[RFC PATCH v2 1/3] mm: add page_counter_margin()

Add a helper to report the minimum remaining chargeable space from a
page counter up to the root of its hierarchy.

This makes the hierarchical margin calculation reusable by callers that
need to know whether a smaller charge could still fit after a larger
charge failed.

Suggested-by: Johannes Weiner <[email protected]>
Signed-off-by: Xueyuan Chen <[email protected]>
---
 include/linux/page_counter.h |  1 +
 mm/page_counter.c            | 20 ++++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/include/linux/page_counter.h b/include/linux/page_counter.h
index d649b6bbbc87..07b7cb12249c 100644
--- a/include/linux/page_counter.h
+++ b/include/linux/page_counter.h
@@ -68,6 +68,7 @@ static inline unsigned long page_counter_read(struct page_counter *counter)
 	return atomic_long_read(&counter->usage);
 }
 
+long page_counter_margin(struct page_counter *counter);
 void page_counter_cancel(struct page_counter *counter, unsigned long nr_pages);
 void page_counter_charge(struct page_counter *counter, unsigned long nr_pages);
 bool page_counter_try_charge(struct page_counter *counter,
diff --git a/mm/page_counter.c b/mm/page_counter.c
index 661e0f2a5127..6e55c7628025 100644
--- a/mm/page_counter.c
+++ b/mm/page_counter.c
@@ -171,6 +171,26 @@ bool page_counter_try_charge(struct page_counter *counter,
 	return false;
 }
 
+/**
+ * page_counter_margin - remaining chargeable pages within hierarchical limits
+ * @counter: counter
+ *
+ * Returns the smallest remaining margin between @counter and the root.
+ */
+long page_counter_margin(struct page_counter *counter)
+{
+	long margin = PAGE_COUNTER_MAX;
+
+	for (; counter; counter = counter->parent) {
+		long limit = READ_ONCE(counter->max);
+		long usage = page_counter_read(counter);
+
+		margin = min(margin, limit - usage);
+	}
+
+	return margin;
+}
+
 /**
  * page_counter_uncharge - hierarchically uncharge pages
  * @counter: counter
-- 
2.47.3