netdev - Re: [PATCH] xen/netfront: improve truesize tracking

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Message-ID: <1355847758.14620.265.camel@zakaz.uk.xensource.com>
Date:	Tue, 18 Dec 2012 16:22:38 +0000
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Eric Dumazet <erdnetdev@...il.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Sander Eikelenboom <linux@...elenboom.it>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	annie li <annie.li@...cle.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>
Subject: Re: [PATCH] xen/netfront: improve truesize tracking

On Tue, 2012-12-18 at 16:13 +0000, Eric Dumazet wrote:
> On Tue, 2012-12-18 at 15:26 +0000, Ian Campbell wrote:
> 
> > So actually we want += PAGE_SIZE * skb_shinfo(skb)->nr_frags ?
> > 
> 
> I dont know what are the real frag sizes in your case.

I think it's a page, see xennet_alloc_rx_buffers and the alloc_page
therein.

> Some drivers allocate a full page for an ethernet frame, others use half
> of a page, it really depends.
> 
> As the frag ABI doesnt contain real size, its ok in this case to account
> the actual frag size.
> 
> (skb->data_len in your driver)

I guess I'm a bit confused by what truesize means again then ;-),
because in that case the original patch is correct although it would
have been less confusing to do:
	skb->truesize += skb->data_len; 
in xennet_poll() and then do the subtraction of
NETFRONT_SKB_CB(skb)->pull_to in handle_incoming_queue() where we
actually do the pull up.

Unless __pskb_pull_tail does that adjustment for us, but if it does I
can't see where.

Ian.

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