virtio_balloon: include disk/file caches memory statistics

Add a new field VIRTIO_BALLOON_S_CACHES to virtio_balloon memory
statistics protocol. The value represents all disk/file caches.

In this case it corresponds to the sum of values
Buffers+Cached+SwapCached from /proc/meminfo.
Signed-off-by: Tomáš Golembiovský <tgolembi@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_balloon.c

@@ -257,11 +257,13 @@ static unsigned int update_balloon_stats(struct virtio_balloon *vb)
 	struct sysinfo i;
 	unsigned int idx = 0;
 	long available;
+	unsigned long caches;
 
 	all_vm_events(events);
 	si_meminfo(&i);
 
 	available = si_mem_available();
+	caches = global_node_page_state(NR_FILE_PAGES);
 
 #ifdef CONFIG_VM_EVENT_COUNTERS
 	update_stat(vb, idx++, VIRTIO_BALLOON_S_SWAP_IN,
@@ -277,6 +279,8 @@ static unsigned int update_balloon_stats(struct virtio_balloon *vb)
 				pages_to_bytes(i.totalram));
 	update_stat(vb, idx++, VIRTIO_BALLOON_S_AVAIL,
 				pages_to_bytes(available));
+	update_stat(vb, idx++, VIRTIO_BALLOON_S_CACHES,
+				pages_to_bytes(caches));
 
 	return idx;
 }

include/uapi/linux/virtio_balloon.h

@@ -52,7 +52,8 @@ struct virtio_balloon_config {
 #define VIRTIO_BALLOON_S_MEMFREE  4   /* Total amount of free memory */
 #define VIRTIO_BALLOON_S_MEMTOT   5   /* Total amount of memory */
 #define VIRTIO_BALLOON_S_AVAIL    6   /* Available memory as in /proc */
-#define VIRTIO_BALLOON_S_NR       7
+#define VIRTIO_BALLOON_S_CACHES   7   /* Disk caches */
+#define VIRTIO_BALLOON_S_NR       8
 
 /*
  * Memory statistics structure.