +-
 mm/page-writeback.c                            |  5 +++-
 mm/vmscan.c                                    |  3 +-
 mm/zpdesc.h                                    |  2 +-
 mm/zsmalloc.c                                  | 15 ++--------
 32 files changed, 127 insertions(+), 106 deletions(-)
---
base-commit: bcd5eb68a6a189497eb26c1b9f622538aa48895d
change-id: 20260728-remove-pg_private-cfe926c7f83c

Best regards,
-- 
Yan, Zi

[PATCH RFC 02/14] perf/ring_buffer: stop using PG_private as AUX page high-order marker

[Zi Yan]

A high-order AUX page sets PG_private on its first page and stores the
order in first_page->private. Stop using PG_private and check
first_page->private for AUX page order only ring buffer and its users.

It prepares for a future commit that remove PG_private.

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: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@redhat.com>
To: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Namhyung Kim <namhyung@kernel.org>
To: Thomas Gleixner <tglx@kernel.org>
To: Borislav Petkov <bp@alien8.de>
To: Dave Hansen <dave.hansen@linux.intel.com>
To: x86@kernel.org
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: James Clark <james.clark@linaro.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: linux-perf-users@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
 arch/x86/events/intel/bts.c | 3 ---
 arch/x86/events/intel/pt.c  | 6 ++----
 kernel/events/ring_buffer.c | 7 +++----
 3 files changed, 5 insertions(+), 11 deletions(-)

diff --git a/arch/x86/events/intel/bts.c b/arch/x86/events/intel/bts.c
index cbac54cb3a9ec..5849392cf26d5 100644
--- a/arch/x86/events/intel/bts.c
+++ b/arch/x86/events/intel/bts.c
@@ -66,9 +66,6 @@ static struct pmu bts_pmu;
 
 static int buf_nr_pages(struct page *page)
 {
-	if (!PagePrivate(page))
-		return 1;
-
 	return 1 << page_private(page);
 }
 
diff --git a/arch/x86/events/intel/pt.c b/arch/x86/events/intel/pt.c
index b5726b50e77db..29d8b4a3560cd 100644
--- a/arch/x86/events/intel/pt.c
+++ b/arch/x86/events/intel/pt.c
@@ -774,8 +774,7 @@ static int topa_insert_pages(struct pt_buffer *buf, int cpu, gfp_t gfp)
 	struct page *p;
 
 	p = virt_to_page(buf->data_pages[buf->nr_pages]);
-	if (PagePrivate(p))
-		order = page_private(p);
+	order = page_private(p);
 
 	if (topa_table_full(topa)) {
 		topa = topa_alloc(cpu, gfp);
@@ -1289,8 +1288,7 @@ static int pt_buffer_try_single(struct pt_buffer *buf, int nr_pages)
 	if (!intel_pt_validate_hw_cap(PT_CAP_single_range_output))
 		goto out;
 
-	if (PagePrivate(p))
-		order = page_private(p);
+	order = page_private(p);
 
 	if (1 << order != nr_pages)
 		goto out;
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 9fe92161715e0..061b5cb222266 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -630,11 +630,10 @@ static struct page *rb_alloc_aux_page(int node, int order)
 		/*
 		 * Communicate the allocation size to the driver:
 		 * if we managed to secure a high-order allocation,
-		 * set its first page's private to this order;
-		 * !PagePrivate(page) means it's just a normal page.
+		 * set its first page's private to this order, otherwise page's
+		 * private remains zero.
 		 */
 		split_page(page, order);
-		SetPagePrivate(page);
 		set_page_private(page, order);
 	}
 
@@ -645,7 +644,7 @@ static void rb_free_aux_page(struct perf_buffer *rb, int idx)
 {
 	struct page *page = virt_to_page(rb->aux_pages[idx]);
 
-	ClearPagePrivate(page);
+	set_page_private(page, 0);
 	__free_page(page);
 }
 

-- 
2.53.0

Re: [PATCH RFC 02/14] perf/ring_buffer: stop using PG_private as AUX page high-order marker

[Usama Arif]

On Fri, 31 Jul 2026 22:13:25 -0400 Zi Yan <ziy@nvidia.com> wrote:

> A high-order AUX page sets PG_private on its first page and stores the
> order in first_page->private. Stop using PG_private and check
> first_page->private for AUX page order only ring buffer and its users.
> 

nit: only "in the" ring buffer?

