Re: [PATCH] splice support #2

[Bodo Eggert <harvested.in.lkml@7eggert.dyndns.org>]

Pavel Machek <pavel@suse.cz> wrote:

>> We should extend the userspace API so that it is prepared to receive
>> 'excess data' via a separate 'flush excess data to' file descriptor:
>> 
>> sys_splice(fd_in, fd_out, fd_flush, size,
>>                    max_flush_size, *bytes_flushed)
> 
> I still see problems with error handling here. You get -EIO to
> userspace. How do you know if it is error reading fd_in, or error
> writing fd_out?

ACK.

Possibly you'll get an error reading from fd_in while also getting an error
flushing the data to fd_out, and an additional error while trying to rescue
the data on fd_flush.

Additionally you'll want to know the amount of transfered bytes even in case
of an error.

struct splice_fds {
        int in_fd,  int in_errno,
        int out_fd, int out_errno,

        /* used on transfer error to rescue the to-be-flushed data: */
        int err_fd, int err_errno,
        /* err_fd == -1 -> don't use */
        int err_size
        /* in: max. size, out: used size, unchanged if no error */
}
int sys_splice(struct splice_fds *fds, size, flags)
/* returns transfered_bytes even in case of an error */

Possible flags:
- Streaming, copied data won't be used too soon.
- Unconditionally use err_fd as a buffer, e.g. because we know that seeking
  from in_fd to out_fd is slow while err_fd is mapped memory.
- fail if err_fd == -1 and an error could cause data loss (out_fd must have
  a buffer like a pipe)

Dangers:
splice(in_fd = /dev/zero, out_fd = /dev/null, ...)
...

</brainstorming>
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