netdev - Re: [Xen-devel] compound skb frag pages appearing in start

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Date:	Wed, 10 Oct 2012 15:31:16 +0200
From:	Sander Eikelenboom <linux@...elenboom.it>
To:	Ian Campbell <Ian.Campbell@...rix.com>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	"Konrad Rzeszutek Wilk" <konrad@...nel.org>,
	xen-devel <xen-devel@...ts.xen.org>
Subject: Re: [Xen-devel] compound skb frag pages appearing in start_xmit


Wednesday, October 10, 2012, 2:29:09 PM, you wrote:

> On Wed, 2012-10-10 at 13:24 +0100, Sander Eikelenboom wrote:
>> Wednesday, October 10, 2012, 12:13:04 PM, you wrote:
>> 
>> > On Tue, 2012-10-09 at 15:40 +0100, Ian Campbell wrote:
>> >> On Tue, 2012-10-09 at 15:27 +0100, Eric Dumazet wrote:
>> >> > On Tue, 2012-10-09 at 15:17 +0100, Ian Campbell wrote:
>> >> > 
>> >> > > Does the higher order pages effectively reduce the number of frags which
>> >> > > are in use? e.g if MAX_SKB_FRAGS is 16, then for order-0 pages you could
>> >> > > have 64K worth of frag data.
>> >> > > 
>> >> > > If we switch to order-3 pages everywhere then can the skb contain 512K
>> >> > > of data, or does the effective maximum number of frags in an skb reduce
>> >> > > to 2?
>> >> > 
>> >> > effective number of frags reduce to 2 or 3
>> >> > 
>> >> > (We still limit GSO packets to ~63536 bytes)
>> >> 
>> >> Great! Then I think the fix is more/less trivial...
>> 
>> > The following seems to work for me.
>> 
>> But it doesn't seem to work for me ... dmesg attached.

>> [  191.777994] ------------[ cut here ]------------
>> [  191.784245] kernel BUG at drivers/net/xen-netback/netback.c:481!

> Looks like that BUG_ON is a little aggressive. It'll trigger if the data
> happens to end on a frame boundary. Hopefully this will fix it for you:

Yes it does !
Thanks .. will recompile and test the netfront case as well

--
Sander

> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
> index d747e30..f2d6b78 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -477,7 +477,7 @@ static void netbk_gop_frag_copy(struct xenvif *vif, struct sk_buff *skb,
>                 size -= bytes;
>  
>                 /* Next frame */
> -               if (offset == PAGE_SIZE) {
> +               if (offset == PAGE_SIZE && size) {
>                         BUG_ON(!PageCompound(page));
>                         page++;
>                         offset = 0;




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