[RFC] Passing stream to userspace

[RFC] Passing stream to userspace

[Jan Schmidt]

Hi there,

I was promised you (viro, hch) have a decided opinion on this topic. The
original mail ("Getting a lot of fs-generated information to user
space") contains a lot of background and is way too long. Point is: I
want to get a bunch of data generated by the kernel (btrfs) to userland
(really).

Matthew Wilcox suggested to use an ioctl changing f_ops.read. Userland
would be like:

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

To avoid doing buffering myself, I suggested passing a fd to the kernel
where it is expected to dump the generated information, like:

> 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 opinions on those or different suggestions? Thanks,
-Jan

Re: [RFC] Passing stream to userspace

[Anand Jain]

Jan,

  I personally like the way we interact with the kernel using the
  /sys or /proc that's a cool way, mainly because no C code is required.
  A generic interface like that (or btrfs-control with more commands
  supported) will enhance troubleshooting and debugging, application
  interface.
  or did I miss anything ?

Cheers, Anand


On 08/12/2011 11:30 PM, Jan Schmidt wrote:
> Hi there,
>
> I was promised you (viro, hch) have a decided opinion on this topic. The
> original mail ("Getting a lot of fs-generated information to user
> space") contains a lot of background and is way too long. Point is: I
> want to get a bunch of data generated by the kernel (btrfs) to userland
> (really).
>
> Matthew Wilcox suggested to use an ioctl changing f_ops.read. Userland
> would be like:
>
>> int fd = open("/mnt/btrfs");
>> ioctl(fd, BTRFS_IOC_STREAM);
>> while (...) {
>> 	read(fd, buf, 4096);
>> 	...
>> }
>> close(fd);
>
> To avoid doing buffering myself, I suggested passing a fd to the kernel
> where it is expected to dump the generated information, like:
>
>> 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 opinions on those or different suggestions? Thanks,
> -Jan
> --
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