netdev - Re: [PATCH net v4] xen-netback: Fix grant ref resolution in RX path

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Date:	Thu, 15 May 2014 12:46:50 +0200
From:	Sander Eikelenboom <linux@...elenboom.it>
To:	Ian Campbell <Ian.Campbell@...rix.com>
CC:	Zoltan Kiss <zoltan.kiss@...rix.com>,
	xen-devel@...ts.xenproject.org, <wei.liu2@...rix.com>,
	<paul.durrant@...rix.com>, <netdev@...r.kernel.org>,
	<david.vrabel@...rix.com>, <davem@...emloft.net>
Subject: Re: [PATCH net v4] xen-netback: Fix grant ref resolution in RX path


Thursday, May 15, 2014, 12:37:49 PM, you wrote:

> On Thu, 2014-05-15 at 11:08 +0100, Zoltan Kiss wrote:
>> The original series for reintroducing grant mapping for netback had a patch [1]
>> to handle receiving of packets from an another VIF. Grant copy on the receiving
>> side needs the grant ref of the page to set up the op.
>> The original patch assumed (wrongly) that the frags array haven't changed. In
>> the case reported by Sander, the sending guest sent a packet where the linear
>> buffer and the first frag were under PKT_PROT_LEN (=128) bytes.
>> xenvif_tx_submit() then pulled up the linear area to 128 bytes, and ditched the
>> first frag. The receiving side had an off-by-one problem when gathered the grant
>> refs.
>> This patch fixes that by checking whether the actual frag's page pointer is the
>> same as the page in the original frag list. It can handle any kind of changes on
>> the original frags array, like:
>> - removing granted frags from the array at any point
>> - adding local pages to the frags list anywhere
>> - reordering the frags
>> It's optimized to the most common case, when there is 1:1 relation between the
>> frags and the list, plus works optimal when frags are removed from the end or
>> the beginning.
>> 
>> [1]: 3e2234: xen-netback: Handle foreign mapped pages on the guest RX path
>> 
>> Reported-by: Sander Eikelenboom <linux@...elenboom.it>
>> Signed-off-by: Zoltan Kiss <zoltan.kiss@...rix.com>

> Acked-by: Ian Campbell <ian.campbell@...rix.com>

> (Remind me, do we need both this and Eric's fix, or is it either/or?)

The regression i reported goes away with Zoltan's fix alone (I tested Zoltan's 
fix v3, without Eric's patch, pulled onto 3.15-rc5).

In earlier tests Eric's patch didn't make a difference in my testcase.

So Zoltan's patch fixes the reported regression.
Eric's patch is a additional correctness fix, independent of the reported regression.

--
Sander



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