netdev - Re: Adding Support for SG,GSO,GRO

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Message-ID: <20101210162328.GA3922337@jupiter.n2.diac24.net>
Date:	Fri, 10 Dec 2010 17:23:28 +0100
From:	David Lamparter <equinox@...c24.net>
To:	Michał Mirosław <mirqus@...il.com>
Cc:	David Lamparter <equinox@...c24.net>,
	David Miller <davem@...emloft.net>, bhutchings@...arflare.com,
	srk@...com, netdev@...r.kernel.org, Jens Axboe <axboe@...nel.dk>
Subject: Re: Adding Support for SG,GSO,GRO

On Fri, Dec 10, 2010 at 05:01:33PM +0100, Michał Mirosław wrote:
> W dniu 10 grudnia 2010 15:31 użytkownik David Lamparter
> <equinox@...c24.net> napisał:
> > On Fri, Dec 10, 2010 at 03:18:11PM +0100, Michał Mirosław wrote:
> >> I'm trying to understand the dependency because it looks artificial for me.
> > sendfile() is defined so that it works asynchronously, that means if you
> > change the data while it is in the queue, you get unpredictable results.
> 
> The question is do we really want good checksum for bogus data?

The data isn't neccessarily bogus. It will be in some state inbetween
old and new. What that means is up to the application.

>                                          Bad checksum in this case is
> actually a good thing as it clearly shows that something is broken in
> the sender and avoids accepting the data as valid at the receiving
> end.

No, because nothing is broken. sendfile() is working as advertised. The
specification of sendfile() is that it sends out data, and it that it
grabs that data at some more or less random point in time. The data is
valid. It is some data that corresponds to what the application wanted
written. It might be a "future" version of the data, but it will not be
random. Unpredictable, yes, in that you won't know where it will choose
old and where new data. But not random or broken.

-David

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