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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-05-13 at 09:13 -0700, Eric Dumazet 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>
> > ---
> 
> 
> The 'cleanup' of stale ubufs should be right after __pskb_pull_tail().
> 
> This is the function that can 'consume frags' after all.

That would be OK for the call to __pskb_pull_tail in netback itself --
but what about any other calls from other bits of the network stack
which don't know about this driver-specific data structure?

> Its not clear that you catch all cases, like skbs being purged in case
> of device dismantle.

Doesn't that go through the normal skb destroy path, as opposed to
manipulating an existing skb?

> 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.

Agreed.

Ian.


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