+ mm-page_alloc-count-guard-pages-as-free-again-when-their-buddy-merges.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/page_alloc: count guard pages as free again when their buddy merges
has been added to the -mm mm-unstable branch.  Its filename is
     mm-page_alloc-count-guard-pages-as-free-again-when-their-buddy-merges.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-page_alloc-count-guard-pages-as-free-again-when-their-buddy-merges.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Rik van Riel <riel@surriel.com>
Subject: mm/page_alloc: count guard pages as free again when their buddy merges
Date: Fri, 16 Oct 2026 12:23:04 -0400

With debug_guardpage_minorder set, expand() turns the unused halves of a
split into guard pages, which page_del_and_expand() leaves out of the free
counts.  Freeing the allocated half credits only that half, and
__free_one_page() then merges the guard buddy with clear_page_guard(),
which no longer touches the counters.

The merged block goes on a free list with the guard pages never counted,
so NR_FREE_PAGES falls behind the free lists by every guard that merges. 
Before commit e0932b6c1f94 ("mm: page_alloc: consolidate free page
accounting"), __set_page_guard() and __clear_page_guard() adjusted the
counts themselves; that commit dropped both adjustments but kept the
subtraction implicit in expand().

Count the guard pages under the merged block's migratetype when the buddy
is cleared.  account_freepages() skips an isolated block as it does for
every other free page, and moving the block off the isolated list counts
it then.

Booting a 16GB VM with debug_pagealloc=on debug_guardpage_minorder=1,
check the difference in free pages reported between /proc/vmstat and
/proc/buddyinfo.

At three points (idle, after a read and file-creation load, after a second
load), check the difference between nr_free_pages and the buddyinfo
numbers in the normal zone, as a number of pages:

                idle   after load   after second load
    unpatched   -130        79205               99121
    patched      -16           45                   0

The small remaining difference seems to be due to the numbers not being
read at exactly the same time, and is also seen without guard pages.

Link: https://lore.kernel.org/20261016122304.5b1eba8e@fangorn
Fixes: e0932b6c1f94 ("mm: page_alloc: consolidate free page accounting")
Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Assisted-by: LLM
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Brendan Jackman <brendan.jackman@linux.dev>
Cc: Kairui Song <kasong@tencent.com>
Cc: Qi Zheng <qi.zheng@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Frank van der Linden <fvdl@google.com>
Cc: <stable@vger.kernel.org>
---

 mm/page_alloc.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/mm/page_alloc.c~mm-page_alloc-count-guard-pages-as-free-again-when-their-buddy-merges
+++ a/mm/page_alloc.c
@@ -971,10 +971,12 @@ static inline void __free_one_page(struc
 		 * Our buddy is free or it is CONFIG_DEBUG_PAGEALLOC guard page,
 		 * merge with it and move up one order.
 		 */
-		if (page_is_guard(buddy))
+		if (page_is_guard(buddy)) {
 			clear_page_guard(zone, buddy, order);
-		else
+			account_freepages(zone, 1 << order, migratetype);
+		} else {
 			__del_page_from_free_list(buddy, zone, order, buddy_mt);
+		}
 
 		if (unlikely(buddy_mt != migratetype)) {
 			/*
_

Patches currently in -mm which might be from riel@surriel.com are

mm-page_alloc-count-guard-pages-as-free-again-when-their-buddy-merges.patch