Re: copy on write for splice() from file to pipe?

On Sat, Feb 11, 2023 at 07:13:44AM -0700, Jens Axboe wrote:

On 2/10/23 8:18?PM, Ming Lei wrote:

quoted

On Fri, Feb 10, 2023 at 02:08:35PM -0800, Linus Torvalds wrote:

quoted

On Fri, Feb 10, 2023 at 1:51 PM Jens Axboe [off-list ref] wrote:

quoted

Speaking of splice/io_uring, Ming posted this today:

https://lore.kernel.org/io-uring/20230210153212.733006-1-ming.lei@redhat.com/ (local)

Ugh. Some of that is really ugly. Both 'ignore_sig' and
'ack_page_consuming' just look wrong. Pure random special cases.

And that 'ignore_sig' is particularly ugly, since the only thing that
sets it also sets SPLICE_F_NONBLOCK.

And the *only* thing that actually then checks that field is
'splice_from_pipe_next()', where there are exactly two
signal_pending() checks that it adds to, and

 (a) the first one is to protect from endless loops

 (b) the second one is irrelevant when  SPLICE_F_NONBLOCK is set

So honestly, just NAK on that series.

I think that instead of 'ignore_sig' (which shouldn't exist), that
first 'signal_pending()' check in splice_from_pipe_next() should just
be changed into a 'fatal_signal_pending()'.

Good point, here the signal is often from task_work_add() called by
io_uring.

Usually you'd use task_sigpending() to distinguis the two, but
fatal_signal_pending() as Linus suggests would also work. The only
concern here is that since you'll be potentially blocking on waiting for
the pipe to be readable - if task does indeed have task_work pending and
that very task_work is the one that will ensure that the pipe is now
readable, then you're waiting condition will never be satisfied.

The 2nd signal_pending() will break the loop to get task_work handled,
so it is safe to only change the 1st one to fatal_signal_pending().


Thanks,
Ming