linux-kernel - [PATCH 3/4] mm/page_alloc: Batch page freeing in decay_pcp

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Message-ID: <20250919195223.1560636-4-joshua.hahnjy@gmail.com>
Date: Fri, 19 Sep 2025 12:52:21 -0700
From: Joshua Hahn <joshua.hahnjy@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Johannes Weiner <hannes@...xchg.org>,
	Chris Mason <clm@...com>,
	Kiryl Shutsemau <kirill@...temov.name>,
	Brendan Jackman <jackmanb@...gle.com>,
	Michal Hocko <mhocko@...e.com>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Zi Yan <ziy@...dia.com>,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	kernel-team@...a.com
Subject: [PATCH 3/4] mm/page_alloc: Batch page freeing in decay_pcp_high

It is possible for pcp->count - pcp->high to be greatly over pcp->batch.
When this happens, we should perform batching to ensure that
free_pcppages_bulk isn't called with too many pages to free at once, and
starve out other threads that need the pcp lock.

Since we are still only freeing the difference between the initial
pcp->count and pcp->high values, there should be no change to how many
pages are freed.

Suggested-by: Chris Mason <clm@...com>
Suggested-by: Andrew Morton <akpm@...ux-foundation.org>
Co-developed-by: Johannes Weiner <hannes@...xchg.org>
Signed-off-by: Joshua Hahn <joshua.hahnjy@...il.com>
---
 mm/page_alloc.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b861b647f184..467e524a99df 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2563,7 +2563,7 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
  */
 bool decay_pcp_high(struct zone *zone, struct per_cpu_pages *pcp)
 {
-	int high_min, to_drain, batch;
+	int high_min, to_drain, to_drain_batched, batch;
 	bool todo;
 
 	high_min = READ_ONCE(pcp->high_min);
@@ -2581,11 +2581,14 @@ bool decay_pcp_high(struct zone *zone, struct per_cpu_pages *pcp)
 	}
 
 	to_drain = pcp->count - pcp->high;
-	if (to_drain > 0) {
+	while (to_drain > 0) {
+		to_drain_batched = min(to_drain, batch);
 		spin_lock(&pcp->lock);
-		free_pcppages_bulk(zone, to_drain, pcp, 0);
+		free_pcppages_bulk(zone, to_drain_batched, pcp, 0);
 		spin_unlock(&pcp->lock);
 		todo = true;
+
+		to_drain -= to_drain_batched;
 	}
 
 	return todo;
-- 
2.47.3