[PATCH] mm/page_alloc: Avoid duplicate NR_FREE_PAGES updates in move_to_free_list()

[PATCH] mm/page_alloc: Avoid duplicate NR_FREE_PAGES updates in move_to_free_list()

[Yajun Deng]

In move_to_free_list(), when a page block changes its migration type,
we need to update free page counts for both the old and new types.
Originally, this was done by two calls to account_freepages(), which
updates NR_FREE_PAGES and also type-specific counters. However, this
causes NR_FREE_PAGES to be updated twice, while the net change is zero
in most cases.

This patch introduces a new function account_freepages_both() that
updates the statistics for both old and new migration types in one go.
It avoids the double update of NR_FREE_PAGES by computing the net change
only when the isolation status changes.

The optimization avoid duplicate NR_FREE_PAGES updates in
move_to_free_list().

Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
---
 mm/page_alloc.c | 37 ++++++++++++++++++++++++++++++-------
 1 file changed, 30 insertions(+), 7 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ebfa07632995..e51d8bd7ab7d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -812,6 +812,16 @@ compaction_capture(struct capture_control *capc, struct page *page,
 }
 #endif /* CONFIG_COMPACTION */
 
+static inline void account_specific_freepages(struct zone *zone, int nr_pages,
+					      int migratetype)
+{
+	if (is_migrate_cma(migratetype))
+		__mod_zone_page_state(zone, NR_FREE_CMA_PAGES, nr_pages);
+	else if (migratetype == MIGRATE_HIGHATOMIC)
+		WRITE_ONCE(zone->nr_free_highatomic,
+			   zone->nr_free_highatomic + nr_pages);
+}
+
 static inline void account_freepages(struct zone *zone, int nr_pages,
 				     int migratetype)
 {
@@ -822,11 +832,25 @@ static inline void account_freepages(struct zone *zone, int nr_pages,
 
 	__mod_zone_page_state(zone, NR_FREE_PAGES, nr_pages);
 
-	if (is_migrate_cma(migratetype))
-		__mod_zone_page_state(zone, NR_FREE_CMA_PAGES, nr_pages);
-	else if (migratetype == MIGRATE_HIGHATOMIC)
-		WRITE_ONCE(zone->nr_free_highatomic,
-			   zone->nr_free_highatomic + nr_pages);
+	account_specific_freepages(zone, nr_pages, migratetype);
+}
+
+static inline void account_freepages_both(struct zone *zone, int nr_pages,
+					  int old_mt, int new_mt)
+{
+	lockdep_assert_held(&zone->lock);
+
+	bool old_isolated = is_migrate_isolate(old_mt);
+	bool new_isolated = is_migrate_isolate(new_mt);
+
+	if (old_isolated != new_isolated)
+		__mod_zone_page_state(zone, NR_FREE_PAGES,
+				      old_isolated ? nr_pages : -nr_pages);
+
+	if (!old_isolated)
+		account_specific_freepages(zone, -nr_pages, old_mt);
+	if (!new_isolated)
+		account_specific_freepages(zone, nr_pages, new_mt);
 }
 
 /* Used for pages not on another list */
@@ -869,8 +893,7 @@ static inline void move_to_free_list(struct page *page, struct zone *zone,
 
 	list_move_tail(&page->buddy_list, &area->free_list[new_mt]);
 
-	account_freepages(zone, -nr_pages, old_mt);
-	account_freepages(zone, nr_pages, new_mt);
+	account_freepages_both(zone, nr_pages, old_mt, new_mt);
 
 	if (order >= pageblock_order &&
 	    is_migrate_isolate(old_mt) != is_migrate_isolate(new_mt)) {
-- 
2.34.1

Re: [PATCH] mm/page_alloc: Avoid duplicate NR_FREE_PAGES updates in move_to_free_list()

[Joshua Hahn]

On Fri,  9 Jan 2026 18:51:21 +0800 Yajun Deng <yajun.deng@linux.dev> wrote:

> In move_to_free_list(), when a page block changes its migration type,
> we need to update free page counts for both the old and new types.
> Originally, this was done by two calls to account_freepages(), which
> updates NR_FREE_PAGES and also type-specific counters. However, this
> causes NR_FREE_PAGES to be updated twice, while the net change is zero
> in most cases.
> 
> This patch introduces a new function account_freepages_both() that
> updates the statistics for both old and new migration types in one go.
> It avoids the double update of NR_FREE_PAGES by computing the net change
> only when the isolation status changes.
> 
> The optimization avoid duplicate NR_FREE_PAGES updates in
> move_to_free_list().

Hi Yajun,

I hope you are doing well, thank you for the patch! I was hoping to better
understand the motivation behind this patch.

From my perspective, I believe that the current state of the code is
not optimal, but it is also not problematic. account_freepages seems like
a relatively cheap function (at the core, it's just some atomic operations).
Personally I also think that semantically, the code currently makes sense;
we are doing the accounting for the old mounttype, then for the new mounttype,
in a way that cancels out. And given that there is still some cases where
the work doesn't end up canceling out due to one of the mounttypes being
MIGRATE_ISOLATE, I think that there is enough purpose in making the two
calls to do the accounting twice.

On the other hand I think there is only one place in the codebase that
will use account_freepages_both, so it might make the burden to understand
the code a bit higher.

What do you think? I don't have a strong stance on whether the performance
effects are big here (if this change indeed has a big performance implication,
then we should definitely go forth with this!) but I do believe the current
code is quite semantically sound and more readable. 

Thank you again for the patch. I hope you have a great day!
Joshua