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

Linus Torvalds
2023-02-10T17:47:52Z

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