> It prepares for a future commit that remove PG_private.
> 
> 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: Peter Zijlstra <peterz@infradead.org>
> To: Ingo Molnar <mingo@redhat.com>
> To: Arnaldo Carvalho de Melo <acme@kernel.org>
> To: Namhyung Kim <namhyung@kernel.org>
> To: Thomas Gleixner <tglx@kernel.org>
> To: Borislav Petkov <bp@alien8.de>
> To: Dave Hansen <dave.hansen@linux.intel.com>
> To: x86@kernel.org
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Cc: Ian Rogers <irogers@google.com>
> Cc: Adrian Hunter <adrian.hunter@intel.com>
> Cc: James Clark <james.clark@linaro.org>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: linux-perf-users@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> ---
>  arch/x86/events/intel/bts.c | 3 ---
>  arch/x86/events/intel/pt.c  | 6 ++----
>  kernel/events/ring_buffer.c | 7 +++----
>  3 files changed, 5 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/events/intel/bts.c b/arch/x86/events/intel/bts.c
> index cbac54cb3a9ec..5849392cf26d5 100644
> --- a/arch/x86/events/intel/bts.c
> +++ b/arch/x86/events/intel/bts.c
> @@ -66,9 +66,6 @@ static struct pmu bts_pmu;
>  
>  static int buf_nr_pages(struct page *page)
>  {
> -	if (!PagePrivate(page))
> -		return 1;
> -


For order 0, 1 << 0 is 1, which is exactly what the !PagePrivate(page) so LGTM.

The commit message probably needs more text but everything else looks good.

Feel free to add

Acked-by: Usama Arif <usama.arif@linux.dev>


>  	return 1 << page_private(page);
>  }
>  
> diff --git a/arch/x86/events/intel/pt.c b/arch/x86/events/intel/pt.c
> index b5726b50e77db..29d8b4a3560cd 100644
> --- a/arch/x86/events/intel/pt.c
> +++ b/arch/x86/events/intel/pt.c
> @@ -774,8 +774,7 @@ static int topa_insert_pages(struct pt_buffer *buf, int cpu, gfp_t gfp)
>  	struct page *p;
>  
>  	p = virt_to_page(buf->data_pages[buf->nr_pages]);
> -	if (PagePrivate(p))
> -		order = page_private(p);
> +	order = page_private(p);
>  
>  	if (topa_table_full(topa)) {
>  		topa = topa_alloc(cpu, gfp);
> @@ -1289,8 +1288,7 @@ static int pt_buffer_try_single(struct pt_buffer *buf, int nr_pages)
>  	if (!intel_pt_validate_hw_cap(PT_CAP_single_range_output))
>  		goto out;
>  
> -	if (PagePrivate(p))
> -		order = page_private(p);
> +	order = page_private(p);
>  
>  	if (1 << order != nr_pages)
>  		goto out;
> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> index 9fe92161715e0..061b5cb222266 100644
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -630,11 +630,10 @@ static struct page *rb_alloc_aux_page(int node, int order)
>  		/*
>  		 * Communicate the allocation size to the driver:
>  		 * if we managed to secure a high-order allocation,
> -		 * set its first page's private to this order;
> -		 * !PagePrivate(page) means it's just a normal page.
> +		 * set its first page's private to this order, otherwise page's
> +		 * private remains zero.
>  		 */
>  		split_page(page, order);
> -		SetPagePrivate(page);
>  		set_page_private(page, order);
>  	}
>  
> @@ -645,7 +644,7 @@ static void rb_free_aux_page(struct perf_buffer *rb, int idx)
>  {
>  	struct page *page = virt_to_page(rb->aux_pages[idx]);
>  
> -	ClearPagePrivate(page);
> +	set_page_private(page, 0);
>  	__free_page(page);
>  }
>  
> 
> -- 
> 2.53.0
> 
> 

Re: [PATCH RFC 02/14] perf/ring_buffer: stop using PG_private as AUX page high-order marker

[Zi Yan]

On Sat Aug 1, 2026 at 10:32 AM EDT, Usama Arif wrote:
> On Fri, 31 Jul 2026 22:13:25 -0400 Zi Yan <ziy@nvidia.com> wrote:
>
>> A high-order AUX page sets PG_private on its first page and stores the
>> order in first_page->private. Stop using PG_private and check
>> first_page->private for AUX page order only ring buffer and its users.
>> 
>
> nit: only "in the" ring buffer?

