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

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

[Rik van Riel]

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>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Dev Jain <dev.jain@arm.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)) {
 			/*

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

[Vlastimil Babka (SUSE)]

On 10/16/26 18:23, Rik van Riel wrote:
> 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>
> Reviewed-by: Zi Yan <ziy@nvidia.com>
> Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
> Reviewed-by: Dev Jain <dev.jain@arm.com>

Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

> ---
>  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)) {
>  			/*
>