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

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Date:   Fri, 10 Feb 2023 13:50:13 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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 2/10/23 1:44 PM, Linus Torvalds wrote:
> 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.

Maybe slightly, as I was not really intending to comment on the strict
splice thing. But yeah I agree on splice, it would not be trivial to do
there. At least with io_uring we have the communication channel we need.
And tracking page refcounts seems iffy and fraught with potential
issues.

-- 
Jens Axboe