virtio_balloon: export 'available' memory to balloon statistics

Add a new field, VIRTIO_BALLOON_S_AVAIL, to virtio_balloon memory
statistics protocol, corresponding to 'Available' in /proc/meminfo.

It indicates to the hypervisor how big the balloon can be inflated
without pushing the guest system to swap.

Signed-off-by: Igor Redko <[e-mail]>
Signed-off-by: Denis V. Lunev <[e-mail]>
Reviewed-by: Roman Kagan <[e-mail]>
Cc: Michael S. Tsirkin <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/virtio/virtio_balloon.c

···
 #include <linux/balloon_compaction.h>
 #include <linux/oom.h>
 #include <linux/wait.h>
+#include <linux/mm.h>
 
 /*
  * Balloon device works in 4K page units.  So each page is pointed to by
···
 	unsigned long events[NR_VM_EVENT_ITEMS];
 	struct sysinfo i;
 	int idx = 0;
+	long available;
 
 	all_vm_events(events);
 	si_meminfo(&i);
+
+	available = si_mem_available();
 
 	update_stat(vb, idx++, VIRTIO_BALLOON_S_SWAP_IN,
 				pages_to_bytes(events[PSWPIN]));
···
 				pages_to_bytes(i.freeram));
 	update_stat(vb, idx++, VIRTIO_BALLOON_S_MEMTOT,
 				pages_to_bytes(i.totalram));
+	update_stat(vb, idx++, VIRTIO_BALLOON_S_AVAIL,
+				pages_to_bytes(available));
 }
 
 /*

include/uapi/linux/virtio_balloon.h

···
 #define VIRTIO_BALLOON_S_MINFLT   3   /* Number of minor faults */
 #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_NR       6
+#define VIRTIO_BALLOON_S_AVAIL    6   /* Available memory as in /proc */
+#define VIRTIO_BALLOON_S_NR       7
 
 /*
  * Memory statistics structure.