netdev - Re: [RFC v2] mac80211: assign needed

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Date:	Tue, 06 May 2008 01:23:55 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	David Miller <davem@...emloft.net>
Cc:	tomasw@...il.com, linville@...driver.com, netdev@...r.kernel.org,
	linux-wireless@...r.kernel.org
Subject: Re: [RFC v2] mac80211: assign needed_headroom/tailroom for netdevs

On Mon, 2008-05-05 at 16:14 -0700, David Miller wrote:

> > Right, that makes sense. But will it ever return false? skb_cloned()
> > returns true often enough, and we only accept linear skbs, but I think I
> > don't understand yet what skb_header_cloned() vs. skb_cloned() refers
> > to. Which exactly is the header space I'm allowed to modify when
> > skb_header_cloned() returns false?
> 
> skb_header_cloned() will return false always if the buffer is not
> cloned.

Right, it's less than skb_cloned().

> If it is cloned, it makes sure the data reference count allows
> for modification of the buffer.

Which buffer? skb->data..skb->tail? And isn't that, in mac80211's case,
at least currently all the buffer anyway?

> For normal traffic, you should see skb_header_cloned() always return
> false unless a network tap like tcpdump is registered.

Interesting point. I'll have to see when socket filters are run,
wpa_supplicant could have a tap open I think. Maybe that's why I'm
seeing so many cloned packets. I think I'll stacktrace skb_clone and
print that out.

johannes

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