[PATCH] pagemap: Update BUDDY flag documentation

[PATCH] pagemap: Update BUDDY flag documentation

[Richard Weinberger]

Since v4.6 the BUDDY flag is set for _all_ pages in the block
and no longer just for the first one.
This change was introduced by:
commit 832fc1de01ae ("/proc/kpageflags: return KPF_BUDDY for "tail" buddy pages")

Strictly speaking, this was an ABI change, but as nobody has noticed
since 2016, let's just update the documentation.

Cc: Vladimir Davydov <vdavydov@virtuozzo.com>>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Richard Weinberger <richard@nod.at>
---
 Documentation/admin-guide/mm/pagemap.rst | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
index e60e9211fd9b2..c57e61b5d8aa8 100644
--- a/Documentation/admin-guide/mm/pagemap.rst
+++ b/Documentation/admin-guide/mm/pagemap.rst
@@ -115,7 +115,8 @@ Short descriptions to the page flags
     A free memory block managed by the buddy system allocator.
     The buddy system organizes free memory in blocks of various orders.
     An order N block has 2^N physically contiguous pages, with the BUDDY flag
-    set for and _only_ for the first page.
+    set for all pages.
+    Before 4.6 only the first page of the block had the flag set.
 15 - COMPOUND_HEAD
     A compound page with order N consists of 2^N physically contiguous pages.
     A compound page with order 2 takes the form of "HTTT", where H donates its
-- 
2.51.0

Re: [PATCH] pagemap: Update BUDDY flag documentation

[Mike Rapoport]

On Sat, Nov 22, 2025 at 10:19:20PM +0100, Richard Weinberger wrote:
> Since v4.6 the BUDDY flag is set for _all_ pages in the block
> and no longer just for the first one.
> This change was introduced by:
> commit 832fc1de01ae ("/proc/kpageflags: return KPF_BUDDY for "tail" buddy pages")
> 
> Strictly speaking, this was an ABI change, but as nobody has noticed
> since 2016, let's just update the documentation.
> 
> Cc: Vladimir Davydov <vdavydov@virtuozzo.com>>
> Cc: Konstantin Khlebnikov <koct9i@gmail.com>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Richard Weinberger <richard@nod.at>

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
>  Documentation/admin-guide/mm/pagemap.rst | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
> index e60e9211fd9b2..c57e61b5d8aa8 100644
> --- a/Documentation/admin-guide/mm/pagemap.rst
> +++ b/Documentation/admin-guide/mm/pagemap.rst
> @@ -115,7 +115,8 @@ Short descriptions to the page flags
>      A free memory block managed by the buddy system allocator.
>      The buddy system organizes free memory in blocks of various orders.
>      An order N block has 2^N physically contiguous pages, with the BUDDY flag
> -    set for and _only_ for the first page.
> +    set for all pages.
> +    Before 4.6 only the first page of the block had the flag set.
>  15 - COMPOUND_HEAD
>      A compound page with order N consists of 2^N physically contiguous pages.
>      A compound page with order 2 takes the form of "HTTT", where H donates its
> -- 
> 2.51.0
> 
> 

-- 
Sincerely yours,
Mike.

Re: [PATCH] pagemap: Update BUDDY flag documentation

[David Hildenbrand (Red Hat)]

On 11/22/25 22:19, Richard Weinberger wrote:
> Since v4.6 the BUDDY flag is set for _all_ pages in the block
> and no longer just for the first one.
> This change was introduced by:
> commit 832fc1de01ae ("/proc/kpageflags: return KPF_BUDDY for "tail" buddy pages")
> 
> Strictly speaking, this was an ABI change, but as nobody has noticed
> since 2016, let's just update the documentation.
> 
> Cc: Vladimir Davydov <vdavydov@virtuozzo.com>>
> Cc: Konstantin Khlebnikov <koct9i@gmail.com>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Richard Weinberger <richard@nod.at>
> ---

Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>

-- 
Cheers

David

Re: [PATCH] pagemap: Update BUDDY flag documentation

[Lorenzo Stoakes]

On Sat, Nov 22, 2025 at 10:19:20PM +0100, Richard Weinberger wrote:
> Since v4.6 the BUDDY flag is set for _all_ pages in the block
> and no longer just for the first one.
> This change was introduced by:
> commit 832fc1de01ae ("/proc/kpageflags: return KPF_BUDDY for "tail" buddy pages")
>
> Strictly speaking, this was an ABI change, but as nobody has noticed
> since 2016, let's just update the documentation.
>
> Cc: Vladimir Davydov <vdavydov@virtuozzo.com>>
> Cc: Konstantin Khlebnikov <koct9i@gmail.com>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Richard Weinberger <richard@nod.at>

Good catch! LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  Documentation/admin-guide/mm/pagemap.rst | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
> index e60e9211fd9b2..c57e61b5d8aa8 100644
> --- a/Documentation/admin-guide/mm/pagemap.rst
> +++ b/Documentation/admin-guide/mm/pagemap.rst
> @@ -115,7 +115,8 @@ Short descriptions to the page flags
>      A free memory block managed by the buddy system allocator.
>      The buddy system organizes free memory in blocks of various orders.
>      An order N block has 2^N physically contiguous pages, with the BUDDY flag
> -    set for and _only_ for the first page.
> +    set for all pages.
> +    Before 4.6 only the first page of the block had the flag set.
>  15 - COMPOUND_HEAD
>      A compound page with order N consists of 2^N physically contiguous pages.
>      A compound page with order 2 takes the form of "HTTT", where H donates its
> --
> 2.51.0
>