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

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Message-ID: <c395bf68-108e-1674-1a1c-4cb26178d87c@kernel.dk>
Date:   Fri, 10 Feb 2023 15:51:49 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     David Laight <David.Laight@...LAB.COM>,
        'Linus Torvalds' <torvalds@...ux-foundation.org>,
        Dave Chinner <david@...morbit.com>
Cc:     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 2/10/23 3:41?PM, David Laight wrote:
> From: Linus Torvalds
>> Sent: 10 February 2023 17:24
> ...
>> 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.
> 
> It is also worth remembering that TCP needs to be able
> to retransmit the data and a much later time.
> So the application must not change the data until it has
> been acked by the remote system.

This has been covered, and:

> I don't think io_uring has any way to indicate anything
> other than 'the data has been accepted by the socket'.

This is wrong and has also been covered.

-- 
Jens Axboe