On Fri, Aug 12, 2011 at 02:14:15PM +0200, Jan Schmidt wrote:

(1) Normally, requests to the file system go through ioctls (on the fd
of the mountpoint) and the result is small enough to be returned when
the ioctl finishes. That said, I thought of passing a user land fd along
with this ioctl to the kernel and make it dump the generated bits there.
Only, I don't see how to turn a fd into a struct file pointer. And I
don't know if that would be considered really ugly by a lot of people.

struct file *filp = fget(fd);
...
fput(filp);

That said, why not have the ioctl mutate the existing fd?
ie in userspace:

int fd = open("/mnt/btrfs");
ioctl(fd, BTRFS_IOC_STREAM);
while (...) {
	read(fd, buf, 4096);
	...
}
close(fd);

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."