[PATCH RFC 14/14] mm/page-flags: remove PG_private
Zi Yan
ziy at nvidia.com
Wed Aug 12 13:10:07 PDT 2026
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 hugetlbfs_reserv.rst is outdated and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 at nvidia.com>
> To: Andrew Morton <akpm at linux-foundation.org>
> To: Baoquan He <baoquan.he at linux.dev>
> To: Mike Rapoport <rppt at kernel.org>
> To: Pasha Tatashin <pasha.tatashin at soleen.com>
> To: Pratyush Yadav <pratyush at kernel.org>
> To: Jonathan Corbet <corbet at lwn.net>
> To: "Matthew Wilcox (Oracle)" <willy at infradead.org>
> To: Jan Kara <jack at suse.cz>
> To: David Hildenbrand <david at kernel.org>
> To: Steven Rostedt <rostedt at goodmis.org>
> To: Masami Hiramatsu <mhiramat at kernel.org>
> Cc: Dave Young <ruirui.yang at linux.dev>
> Cc: Shuah Khan <skhan at linuxfoundation.org>
> Cc: Lorenzo Stoakes <ljs at kernel.org>
> Cc: "Liam R. Howlett" <liam at infradead.org>
> Cc: Vlastimil Babka <vbabka at kernel.org>
> Cc: Suren Baghdasaryan <surenb at google.com>
> Cc: Michal Hocko <mhocko at suse.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> Cc: kexec at lists.infradead.org
> Cc: linux-doc at vger.kernel.org
> Cc: linux-kernel at vger.kernel.org
> Cc: linux-fsdevel at vger.kernel.org
> Cc: linux-mm at kvack.org
> Cc: linux-trace-kernel at 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 | 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@nvidia.com/
<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
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