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

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Message-ID: <CAHk-=wjzqrD5wrfeaU390bXEEBY2JF-oKmFN4fREzgyXsbQRTQ@mail.gmail.com>
Date:   Fri, 10 Feb 2023 14:08:35 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jens Axboe <axboe@...nel.dk>, Ming Lei <ming.lei@...hat.com>
Cc:     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>
Subject: Re: copy on write for splice() from file to pipe?

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()'.

But that 'ack_page_consuming' thing looks even more disgusting, and
since I'm not sure why it even exists, I don't know what it's doing
wrong.

Let's agree not to make splice() worse, while people are talking about
how bad it already is, ok?

                Linus