fuse: allow batching of FORGET requests

Terje Malmedal reports that a fuse filesystem with 32 million inodes
on a machine with lots of memory can take up to 30 minutes to process
FORGET requests when all those inodes are evicted from the icache.

To solve this, create a BATCH_FORGET request that allows up to about
8000 FORGET requests to be sent in a single message.

This request is only sent if userspace supports interface version 7.16
or later, otherwise fall back to sending individual FORGET messages.

Reported-by: Terje Malmedal <terje.malmedal@usit.uio.no>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

include/linux/fuse.h

@@ -41,6 +41,9 @@
  * 7.15
  *  - add store notify
  *  - add retrieve notify
+ *
+ * 7.16
+ *  - add BATCH_FORGET request
  */
 
 #ifndef _LINUX_FUSE_H
@@ -72,7 +75,7 @@
 #define FUSE_KERNEL_VERSION 7
 
 /** Minor version number of this interface */
-#define FUSE_KERNEL_MINOR_VERSION 15
+#define FUSE_KERNEL_MINOR_VERSION 16
 
 /** The node ID of the root inode */
 #define FUSE_ROOT_ID 1
@@ -256,6 +259,7 @@ enum fuse_opcode {
 	FUSE_IOCTL         = 39,
 	FUSE_POLL          = 40,
 	FUSE_NOTIFY_REPLY  = 41,
+	FUSE_BATCH_FORGET  = 42,
 
 	/* CUSE specific operations */
 	CUSE_INIT          = 4096,
@@ -290,6 +294,16 @@ struct fuse_forget_in {
 	__u64	nlookup;
 };
 
+struct fuse_forget_one {
+	__u64	nodeid;
+	__u64	nlookup;
+};
+
+struct fuse_batch_forget_in {
+	__u32	count;
+	__u32	dummy;
+};
+
 struct fuse_getattr_in {
 	__u32	getattr_flags;
 	__u32	dummy;