Re: [Xen-devel] [PATCH net RFC] xen-netback: Fix grant ref resolution in RX path

[Sander Eikelenboom <linux@xxxxxxxxxxxxxx>]

Tuesday, May 13, 2014, 6:13:55 PM, you wrote:

> On Tue, 2014-05-13 at 15:31 +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 beginning or the end
>> - adding local pages to the frags list
>> To keep it optimized to the most common cases, it doesn't handle when the 
>> order
>> of the original frags changed. That would require ubuf to be reseted to the
>> beginning of the chain (skb_shinfo(skb)->destructor_arg), and reiterating
>> through the list every time.
>> 
>> OPEN QUESTIONS:
>> - Is it a safe assumption that nothing changes the order of the original 
>> frags?
>>   Removing them from the array or injecting new pages anywhere is not a 
>> problem.
>> - I used UINT_MAX as a kind of INVALID_GRANT_REF, however there is no such 
>> thing
>>   in the grant mapping API. Should we codify this or is it better if we just
>>   find another way to distinguish whether a frag is local or not?
>> - Should this fix go to David's net tree or directly to the mainline tree? Or
>>   both?
>> 
>> [1]: 3e2234: xen-netback: Handle foreign mapped pages on the guest RX path
>> 
>> Reported-by: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
>> Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
>> ---

Will test the patch tomorrow and see how this works out for me.

> The 'cleanup' of stale ubufs should be right after __pskb_pull_tail().

> This is the function that can 'consume frags' after all.

> Its not clear that you catch all cases, like skbs being purged in case
> of device dismantle.
Hmm that's quite important since a partial fix only makes the potential 
remaining cases even harder to spot and debug i'm afraid.

> I am not saying your patch is wrong, only that it adds yet an obscure
> thing with no comments. In two years, nobody will understand this.

--
Sander




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