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

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Date:   Sat, 11 Feb 2023 23:05:35 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Chinner <david@...morbit.com>,
        Matthew Wilcox <willy@...radead.org>,
        Stefan Metzmacher <metze@...ba.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API Mailing List <linux-api@...r.kernel.org>,
        io-uring <io-uring@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Samba Technical <samba-technical@...ts.samba.org>,
        ming.lei@...hat.com
Subject: 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:
> > 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@...nel.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