public inbox for [email protected]
 help / color / mirror / Atom feed
From: Linus Torvalds <[email protected]>
To: Dave Chinner <[email protected]>
Cc: Stefan Metzmacher <[email protected]>, Jens Axboe <[email protected]>,
	linux-fsdevel <[email protected]>,
	Linux API Mailing List <[email protected]>,
	io-uring <[email protected]>,
	"[email protected]" <[email protected]>,
	Al Viro <[email protected]>,
	Samba Technical <[email protected]>
Subject: Re: copy on write for splice() from file to pipe?
Date: Fri, 10 Feb 2023 09:47:28 -0800	[thread overview]
Message-ID: <CAHk-=wgXvRKwsHUjA9T9Tw6n5x1pCO6B+4kk0GAx+oQ5qhUyRw@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wj6jd0JWtxO0JvjYUgKfnGEj4BzPVOfY+4_=-0iiGh0tw@mail.gmail.com>

On Fri, Feb 10, 2023 at 9:23 AM Linus Torvalds
<[email protected]> 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

  reply	other threads:[~2023-02-10 17:47 UTC|newest]

Thread overview: 64+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-02-09 13:55 copy on write for splice() from file to pipe? Stefan Metzmacher
2023-02-09 14:11 ` Matthew Wilcox
2023-02-09 14:29   ` Stefan Metzmacher
2023-02-09 16:41 ` Linus Torvalds
2023-02-09 19:17   ` Stefan Metzmacher
2023-02-09 19:36     ` Linus Torvalds
2023-02-09 19:48       ` Linus Torvalds
2023-02-09 20:33         ` Jeremy Allison
2023-02-10 20:45         ` Stefan Metzmacher
2023-02-10 20:51           ` Linus Torvalds
2023-02-10  2:16   ` Dave Chinner
2023-02-10  4:06     ` Dave Chinner
2023-02-10  4:44       ` Matthew Wilcox
2023-02-10  6:57         ` Dave Chinner
2023-02-10 15:14           ` Andy Lutomirski
2023-02-10 16:33             ` Linus Torvalds
2023-02-10 17:57               ` Andy Lutomirski
2023-02-10 18:19                 ` Jeremy Allison
2023-02-10 19:29                   ` Stefan Metzmacher
2023-02-10 18:37                 ` Linus Torvalds
2023-02-10 19:01                   ` Andy Lutomirski
2023-02-10 19:18                     ` Linus Torvalds
2023-02-10 19:27                       ` Jeremy Allison
2023-02-10 19:42                         ` Stefan Metzmacher
2023-02-10 19:42                         ` Linus Torvalds
2023-02-10 19:54                           ` Stefan Metzmacher
2023-02-10 19:29                       ` Linus Torvalds
2023-02-13  9:07                         ` Herbert Xu
2023-02-10 19:55                       ` Andy Lutomirski
2023-02-10 20:27                         ` Linus Torvalds
2023-02-10 20:32                           ` Jens Axboe
2023-02-10 20:36                             ` Linus Torvalds
2023-02-10 20:39                               ` Jens Axboe
2023-02-10 20:44                                 ` Linus Torvalds
2023-02-10 20:50                                   ` Jens Axboe
2023-02-10 21:14                                     ` Andy Lutomirski
2023-02-10 21:27                                       ` Jens Axboe
2023-02-10 21:51                                         ` Jens Axboe
2023-02-10 22:08                                           ` Linus Torvalds
2023-02-10 22:16                                             ` Jens Axboe
2023-02-10 22:17                                             ` Linus Torvalds
2023-02-10 22:25                                               ` Jens Axboe
2023-02-10 22:35                                                 ` Linus Torvalds
2023-02-10 22:51                                                   ` Jens Axboe
2023-02-11  3:18                                             ` Ming Lei
2023-02-11  6:17                                               ` Ming Lei
2023-02-11 14:13                                               ` Jens Axboe
2023-02-11 15:05                                                 ` Ming Lei
2023-02-11 15:33                                                   ` Jens Axboe
2023-02-11 18:57                                                     ` Linus Torvalds
2023-02-12  2:46                                                       ` Jens Axboe
2023-02-10  4:47       ` Linus Torvalds
2023-02-10  6:19         ` Dave Chinner
2023-02-10 17:23           ` Linus Torvalds
2023-02-10 17:47             ` Linus Torvalds [this message]
2023-02-13  9:28               ` Herbert Xu
2023-02-10 22:41             ` David Laight
2023-02-10 22:51               ` Jens Axboe
2023-02-13  9:30               ` Herbert Xu
2023-02-13  9:25           ` Herbert Xu
2023-02-13 18:01             ` Andy Lutomirski
2023-02-14  1:22               ` Herbert Xu
2023-02-17 23:13                 ` Andy Lutomirski
2023-02-20  4:54                   ` Herbert Xu

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='CAHk-=wgXvRKwsHUjA9T9Tw6n5x1pCO6B+4kk0GAx+oQ5qhUyRw@mail.gmail.com' \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox