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

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Message-ID: <CAHk-=wjUjtLjLbdTz=AzvGekyU1xiSL-wAAb7_j_XoT9t4o1vQ@mail.gmail.com>
Date:   Fri, 10 Feb 2023 14:17:55 -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 2:08 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
>  (a) the first one is to protect from endless loops

Just to clarify: they're not "endless loops" per se, but we have
splice sources and destinations that always succeed, like /dev/zero
and /dev/null.

So things like "sendfile()" that are happy to just repeat until done
do need to have some kind of signal handling even for the case when
we're not actually waiting for data. That's what that whole

        /*
         * Check for signal early to make process killable when there are
         * always buffers available
         */

this is all about. See commit c725bfce7968 ("vfs: Make sendfile(2)
killable even better") for a less obvious example than that
"zero->null" kind of thing.

(I actually suspect that /dev/zero no longer works as a splice source,
since we disabled the whole "fall back to regular IO" that Christoph
did in 36e2c7421f02 "fs: don't allow splice read/write without
explicit ops").

            Linus