linux-kernel - Re: [PATCH] pagemap: Update BUDDY flag documentation

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Message-ID: <ccd47ac6-439b-4c17-91fc-15c4f007af80@lucifer.local>
Date: Mon, 24 Nov 2025 10:51:03 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Richard Weinberger <richard@....at>
Cc: linux-mm@...ck.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, corbet@....net, mhocko@...e.com,
        surenb@...gle.com, rppt@...nel.org, vbabka@...e.cz,
        Liam.Howlett@...cle.com, david@...nel.org, akpm@...ux-foundation.org,
        Vladimir Davydov <vdavydov@...tuozzo.com>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] pagemap: Update BUDDY flag documentation

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@...tuozzo.com>>
> Cc: Konstantin Khlebnikov <koct9i@...il.com>
> Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Richard Weinberger <richard@....at>

Good catch! LGTM, so:

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