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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 10 Feb 2023 09:47:28 -0800
Message-ID: <CAHk-=wgXvRKwsHUjA9T9Tw6n5x1pCO6B+4kk0GAx+oQ5qhUyRw@mail.gmail.com>
Subject: Re: copy on write for splice() from file to pipe?
To:     Dave Chinner <david@fromorbit.com>
Cc:     Stefan Metzmacher <metze@samba.org>, Jens Axboe <axboe@kernel.dk>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux API Mailing List <linux-api@vger.kernel.org>,
        io-uring <io-uring@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Samba Technical <samba-technical@lists.samba.org>
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On Fri, Feb 10, 2023 at 9:23 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> And when it comes to networking, in general things like TCP checksums
> etc should be ok even with data that isn't stable.  When doing things
> by hand, networking should always use the "copy-and-checksum"
> functions that do the checksum while copying (so even if the source
> data changes, the checksum is going to be the checksum for the data
> that was copied).
>
> And in many (most?) smarter network cards, the card itself does the
> checksum, again on the data as it is transferred from memory.
>
> So it's not like "networking needs a stable source" is some really
> _fundamental_ requirement for things like that to work.
>
> But it may well be that we have situations where some network driver
> does the checksumming separately from then copying the data.

Ok, so I decided to try to take a look.

Somebody who actually does networking (and drivers in particular)
should probably check this, but it *looks* like the IPv4 TCP case
(just to pick the ony I looked at) gores through
tcp_sendpage_locked(), which does

        if (!(sk->sk_route_caps & NETIF_F_SG))
                return sock_no_sendpage_locked(sk, page, offset, size, flags);

which basically says "if you can't handle fragmented socket buffers,
do that 'no_sendpage' case".

So that will basically end up just falling back to a kernel
'sendmsg()', which does a copy and then it's stable.

But for the networks that *can* handle fragmented socket buffers, it
then calls do_tcp_sendpages() instead, which just creates a skb
fragment of the page (with tcp_build_frag()).

I wonder if that case should just require NETIF_F_HW_CSUM?

              Linus