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

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Date:   Fri, 10 Feb 2023 12:36:58 -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 12:32 PM Jens Axboe <axboe@...nel.dk> wrote:
>
> No, we very much do have that for io_uring zerocopy sends, which was in
> the bit below you snipped from the reply. It'll tell you when data has
> been sent out, and when the data has been acked.

Hmm. splice() itself definitely doesn't have that data - there's no
"io context" for it.

There is only the pipe buffers, and they are released when the data
has been accepted - which is not the same as used (eg the networking
layer just takes another ref to the page and says "I'm done").

Maybe adding some io context to the pipe buffer would be possible, but
it's certainly not obvious how, without changing splice() semantics
completely.

             Linus