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

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Date:   Fri, 10 Feb 2023 12:44:56 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jens Axboe <axboe@...nel.dk>
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 12:39 PM Jens Axboe <axboe@...nel.dk> wrote:
>
> Right, I'm referencing doing zerocopy data sends with io_uring, using
> IORING_OP_SEND_ZC. This isn't from a file, it's from a memory location,
> but the important bit here is the split notifications and how you
> could wire up a OP_SENDFILE similarly to what Andy described.

Sure, I think it's much more reasonable with io_uring than with splice itself.

So I was mainly just reacting to the "strict-splice" thing where Andy
was talking about tracking the page refcounts. I don't think anything
like that can be done at a splice() level, but higher levels that
actually know about the whole IO might be able to do something like
that.

Maybe we're just talking past each other.

             Linus