netdev - Re: [PATCH] net: add destructor for skb data (rewritten)

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Date:	Sat, 19 Apr 2008 02:35:24 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	rusty@...tcorp.com.au
Cc:	netdev@...r.kernel.org, maxk@...lcomm.com,
	herbert@...dor.apana.org.au
Subject: Re: [PATCH] net: add destructor for skb data (rewritten)

From: Rusty Russell <rusty@...tcorp.com.au>
Date: Fri, 18 Apr 2008 14:21:25 +1000

> If we want to notify something when an skb is truly finished (such as
> for tun vringfd support), we need a destructor on the data.
> 
> This turns out to be slightly non-trivial as fragments from one skb
> get copied to another skb: if the first skb has a destructor (or its
> parent does) we need to keep a reference to it and destroy it only
> when (all the) children are destroyed.  We add an 'orig' pointer to
> the skb_shared_info to do this.
> 
> But there's currently no way to get from the shinfo to the head (to
> kfree it), so we add a 'len' field.  A better alternative to this
> might be to move the skb_shared_info to before the head of the skb data.
> 
> Note that the destructor is responsible for calling kfree: for the tun
> device, this is critical since the destructor can be called from any
> context and it has to do a copy_to_user, so it queues the skb.
> 
> Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>

I'm mostly ambivalent but I will say I'm not happy about all of this
extra state you're adding even though it's "only" to the SKB data
shared-info struct and not sk_buff properly.

Does this handle SKB frags of arbitrary depth?  SKB's can be nested to
arbitrary depths via the frag mechanism.
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