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

["David Hildenbrand (Arm)" <david@kernel.org>]

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 hugetlbfs_reserv.rst is outdated and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