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

[Zi Yan]

On Fri Jul 31, 2026 at 10:13 PM EDT, 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.
>
> 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 hugetlbfs_reserv.rst is outdated and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@xxxxxxxxxx>
> To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> To: Baoquan He <baoquan.he@xxxxxxxxx>
> To: Mike Rapoport <rppt@xxxxxxxxxx>
> To: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
> To: Pratyush Yadav <pratyush@xxxxxxxxxx>
> To: Jonathan Corbet <corbet@xxxxxxx>
> To: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> To: Jan Kara <jack@xxxxxxx>
> To: David Hildenbrand <david@xxxxxxxxxx>
> To: Steven Rostedt <rostedt@xxxxxxxxxxx>
> To: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Cc: Dave Young <ruirui.yang@xxxxxxxxx>
> Cc: Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx>
> Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> Cc: "Liam R. Howlett" <liam@xxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxxxxx>
> Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: kexec@xxxxxxxxxxxxxxxxxxx
> Cc: linux-doc@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-trace-kernel@xxxxxxxxxxxxxxx
> ---
>  Documentation/admin-guide/kdump/vmcoreinfo.rst |  2 +-
>  Documentation/filesystems/vfs.rst              |  6 +++---
>  include/linux/page-flags.h                     | 19 ++-----------------
>  include/trace/events/mmflags.h                 |  2 +-
>  kernel/vmcore_info.c                           |  1 -
>  5 files changed, 7 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 e7677423a20f7..5cc07f82fe371 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 0e3628ea080c4..eb2961ed61018 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 */

Sashiko asked how to detect leaked filesystem private data during page
free time after PG_private is removed.

Answer:

After the conversion, folio/page should have an elevated refcount whenever
->private is set. That would help detect leaked private data. I tried to
enforce ->private needs to be NULL at page free time[1], but that might
cause trouble for certain use cases.

[1] https://lore.kernel.org/all/20260223032641.1859381-1-ziy@xxxxxxxxxx/

<snip>

> 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)

Sashiko asked if removing PG_private from vmcoreinfo breaks back
compatiblity for related userspace tools.

Answer:

Userspace tools will adapt to the changes.


-- 
Best Regards,
Yan, Zi