[PATCH v5 1/2] xenbus/backend: Add memory pressure handler callback

[SeongJae Park <sj38.park@gmail.com>]

Granting pages consumes backend system memory.  In systems configured
with insufficient spare memory for those pages, it can cause a memory
pressure situation.  However, finding the optimal amount of the spare
memory is challenging for large systems having dynamic resource
utilization patterns.  Also, such a static configuration might lack a
flexibility.

To mitigate such problems, this commit adds a memory reclaim callback to
'xenbus_driver'.  Using this facility, 'xenbus' would be able to monitor
a memory pressure and request specific devices of specific backend
drivers which causing the given pressure to voluntarily release its
memory.

That said, this commit simply requests every callback registered driver
to release its memory for every domain, rather than issueing the
requests to the drivers and the domain in charge.  Such things will be
done in a futur.  Also, this commit focuses on memory only.  However, it
would be ablt to be extended for general resources.

Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 drivers/xen/xenbus/xenbus_probe_backend.c | 31 +++++++++++++++++++++++
 include/xen/xenbus.h                      |  1 +
 2 files changed, 32 insertions(+)

diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
index b0bed4faf44c..5a5ba29e39df 100644
--- a/drivers/xen/xenbus/xenbus_probe_backend.c
+++ b/drivers/xen/xenbus/xenbus_probe_backend.c
@@ -248,6 +248,34 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
 	return NOTIFY_DONE;
 }
 
+static int xenbus_backend_reclaim(struct device *dev, void *data)
+{
+	struct xenbus_driver *drv;
+	if (!dev->driver)
+		return -ENOENT;
+	drv = to_xenbus_driver(dev->driver);
+	if (drv && drv->reclaim)
+		drv->reclaim(to_xenbus_device(dev));
+	return 0;
+}
+
+/*
+ * Returns 0 always because we are using shrinker to only detect memory
+ * pressure.
+ */
+static unsigned long xenbus_backend_shrink_count(struct shrinker *shrinker,
+				struct shrink_control *sc)
+{
+	bus_for_each_dev(&xenbus_backend.bus, NULL, NULL,
+			xenbus_backend_reclaim);
+	return 0;
+}
+
+static struct shrinker xenbus_backend_shrinker = {
+	.count_objects = xenbus_backend_shrink_count,
+	.seeks = DEFAULT_SEEKS,
+};
+
 static int __init xenbus_probe_backend_init(void)
 {
 	static struct notifier_block xenstore_notifier = {
@@ -264,6 +292,9 @@ static int __init xenbus_probe_backend_init(void)
 
 	register_xenstore_notifier(&xenstore_notifier);
 
+	if (register_shrinker(&xenbus_backend_shrinker))
+		pr_warn("shrinker registration failed\n");
+
 	return 0;
 }
 subsys_initcall(xenbus_probe_backend_init);
diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
index 869c816d5f8c..cdb075e4182f 100644
--- a/include/xen/xenbus.h
+++ b/include/xen/xenbus.h
@@ -104,6 +104,7 @@ struct xenbus_driver {
 	struct device_driver driver;
 	int (*read_otherend_details)(struct xenbus_device *dev);
 	int (*is_ready)(struct xenbus_device *dev);
+	unsigned (*reclaim)(struct xenbus_device *dev);
 };
 
 static inline struct xenbus_driver *to_xenbus_driver(struct device_driver *drv)
-- 
2.17.1