netdev - Re: [Xen-devel] [3.15-rc3] Bisected: xen-netback mangles packets between two guests on a bridge since merge of "TX grant mapping with SKBTX_DEV

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Message-ID: <5363AFED.5080601@citrix.com>
Date:	Fri, 2 May 2014 15:47:09 +0100
From:	Zoltan Kiss <zoltan.kiss@...rix.com>
To:	Sander Eikelenboom <linux@...elenboom.it>
CC:	Eric Dumazet <eric.dumazet@...il.com>, <netdev@...r.kernel.org>,
	<xen-devel@...ts.xen.org>, Ian Campbell <Ian.Campbell@...rix.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [Xen-devel] [3.15-rc3] Bisected: xen-netback mangles packets
 between two guests on a bridge since merge of "TX grant mapping with SKBTX_DEV_ZEROCOPY
 instead of copy" series.

On 02/05/14 15:06, Sander Eikelenboom wrote:
>
> Friday, May 2, 2014, 4:00:40 PM, you wrote:
>
>>>> Let's see what that does different and if that gives an insight in what is going
>>>> wrong.
>>>
>>> The debug patch that i used is attached.
>>> Here is some of the output, skb->data_len size sometimes varies slightly as does skb->truesize (is that allowed to become > 64k ?).
>> Yes, truesize can be over 64K, as it includes the aligned size of the
>> data itself (64k max), struct skbuff and struct shared_info.
>> However my calculation for the truesize might be wrong: I count all the
>> frags as full sized pages (skb->truesize += i * PAGE_SIZE), but the last
>> page is not necessarily used entirely by that frag.
>> I had the idea that truesize should include the size of the whole page
>> as we reserved it with alloc_page for this skb, but it might not be true.
> Btw. with you "skb->>truesize -= nskb->data_len;" version truesize should
>> end up around 128k, something entirely wrong. Can you try with
> "skb->>truesize -= nskb->data_len;" in that line?
>> My repro env is underway in the meantime ...
>
> Erhmm i don't get this last part, first you entitle
> "skb->truesize -= nskb->data_len;" as being "entirely wrong" (which is probably true)
>
> Then you want me to try just that ? .. or did you mean something else ?

Sorry, I was misleading and wrong. Can you try out this scenario with 
the attached patch?

Thanks,

Zoli


View attachment "truesize.patch" of type "text/x-patch" (1110 bytes)