splice status

[Jens Axboe <axboe@suse.de>]

Hi,

Since a lot of splice/tee stuff has been merged, I thought I'd post a
little status report and other potentially useful info.

- splice interfaces should be stable now, I don't envision any further
  changes to the ->splice_read or ->splice_write file_operations hooks
  or the splice syscall. splice now accepts an input or output offset
  like sendfile(), so it doesn't have to rely on ->f_pos in the file
  structure.

- Ditto for the sys_tee syscall.

- sendfile() will be replaced with splice(). sys_sendfile will remain of
  course, this is only an internal thing. The current do_splice_direct()
  is a sendfile() helper. The splice branch in the block git repo has
  a patch to remove generic_file_sendfile() and all it's users by
  converting them to ->splice_read(). There's also a patch there that
  fixes up loop. The only remaining users of the file_operations
  .sendfile hook are nfds/shmem/ext2-xip/relay. That still needs doing.
  The current plan is to merge this stuff post 2.6.17.

I have a little collection of splice test tools that people may find
useful to play with this stuff. It's in a git repo here:

        git://brick.kernel.dk/data/git/splice.git

and snapshots are generated every hour on changes and can be fetched
from

        http://brick.kernel.dk/snaps/

There are tools there to test both splice and tee, a little README
explains the basic principle of them. I'd appreciate people testing and
playing with these tools, just in case we still have some bugs lurking.

Finally, known bugs:

- Some smallish splice reads are buggy. Patch is in splice branch and
  will hopefully be merged whenever Linus gets in front of his computer.

- The ->splice_pipe cache needs to be initialized to NULL on forks. Only
  affects do_splice_direct() usage, so not a problem in current kernels.
  Patch also Linus bound today.


-- 
Jens Axboe