[PATCH v2] mm/page_alloc: Avoid duplicate NR_FREE_PAGES updates in move_to_free_list()

[PATCH v2] 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 adds a condition that updates the NR_FREE_PAGES only if one of
the two types is the isolate type. This avoids NR_FREE_PAGES being
updates twice.

The optimization avoid duplicate NR_FREE_PAGES updates in
move_to_free_list().

Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
Suggested-by: Joshua Hahn <joshua.hahnjy@gmail.com>
---
  v2: remove account_freepages_both().
  v1: https://lore.kernel.org/all/20260109105121.328780-1-yajun.deng@linux.dev/
---
 mm/page_alloc.c | 38 +++++++++++++++++++++++++-------------
 1 file changed, 25 insertions(+), 13 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ebfa07632995..d56e94eb4914 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 +832,7 @@ 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);
 }
 
 /* Used for pages not on another list */
@@ -861,6 +867,8 @@ static inline void move_to_free_list(struct page *page, struct zone *zone,
 {
 	struct free_area *area = &zone->free_area[order];
 	int nr_pages = 1 << order;
+	bool old_isolated = is_migrate_isolate(old_mt);
+	bool new_isolated = is_migrate_isolate(new_mt);
 
 	/* Free page moving can fail, so it happens before the type update */
 	VM_WARN_ONCE(get_pageblock_migratetype(page) != old_mt,
@@ -869,14 +877,18 @@ 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);
-
-	if (order >= pageblock_order &&
-	    is_migrate_isolate(old_mt) != is_migrate_isolate(new_mt)) {
-		if (!is_migrate_isolate(old_mt))
-			nr_pages = -nr_pages;
-		__mod_zone_page_state(zone, NR_FREE_PAGES_BLOCKS, nr_pages);
+	if (!old_isolated)
+		account_specific_freepages(zone, -nr_pages, old_mt);
+	if (!new_isolated)
+		account_specific_freepages(zone, nr_pages, new_mt);
+
+	/* Only update NR_FREE_PAGES if one of them is isolate type */
+	if (old_isolated != new_isolated) {
+		nr_pages = old_isolated ? nr_pages : -nr_pages;
+		__mod_zone_page_state(zone, NR_FREE_PAGES, nr_pages);
+		if (order >= pageblock_order)
+			__mod_zone_page_state(zone, NR_FREE_PAGES_BLOCKS,
+					      nr_pages);
 	}
 }
 
-- 
2.34.1

Re: [PATCH v2] mm/page_alloc: Avoid duplicate NR_FREE_PAGES updates in move_to_free_list()

[Johannes Weiner]

On Mon, Jan 12, 2026 at 08:16:14PM +0800, Yajun Deng 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 adds a condition that updates the NR_FREE_PAGES only if one of
> the two types is the isolate type. This avoids NR_FREE_PAGES being
> updates twice.
> 
> The optimization avoid duplicate NR_FREE_PAGES updates in
> move_to_free_list().
> 
> Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
> Suggested-by: Joshua Hahn <joshua.hahnjy@gmail.com>

I'm not a fan of this.

The code ends up more complicated, more lines, and fragile because the
accounting decisions are now spread out over multiple places (again).

Is it worth it? move_to_free_list() is used in page isolation, which
has to do the accounting anyway; and migratetype fallbacks, which we
are trying to avoid as much as possible. So this path shouldn't be all
that hot to begin with.

Simplicity & maintainability trumps here, IMO, unless you have hard
data showing this is worth the pain.

Re: [PATCH v2] mm/page_alloc: Avoid duplicate NR_FREE_PAGES updates in move_to_free_list()

[Yajun Deng]

January 12, 2026 at 10:50 PM, "Johannes Weiner" <hannes@cmpxchg.org mailto:hannes@cmpxchg.org?to=%22Johannes%20Weiner%22%20%3Channes%40cmpxchg.org%3E > wrote:


> 
> On Mon, Jan 12, 2026 at 08:16:14PM +0800, Yajun Deng 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 adds a condition that updates the NR_FREE_PAGES only if one of
> >  the two types is the isolate type. This avoids NR_FREE_PAGES being
> >  updates twice.
> >  
> >  The optimization avoid duplicate NR_FREE_PAGES updates in
> >  move_to_free_list().
> >  
> >  Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
> >  Suggested-by: Joshua Hahn <joshua.hahnjy@gmail.com>
> > 
> I'm not a fan of this.
> 
> The code ends up more complicated, more lines, and fragile because the
> accounting decisions are now spread out over multiple places (again).
> 
How about V1? It will introduce account_freepages_both().
https://lore.kernel.org/all/20260109105121.328780-1-yajun.deng@linux.dev/

> Is it worth it? move_to_free_list() is used in page isolation, which
> has to do the accounting anyway; and migratetype fallbacks, which we
> are trying to avoid as much as possible. So this path shouldn't be all
> that hot to begin with.
> 
Not all cases are of the isolation type. There are indeed duplicate caculations.
move_freepages_block() will be called in __isolate_free_page().
Both old_mt and new_mt are mergeable type in this case.

> Simplicity & maintainability trumps here, IMO, unless you have hard
> data showing this is worth the pain.
>
I'm tring to do it, but I haven't yet.

Re: [PATCH v2] mm/page_alloc: Avoid duplicate NR_FREE_PAGES updates in move_to_free_list()

[Joshua Hahn]

On Mon, 12 Jan 2026 20:16:14 +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 adds a condition that updates the NR_FREE_PAGES only if one of
> the two types is the isolate type. This avoids NR_FREE_PAGES being
> updates twice.
> 
> The optimization avoid duplicate NR_FREE_PAGES updates in
> move_to_free_list().
> 
> Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
> Suggested-by: Joshua Hahn <joshua.hahnjy@gmail.com>

Hello Yajun,

LGTM. Thank you for kindly accepting my suggestion!
Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>