netdev - Re: [PATCH RFC] solos-pci: Fix BUG() with shared skb

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Message-Id: <20130904.175147.516807555661755710.davem@davemloft.net>
Date:	Wed, 04 Sep 2013 17:51:47 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	dwmw2@...radead.org
Cc:	netdev@...r.kernel.org, simon@...e.lp0.eu,
	linux-atm-general@...ts.sourceforge.net, nathan@...verse.com.au
Subject: Re: [PATCH RFC] solos-pci: Fix BUG() with shared skb

From: David Woodhouse <dwmw2@...radead.org>
Date: Wed, 04 Sep 2013 21:41:15 +0100

> On Wed, 2013-09-04 at 14:30 -0400, David Miller wrote:
>> skb_realloc_headroom() should do everything you need.
> 
> Great, thanks! Something like this then... ? 
> 
> Do I really need the truesize check? And if so, is there a better way to
> handle the ATM accounting? It just *happens* to be the case that the the
> br2684_pop() and pppoatm_pop() functions don't mind being bypassed in
> this fashion, and we should probably get away with tweaking the core ATM
> accounting directly like this. Doesn't make me happy though...
 ...
> +		nskb = skb_realloc_headroom(skb, sizeof(*header));
> +		if (!nskb) {
> +			solos_pop(vcc, skb);
> +			return -ENOMEM;
> +		}
> +		if (skb->truesize != nskb->truesize)
> +			atm_force_charge(vcc, nskb->truesize - skb->truesize);

My understanding is that truesize will not be changed by calls to
skb_realloc_headroom(), because if it did then every ethernet driver
would be screwed as the skb->truesize adjustment would corrupt socket
memory accounting.

The only thing you need to be mindful is that after the
skb_realloc_headroom() call all skb data pointers change, and
therefore things like packet pointers are now stale and need to be
recalculated.

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