On 12.08.2011 14:31, Matthew Wilcox wrote:

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

quoted

(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);

After thinking twice, this has a drawback: I'd have to track state
between two read(2) calls, waiting for userspace to "pull" more data.

In contrast, I'd rather use a "push" like approach, having the buffering
done for me. Getting back to my suggestion (1, giving a fd to the kernel
where the output is expected), that should look like this in userspace:

-
int fd;
int pipefd[2];
struct io_args io_args;

fd = open("/mnt/btrfs");
pipe(pipefd);
io_agrs.dest = pipefd[0];

/* thread 1 */
ioctl(fd, BTRFS_IOC_STREAM, &io_args);
/* thread 2 */
while (...) {
	read(pipefd[1], buf, 4096);
	...
}
-

Any suggestions and opinions appreciated. Thanks,
-Jan