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

Ming Lei
2023-02-11T15:06:41Z

On Sat, Feb 11, 2023 at 07:13:44AM -0700, Jens Axboe wrote:
> On 2/10/23 8:18?PM, Ming Lei wrote:
> > On Fri, Feb 10, 2023 at 02:08:35PM -0800, Linus Torvalds wrote:
> >> On Fri, Feb 10, 2023 at 1:51 PM Jens Axboe <axboe@kernel.dk> wrote:
> >>>
> >>> Speaking of splice/io_uring, Ming posted this today:
> >>>
> >>> https://lore.kernel.org/io-uring/20230210153212.733006-1-ming.lei@redhat.com/
> >>
> >> 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