[PATCH v3 1/1] Create debugfs file with virtio balloon usage information

[Alexander Atanasov]

Allow the guest to know how much it is ballooned by the host.
It is useful when debugging out of memory conditions.

When host gets back memory from the guest it is accounted
as used memory in the guest but the guest have no way to know
how much it is actually ballooned.

Signed-off-by: Alexander Atanasov <alexander.atanasov@xxxxxxxxxxxxx>
---
 drivers/virtio/virtio_balloon.c | 78 +++++++++++++++++++++++++++++++++
 1 file changed, 78 insertions(+)

V2:
 - fixed coding style
 - removed pretty print
V3:
 - removed dublicate of features
 - comment about balooned_pages more clear
 - convert host pages to balloon pages

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index b9737da6c4dd..65964afd4f13 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -10,6 +10,7 @@
 #include <linux/virtio_balloon.h>
 #include <linux/swap.h>
 #include <linux/workqueue.h>
+#include <linux/debugfs.h>
 #include <linux/delay.h>
 #include <linux/slab.h>
 #include <linux/module.h>
@@ -731,6 +732,78 @@ static void report_free_page_func(struct work_struct *work)
 	}
 }
 
+/*
+ * DEBUGFS Interface
+ */
+#ifdef CONFIG_DEBUG_FS
+
+#define guest_to_balloon_pages(i) ((i)*VIRTIO_BALLOON_PAGES_PER_PAGE)
+
+/**
+ * virtio_balloon_debug_show - shows statistics of balloon operations.
+ * @f: pointer to the &struct seq_file.
+ * @offset: ignored.
+ *
+ * Provides the statistics that can be accessed in virtio-balloon in the debugfs.
+ *
+ * Return: zero on success or an error code.
+ */
+
+static int virtio_balloon_debug_show(struct seq_file *f, void *offset)
+{
+	struct virtio_balloon *b = f->private;
+	u32 num_pages;
+	struct sysinfo i;
+
+	si_meminfo(&i);
+
+	seq_printf(f, "%-22s: %d\n", "page_size", 4096);
+
+	virtio_cread_le(b->vdev, struct virtio_balloon_config, actual,
+			&num_pages);
+	/*
+	 * Pages allocated by host from the guest memory.
+	 * Host inflates the balloon to get more memory.
+	 * Guest needs to deflate the balloon to get more memory.
+	 */
+	seq_printf(f, "%-22s: %u\n", "ballooned_pages", num_pages);
+
+	/* Total Memory for the guest from host */
+	seq_printf(f, "%-22s: %lu\n", "total_pages",
+			guest_to_balloon_pages(i.totalram));
+
+	/* Current memory for the guest */
+	seq_printf(f, "%-22s: %lu\n", "current_pages",
+			guest_to_balloon_pages(i.totalram) - num_pages);
+
+	return 0;
+}
+
+DEFINE_SHOW_ATTRIBUTE(virtio_balloon_debug);
+
+static void  virtio_balloon_debugfs_init(struct virtio_balloon *b)
+{
+	debugfs_create_file("virtio-balloon", 0444, NULL, b,
+			    &virtio_balloon_debug_fops);
+}
+
+static void  virtio_balloon_debugfs_exit(struct virtio_balloon *b)
+{
+	debugfs_remove(debugfs_lookup("virtio-balloon", NULL));
+}
+
+#else
+
+static inline void virtio_balloon_debugfs_init(struct virtio_balloon *b)
+{
+}
+
+static inline void virtio_balloon_debugfs_exit(struct virtio_balloon *b)
+{
+}
+
+#endif	/* CONFIG_DEBUG_FS */
+
 #ifdef CONFIG_BALLOON_COMPACTION
 /*
  * virtballoon_migratepage - perform the balloon page migration on behalf of
@@ -1019,6 +1092,9 @@ static int virtballoon_probe(struct virtio_device *vdev)
 
 	if (towards_target(vb))
 		virtballoon_changed(vdev);
+
+	virtio_balloon_debugfs_init(vb);
+
 	return 0;
 
 out_unregister_oom:
@@ -1065,6 +1141,8 @@ static void virtballoon_remove(struct virtio_device *vdev)
 {
 	struct virtio_balloon *vb = vdev->priv;
 
+	virtio_balloon_debugfs_exit(vb);
+
 	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING))
 		page_reporting_unregister(&vb->pr_dev_info);
 	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
-- 
2.25.1