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
next prev parent 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