Re: [Xen-devel] [PATCH v2 1/1] xen/netback: correctly calculate required slots of skb.

[annie li <annie.li@xxxxxxxxxx>]

On 2013-7-11 10:18, David Miller wrote:
> From: Annie Li <annie.li@xxxxxxxxxx>
> Date: Wed, 10 Jul 2013 17:15:11 +0800
>
> > When counting required slots for skb, netback directly uses DIV_ROUND_UP to get
> > slots required by header data. This is wrong when offset in the page of header
> > data is not zero, and is also inconsistent with following calculation for
> > required slot in netbk_gop_skb.
> >
> > In netbk_gop_skb, required slots are calculated based on offset and len in page
> > of header data. It is possible that required slots here is larger than the one
> > calculated in earlier netbk_count_requests. This inconsistency directly results
> > in rx_req_cons_peek and xen_netbk_rx_ring_full judgement are wrong.
> >
> > Then it comes to situation the ring is actually full, but netback thinks it is
> > not and continues to create responses. This results in response overlaps request
> > in the ring, then grantcopy gets wrong grant reference and throws out error,
> > for example "(XEN) grant_table.c:1763:d0 Bad grant reference 2949120", the
> > grant reference is invalid value here. Netback returns XEN_NETIF_RSP_ERROR(-1)
> > to netfront when grant copy status is error, then netfront gets rx->status
> > (the status is -1, not really data size now), and throws out error,
> > "kernel: net eth1: rx->offset: 0, size: 4294967295". This issue can be 
> > reproduced
> > by doing gzip/gunzip in nfs share with mtu = 9000, the guest would panic after
> > running such test for a while.
> >
> > This patch is based on 3.10-rc7.
> >
> > Signed-off-by: Annie Li <annie.li@xxxxxxxxxx>
> This patch looks good to me, but I'd like to see some reviews from other
> experts in this area.
>
> In the future I'd really like to see this code either use PAGE_SIZE
> everywhere or MAX_BUFFER_OFFSET everywhere, in the buffer chopping
> code.
>
> I think using both leads to confusion and makes this code harder to
> read.
True, I had the confusion too.

>   I prefer MAX_BUFFER_OFFSET because it gives the indication that
> what this value represents is the modulus upon which we must chop up
> RX buffers in this driver.
Would PAGE_SIZE be more straight? MAX_BUFFER_OFFSET gives an idea of 
offset instead of length.
Anyway, making it consistent is a good idea.

Thanks
Annie
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