Will fix it.
>
>> It prepares for a future commit that remove PG_private.
>> 
>> 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: Peter Zijlstra <peterz@infradead.org>
>> To: Ingo Molnar <mingo@redhat.com>
>> To: Arnaldo Carvalho de Melo <acme@kernel.org>
>> To: Namhyung Kim <namhyung@kernel.org>
>> To: Thomas Gleixner <tglx@kernel.org>
>> To: Borislav Petkov <bp@alien8.de>
>> To: Dave Hansen <dave.hansen@linux.intel.com>
>> To: x86@kernel.org
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
>> Cc: Jiri Olsa <jolsa@kernel.org>
>> Cc: Ian Rogers <irogers@google.com>
>> Cc: Adrian Hunter <adrian.hunter@intel.com>
>> Cc: James Clark <james.clark@linaro.org>
>> Cc: "H. Peter Anvin" <hpa@zytor.com>
>> Cc: linux-perf-users@vger.kernel.org
>> Cc: linux-kernel@vger.kernel.org
>> ---
>>  arch/x86/events/intel/bts.c | 3 ---
>>  arch/x86/events/intel/pt.c  | 6 ++----
>>  kernel/events/ring_buffer.c | 7 +++----
>>  3 files changed, 5 insertions(+), 11 deletions(-)
>> 
>> diff --git a/arch/x86/events/intel/bts.c b/arch/x86/events/intel/bts.c
>> index cbac54cb3a9ec..5849392cf26d5 100644
>> --- a/arch/x86/events/intel/bts.c
>> +++ b/arch/x86/events/intel/bts.c
>> @@ -66,9 +66,6 @@ static struct pmu bts_pmu;
>>  
>>  static int buf_nr_pages(struct page *page)
>>  {
>> -	if (!PagePrivate(page))
>> -		return 1;
>> -
>
>
> For order 0, 1 << 0 is 1, which is exactly what the !PagePrivate(page) so LGTM.
>
> The commit message probably needs more text but everything else looks good.
>
> Feel free to add
>
> Acked-by: Usama Arif <usama.arif@linux.dev>

Thanks. Will add order 0 explanation to the commit message.
>
>
>>  	return 1 << page_private(page);
>>  }
>>  
>> diff --git a/arch/x86/events/intel/pt.c b/arch/x86/events/intel/pt.c
>> index b5726b50e77db..29d8b4a3560cd 100644
>> --- a/arch/x86/events/intel/pt.c
>> +++ b/arch/x86/events/intel/pt.c
>> @@ -774,8 +774,7 @@ static int topa_insert_pages(struct pt_buffer *buf, int cpu, gfp_t gfp)
>>  	struct page *p;
>>  
>>  	p = virt_to_page(buf->data_pages[buf->nr_pages]);
>> -	if (PagePrivate(p))
>> -		order = page_private(p);
>> +	order = page_private(p);
>>  
>>  	if (topa_table_full(topa)) {
>>  		topa = topa_alloc(cpu, gfp);
>> @@ -1289,8 +1288,7 @@ static int pt_buffer_try_single(struct pt_buffer *buf, int nr_pages)
>>  	if (!intel_pt_validate_hw_cap(PT_CAP_single_range_output))
>>  		goto out;
>>  
>> -	if (PagePrivate(p))
>> -		order = page_private(p);
>> +	order = page_private(p);
>>  
>>  	if (1 << order != nr_pages)
>>  		goto out;
>> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
>> index 9fe92161715e0..061b5cb222266 100644
>> --- a/kernel/events/ring_buffer.c
>> +++ b/kernel/events/ring_buffer.c
>> @@ -630,11 +630,10 @@ static struct page *rb_alloc_aux_page(int node, int order)
>>  		/*
>>  		 * Communicate the allocation size to the driver:
>>  		 * if we managed to secure a high-order allocation,
>> -		 * set its first page's private to this order;
>> -		 * !PagePrivate(page) means it's just a normal page.
>> +		 * set its first page's private to this order, otherwise page's
>> +		 * private remains zero.
>>  		 */
>>  		split_page(page, order);
>> -		SetPagePrivate(page);
>>  		set_page_private(page, order);
>>  	}
>>  
>> @@ -645,7 +644,7 @@ static void rb_free_aux_page(struct perf_buffer *rb, int idx)
>>  {
>>  	struct page *page = virt_to_page(rb->aux_pages[idx]);
>>  
>> -	ClearPagePrivate(page);
>> +	set_page_private(page, 0);
>>  	__free_page(page);
>>  }
>>  
>> 
>> -- 
>> 2.53.0
>> 
>> 




-- 
Best Regards,
Yan, Zi