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

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.
> - 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>
> ---

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.

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.

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