Subject: Re: several packets in a single buffer in Rx
From: Ben Hutchings <bhutchings@solarflare.com>
To: Tomas Winkler <tomasw@gmail.com>
Cc: Emmanuel Grumbach <egrumbach@gmail.com>,
	=?UTF-8?Q?Micha=C5=82_Miros=C5=82aw?= <mirqus@gmail.com>,
	netdev@vger.kernel.org, linux-wireless@vger.kernel.org,
	Johannes Berg <johannes@sipsolutions.net>,
	"Guy, Wey-Yi" <wey-yi.w.guy@intel.com>, guy.cohen@intel.com
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Date: Mon, 16 May 2011 21:15:12 +0100
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On Mon, 2011-05-16 at 23:04 +0300, Tomas Winkler wrote:
> 2011/5/16 Emmanuel Grumbach <egrumbach@gmail.com>:
> > 2011/5/16 Michał Mirosław <mirqus@gmail.com>:
> >> W dniu 16 maja 2011 14:59 użytkownik Emmanuel Grumbach
> >> <egrumbach@gmail.com> napisał:
> >>> 2011/5/16 Michał Mirosław <mirqus@gmail.com>:
> >>>> 2011/5/16 Emmanuel Grumbach <egrumbach@gmail.com>:
> >>>>> I would like to be able to deliver the same page several times to the
> >>>>> stack without having the stack consume it before the last time I
> >>>>> deliver it.
> >>>>> Of course I would like to avoid cloning it.
> >>>>
> >>>> Just do get_page() on the page having another packet in it before
> >>>> passing skb up.
> >>>>
> >>>
> >>> I can see the path:
> >>> __kfree_skb -> skb_release_all -> skb_release_data -> put_page
> >>> put_page will free the page iff the _count variable reaches 0. Of course,
> >>> _count is incremented by get_page.
> >>>
> >>> I will give it try.
> >>>
> >>> I understand that this will work regardless the order given to
> >>> alloc_pages right ?
> >>
> >> Yes. Remember that if you put a lot of packets in a big-order page
> >> then the memory will be freed only after all packets are freed.
> >
> > Sure. Thanks for the help.
> 
> How it is ensured that skb manipulation won't corrupt another packet
> on the same page?

Fragments attached to an skb are always treated as read-only.  The
headers will be copied into the skb's header buffer and may be modified
there.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.