netdev - Re: [PATCH 2/2] nbd: change a parameter's type to remove a memcpy call

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Date:	Fri, 20 Jul 2007 09:46:38 +0800
From:	"rae l" <crquan@...il.com>
To:	"Paul Clements" <paul.clements@...eleye.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Pavel Machek" <pavel@....cz>,
	"Steven Whitehouse" <steve@...gwyn.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	debra <chdebra@...il.com>
Subject: Re: [PATCH 2/2] nbd: change a parameter's type to remove a memcpy call

On 7/20/07, Paul Clements <paul.clements@...eleye.com> wrote:
> Denis Cheng wrote:
> > this memcpy looks so strange, in fact it's merely a pointer dereference,
> > so I change the parameter's type to refer it more directly,
> > this could make the memcpy not needed anymore.
> >
> > in the function nbd_read_stat where nbd_find_request is only once called,
> > the parameter served should be transformed accordingly.
>
> This is really a matter of preference. The generated code ends up being
> about the same, I think, while your patch makes the call to
> nbd_find_request kind of obtuse. Also, the memcpy's are balanced between
> send_req and find_request, so you can quickly see how the data is being
> transferred (from req into handle, and then back again). Your patch
> makes this less clear, at least to me.
With one explicit memcpy stripped out, I think it's more clear to
nbd_find_request.
In nbd_read_stat, the cast to (struct request **) is not apparent, I
must admit; but I think the best solution is declaring other few
structs to make it clear, it's due to the lack of description of nbd
client and server communication protocol.

BTW, I think the nbd driver needs a clear documentation (its main site
http://nbd.sourceforge.net/ does not give it):

1. When nbd_find_request is needed to call, the 8 byte memory of char
handle[8] field in struct nbd_reply actually stores a pointer (struct
request *), that pointer is received from the network. Since a pointer
is only meaningful to the host, transfering it over the network will
be unreliable, I don't think it's a good design,

-- 
Denis Cheng
Linux Application Developer

"One of my most productive days was throwing away 1000 lines of code."
 - Ken Thompson.
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