netdev - Re: Socket buffer sizes with autotuning

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Message-ID: <d1c2719f0805071716r1aa5ca71tc0da19296e0248e@mail.gmail.com>
Date:	Wed, 7 May 2008 17:16:08 -0700
From:	"Jerry Chu" <hkchu@...gle.com>
To:	"David Miller" <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: Socket buffer sizes with autotuning

On Wed, May 7, 2008 at 2:20 PM, David Miller <davem@...emloft.net> wrote:
> From: "Jerry Chu" <hkchu@...gle.com>
>  Date: Wed, 7 May 2008 11:54:01 -0700
>
>
>  > This is one of many things about skb that I still don't completely understand.
>  > Why in tcp_transmit_skb() we'll have to pskb_copy() if skb_cloned()?
>
>  The other clone holder owns the packet header area.  All packets on
>  the retransmit queue of TCP are headerless.  The call sites down
>  into the device add the headers.
>
>  Therefore we cannot have two paths modifying the headers at the same
>  time.

Oh I see. I think I confused pskb_copy() with skb_copy() and kept thinking
why do we need to copy the whole thing...

>
>
>  > Can't we clone a skb mulitple times? Is it due to some special optimization
>  > from skb->fclone stuff... that imposes this restriction?
>
>  No, it has nothing to do with fclone.  It has to do with what instance
>  owns the packet header area in front of the user's TCP data.

Thanks for explaining.

Jerry

>
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