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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CALCETrVx4cj7KrhaevtFN19rf=A6kauFTr7UPzQVage0MsBLrg@mail.gmail.com>
Date:   Fri, 10 Feb 2023 13:14:46 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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 12:50 PM Jens Axboe <axboe@...nel.dk> wrote:
>
> 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.
>

Hmm.

Are there any real-world use cases for zero-copy splice() that
actually depend on splicing from a file to a pipe and then later from
the pipe to a socket (or file or whatever)?  Or would everything
important be covered by a potential new io_uring operation that copies
from one fd directly to another fd?

Maybe I'm getting far ahead of myself.