fuse: support clients that don't implement 'open'

open/release operations require userspace transitions to keep track
of the open count and to perform any FS-specific setup.  However,
for some purely read-only FSs which don't need to perform any setup
at open/release time, we can avoid the performance overhead of
calling into userspace for open/release calls.

This patch adds the necessary support to the fuse kernel modules to prevent
open/release operations from hitting in userspace. When the client returns
ENOSYS, we avoid sending the subsequent release to userspace, and also
remember this so that future opens also don't trigger a userspace
operation.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/fuse/file.c

@@ -127,7 +127,15 @@ static void fuse_file_put(struct fuse_file *ff, bool sync)
 	if (atomic_dec_and_test(&ff->count)) {
 		struct fuse_req *req = ff->reserved_req;
 
-		if (sync) {
+		if (ff->fc->no_open) {
+			/*
+			 * Drop the release request when client does not
+			 * implement 'open'
+			 */
+			req->background = 0;
+			path_put(&req->misc.release.path);
+			fuse_put_request(ff->fc, req);
+		} else if (sync) {
 			req->background = 0;
 			fuse_request_send(ff->fc, req);
 			path_put(&req->misc.release.path);
@@ -144,27 +152,36 @@ static void fuse_file_put(struct fuse_file *ff, bool sync)
 int fuse_do_open(struct fuse_conn *fc, u64 nodeid, struct file *file,
 		 bool isdir)
 {
-	struct fuse_open_out outarg;
 	struct fuse_file *ff;
-	int err;
 	int opcode = isdir ? FUSE_OPENDIR : FUSE_OPEN;
 
 	ff = fuse_file_alloc(fc);
 	if (!ff)
 		return -ENOMEM;
 
+	ff->fh = 0;
+	ff->open_flags = FOPEN_KEEP_CACHE; /* Default for no-open */
+	if (!fc->no_open || isdir) {
+		struct fuse_open_out outarg;
+		int err;
+
 		err = fuse_send_open(fc, nodeid, file, opcode, &outarg);
-	if (err) {
+		if (!err) {
+			ff->fh = outarg.fh;
+			ff->open_flags = outarg.open_flags;
+
+		} else if (err != -ENOSYS || isdir) {
 			fuse_file_free(ff);
 			return err;
+		} else {
+			fc->no_open = 1;
+		}
 	}
 
 	if (isdir)
-		outarg.open_flags &= ~FOPEN_DIRECT_IO;
+		ff->open_flags &= ~FOPEN_DIRECT_IO;
 
-	ff->fh = outarg.fh;
 	ff->nodeid = nodeid;
-	ff->open_flags = outarg.open_flags;
 	file->private_data = fuse_file_get(ff);
 
 	return 0;

fs/fuse/fuse_i.h

@@ -485,6 +485,9 @@ struct fuse_conn {
 	 * and hence races in setting them will not cause malfunction
 	 */
 
+	/** Is open/release not implemented by fs? */
+	unsigned no_open:1;
+
 	/** Is fsync not implemented by fs? */
 	unsigned no_fsync:1;