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 hugetlbfs_reserv.rst is outdated and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@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                 |  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