fuse: add atomic open+truncate support

This patch allows fuse filesystems to implement open(..., O_TRUNC) as a single
request, instead of separate truncate and open requests.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fuse/dir.c

@@ -1083,6 +1083,9 @@ static int fuse_do_setattr(struct dentry *entry, struct iattr *attr,
 			return err;
 	}
 
+	if ((attr->ia_valid & ATTR_OPEN) && fc->atomic_o_trunc)
+		return 0;
+
 	if (attr->ia_valid & ATTR_SIZE) {
 		unsigned long limit;
 		if (IS_SWAPFILE(inode))

fs/fuse/file.c

@@ -28,7 +28,9 @@ static int fuse_send_open(struct inode *inode, struct file *file, int isdir,
 		return PTR_ERR(req);
 
 	memset(&inarg, 0, sizeof(inarg));
-	inarg.flags = file->f_flags & ~(O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC);
+	inarg.flags = file->f_flags & ~(O_CREAT | O_EXCL | O_NOCTTY);
+	if (!fc->atomic_o_trunc)
+		inarg.flags &= ~O_TRUNC;
 	req->in.h.opcode = isdir ? FUSE_OPENDIR : FUSE_OPEN;
 	req->in.h.nodeid = get_node_id(inode);
 	req->in.numargs = 1;

fs/fuse/fuse_i.h

@@ -320,6 +320,9 @@ struct fuse_conn {
 	/** Do readpages asynchronously?  Only set in INIT */
 	unsigned async_read : 1;
 
+	/** Do not send separate SETATTR request before open(O_TRUNC)  */
+	unsigned atomic_o_trunc : 1;
+
 	/*
 	 * The following bitfields are only for optimization purposes
 	 * and hence races in setting them will not cause malfunction

fs/fuse/inode.c

@@ -531,6 +531,8 @@ static void process_init_reply(struct fuse_conn *fc, struct fuse_req *req)
 				fc->async_read = 1;
 			if (!(arg->flags & FUSE_POSIX_LOCKS))
 				fc->no_lock = 1;
+			if (arg->flags & FUSE_ATOMIC_O_TRUNC)
+				fc->atomic_o_trunc = 1;
 		} else {
 			ra_pages = fc->max_read / PAGE_CACHE_SIZE;
 			fc->no_lock = 1;
@@ -553,7 +555,8 @@ static void fuse_send_init(struct fuse_conn *fc, struct fuse_req *req)
 	arg->major = FUSE_KERNEL_VERSION;
 	arg->minor = FUSE_KERNEL_MINOR_VERSION;
 	arg->max_readahead = fc->bdi.ra_pages * PAGE_CACHE_SIZE;
-	arg->flags |= FUSE_ASYNC_READ | FUSE_POSIX_LOCKS | FUSE_FILE_OPS;
+	arg->flags |= FUSE_ASYNC_READ | FUSE_POSIX_LOCKS | FUSE_FILE_OPS |
+		FUSE_ATOMIC_O_TRUNC;
 	req->in.h.opcode = FUSE_INIT;
 	req->in.numargs = 1;
 	req->in.args[0].size = sizeof(*arg);

include/linux/fuse.h

@@ -101,6 +101,7 @@ struct fuse_file_lock {
 #define FUSE_ASYNC_READ		(1 << 0)
 #define FUSE_POSIX_LOCKS	(1 << 1)
 #define FUSE_FILE_OPS		(1 << 2)
+#define FUSE_ATOMIC_O_TRUNC	(1 << 3)
 
 /**
  * Release flags