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

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Date:   Sat, 11 Feb 2023 14:17:18 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jens Axboe <axboe@...nel.dk>, 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 11:18:38AM +0800, 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.
> 
> > 
> > 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.
> 
> The motivation is for confirming that if the produced buffer can be used
> for READ or WRITE. Another way could be to add PIPE_BUF_FLAG_MAY_READ[WRITE].

BTW, I meant the added flags are source/sink private flags, which are
not used by generic pipe/splice code, just used by the actual source and
sink subsystem.

thanks,
Ming