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

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Message-ID: <20251122211920.3410371-1-richard@nod.at>
Date: Sat, 22 Nov 2025 22:19:20 +0100
From: Richard Weinberger <richard@....at>
To: linux-mm@...ck.org
Cc: 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,
	lorenzo.stoakes@...cle.com,
	david@...nel.org,
	akpm@...ux-foundation.org,
	Richard Weinberger <richard@....at>,
	Vladimir Davydov <vdavydov@...tuozzo.com>,
	Konstantin Khlebnikov <koct9i@...il.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH] pagemap: Update BUDDY flag documentation

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>
---
 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