[RFC PATCH 1/5] mm/page_alloc: count guard pages as free again when their buddy merges

[Rik van Riel <riel@surriel.com>]

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.

Fixes: e0932b6c1f94 ("mm: page_alloc: consolidate free page accounting")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Rik van Riel <riel@surriel.com>
---
 mm/page_alloc.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 12fac9084c483..4658af97be010 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -988,10 +988,12 @@ static inline void __free_one_page(struct page *page,
 		 * 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)) {
 			/*
-- 
2.53.0-Meta