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

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Message-ID: <1dd85095-c18c-ed3e-38b7-02f4d13d9bd6@kernel.dk>
Date:   Fri, 10 Feb 2023 13:32:02 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>
Cc:     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 2/10/23 1:27 PM, Linus Torvalds wrote:
> On Fri, Feb 10, 2023 at 11:56 AM Andy Lutomirski <luto@...nel.org> wrote:
>>
>> i think this is almost exactly what Jeremy and Stefan are asking for
>> re: notification when the system is done with a zero-copy send:
> 
> Yeah, right now it's all just "incremented page counts", I think.
> 
> Even the pipe code itself doesn't know about writes that have already
> been done, but that are pending elsewhere.
> 
> You'd have to ask the target file descriptor itself about "how much do
> you have pending" or something.

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. Or what am I missing
here? Late to this thread, and there's a ton of stuff go to through.

-- 
Jens Axboe