[PATCH 3/6 v3] mm/page_counter: introduce stock drain APIs

[Joshua Hahn <joshua.hahnjy@gmail.com>]

Introduce page_counter variants to replace memcg stock draining
functions.

page_counter_drain_stock_local() drains the stock of the local CPU,
taking a local stock lock to serialize against concurrent charges.

page_counter_drain_stock_cpu() does the same, but without taking a local
lock. This is possible because it will only be called from the CPU
hotplug path, where the CPU is dead and there cannot be any more charges.

Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
---
 include/linux/page_counter.h |  3 +++
 mm/page_counter.c            | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 37 insertions(+)

diff --git a/include/linux/page_counter.h b/include/linux/page_counter.h
index c92bb2ee2a581..4a88874019af0 100644
--- a/include/linux/page_counter.h
+++ b/include/linux/page_counter.h
@@ -111,6 +111,9 @@ static inline void page_counter_reset_watermark(struct page_counter *counter)
 int page_counter_alloc_stock(struct page_counter *counter, unsigned int batch);
 void page_counter_disable_stock(struct page_counter *counter);
 void page_counter_free_stock(struct page_counter *counter);
+void page_counter_drain_stock_local(struct page_counter *counter);
+void page_counter_drain_stock_cpu(struct page_counter *counter,
+				  unsigned int cpu);
 
 #if IS_ENABLED(CONFIG_MEMCG) || IS_ENABLED(CONFIG_CGROUP_DMEM)
 void page_counter_calculate_protection(struct page_counter *root,
diff --git a/mm/page_counter.c b/mm/page_counter.c
index 1a71de4f43fd0..7e7eb683472d9 100644
--- a/mm/page_counter.c
+++ b/mm/page_counter.c
@@ -378,6 +378,40 @@ void page_counter_free_stock(struct page_counter *counter)
 	counter->stock = NULL;
 }
 
+void page_counter_drain_stock_local(struct page_counter *counter)
+{
+	struct page_counter_stock *stock;
+	unsigned long nr_pages;
+
+	if (!counter->stock)
+		return;
+
+	local_lock(&counter->stock->lock);
+	stock = this_cpu_ptr(counter->stock);
+	nr_pages = stock->nr_pages;
+	stock->nr_pages = 0;
+	local_unlock(&counter->stock->lock);
+
+	if (nr_pages)
+		page_counter_uncharge(counter, nr_pages);
+}
+
+void page_counter_drain_stock_cpu(struct page_counter *counter,
+				  unsigned int cpu)
+{
+	struct page_counter_stock *stock;
+	unsigned long nr_pages;
+
+	if (!counter->stock)
+		return;
+
+	stock = per_cpu_ptr(counter->stock, cpu);
+	nr_pages = stock->nr_pages;
+	if (nr_pages) {
+		stock->nr_pages = 0;
+		page_counter_uncharge(counter, nr_pages);
+	}
+}
 
 #if IS_ENABLED(CONFIG_MEMCG) || IS_ENABLED(CONFIG_CGROUP_DMEM)
 /*
-- 
2.53.0-Meta