add support for FUSE_NOTIFY_PRUNE

Add fuse_lowlevel_notify_prune() helper, which sends FUSE_NOTIFY_PRUNE
notification to kernel indicating that the server side's inode caches
with resources e.g. file handle could be cleaned up if the corresponding
dentry/inode caches (with dangling references) at the kernel side could
be pruned out.

This is a best-effort operation as inodes with active references are
skipped.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>

doc/README.notifications

@@ -27,6 +27,7 @@ notifications:
 | RETRIEVE    | fuse_lowlevel_notify_retrieve    |
 | DELETE      | fuse_lowlevel_notify_delete      |
 | RESEND      | -                                |
+| PRUNE       | fuse_lowlevel_notify_prune       |
 |-------------+----------------------------------|
 
 One important restriction is that these asynchronous operations SHALL NOT be

include/fuse_kernel.h

@@ -239,6 +239,7 @@
  *  7.45
  *  - add FUSE_COPY_FILE_RANGE_64
  *  - add struct fuse_copy_file_range_out
+ *  - add FUSE_NOTIFY_PRUNE
  */
 
 #ifndef _LINUX_FUSE_H
@@ -680,7 +681,7 @@ enum fuse_notify_code {
 	FUSE_NOTIFY_DELETE = 6,
 	FUSE_NOTIFY_RESEND = 7,
 	FUSE_NOTIFY_INC_EPOCH = 8,
-	FUSE_NOTIFY_CODE_MAX,
+	FUSE_NOTIFY_PRUNE = 9,
 };
 
 /* The read buffer is required to be at least 8k, but may be much larger */
@@ -1119,6 +1120,12 @@ struct fuse_notify_retrieve_in {
 	uint64_t	dummy4;
 };
 
+struct fuse_notify_prune_out {
+	uint32_t	count;
+	uint32_t	padding;
+	uint64_t	spare;
+};
+
 struct fuse_backing_map {
 	int32_t		fd;
 	uint32_t	flags;

include/fuse_lowlevel.h

@@ -1954,6 +1954,26 @@ int fuse_lowlevel_notify_store(struct fuse_session *se, fuse_ino_t ino,
 int fuse_lowlevel_notify_retrieve(struct fuse_session *se, fuse_ino_t ino,
 				  size_t size, off_t offset, void *cookie);
 
+/**
+ * Notify to prune kernel's own dentry/inode caches
+ *
+ * Some fuse servers need to prune their caches, which can only be done if the
+ * kernel's own dentry/inode caches are pruned first to avoid dangling
+ * references.  Once dangling dentry/inode cache gets pruned, the inode gets
+ * evicted and thus FUSE_FORGET will be sent to fuse server.  On receiving
+ * FUSE_FORGET, fuse server can free their own inode cache with resources, e.g.
+ * corresponding file handle.
+ *
+ * The notification takes an array of node IDs to try and get rid of.  It is
+ * best-effort as inodes with active references are skipped.
+ *
+ * @param se the session object
+ * @param nodeids the array of node IDs to be pruned
+ * @param count the length of the nodeids array
+ * @return zero for success, -errno for failure
+ */
+int fuse_lowlevel_notify_prune(struct fuse_session *se,
+			       fuse_ino_t *nodeids, uint32_t count);
 
 /* ----------------------------------------------------------- *
  * Utility functions					       *

lib/fuse_lowlevel.c

@@ -3384,6 +3384,30 @@ int fuse_lowlevel_notify_retrieve(struct fuse_session *se, fuse_ino_t ino,
 	return err;
 }
 
+int fuse_lowlevel_notify_prune(struct fuse_session *se,
+			       fuse_ino_t *nodeids, uint32_t count)
+{
+	struct fuse_notify_prune_out outarg;
+	struct iovec iov[3];
+
+	if (!se)
+		return -EINVAL;
+
+	if (se->conn.proto_minor < 45)
+		return -ENOSYS;
+
+	outarg.count = count;
+	outarg.padding = 0;
+	outarg.spare = 0;
+
+	iov[1].iov_base = &outarg;
+	iov[1].iov_len = sizeof(outarg);
+	iov[2].iov_base = (void *)nodeids;
+	iov[2].iov_len = sizeof(fuse_ino_t) * count;
+
+	return send_notify_iov(se, FUSE_NOTIFY_PRUNE, iov, 3);
+}
+
 void *fuse_req_userdata(fuse_req_t req)
 {
 	return req->se->userdata;

lib/fuse_versionscript

@@ -226,6 +226,7 @@ FUSE_3.19 {
 	global:
 		fuse_session_start_teardown_watchdog;
 		fuse_session_stop_teardown_watchdog;
+		fuse_lowlevel_notify_prune;
 } FUSE_3.18;
 
 # Local Variables: