Re: [Xen-devel] netback BUG_ON when using copy_skb=1

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.10.13 at 06:13, jerry <jerry.lilijun@xxxxxxxxxx> wrote:
> Hi Wei Liu,
> 
> I am doing some network performance on Xen4.1.2 and kernel 3.0, and get a 
> crash with BUG_ON(netbk->mmap_pages[idx] != page) in netbk_gop_frag() 
> accidentally.
> 
> By analyzing the module drivers/xen/netback,

You aren't looking at the upstream driver, are you? If so, Wei is
very likely the wrong addressee.

Assuming that you instead talk of the SLE11 kernel, I can only
point out that a problem in that code was found and fixed a
couple of months ago (resulting in the BUG_ON() you quoted not
being there anymore), so you're simply not looking at up-to-date
code.

Jan

> I think the reason is as 
> follows when sending packets from VM1 to VM2:
> 1) The two netback thread(the first for VM1 sending, second for VM2 
> receiving) run concurrently.
> 2) In first netback thread, it will do delayed copy from a foreign granted 
> page to local memory when some outstanding packets have been pending too 
> long( above half of one HZ).
>    Then netbk->mmap_pages[idx] will be replaced with new allocated page.
> 3) If the packets are forwarded to VM2 by virtual switch, netbk_gop_frag() 
> will be called in second netback thread.
>    And that function will judge whether the pages in skb frags[] is foreign 
> in order to make sure how to do grant copy.
> 4) If the page replacing was done after the page foreign judge in 
> netbk_gop_frag(), the BUG will be invoked because the page from skb frags[] 
> are different with mmap_pages[idx].
> 
> I tried to using spin_lock to protect the page accessing, but no appropriate 
> solutions was found.
> How to fix this problem?  Would you like to share some opinions?
> 
> In addition, I have tried to turn off copy_skb. Then the vif netdevice may 
> not be released after shutting down VM,
> that's because outstanding packets hold the reference count of the device 
> too long for some unknown reason.
> The reason may be that the NIC does not release packets after DMA.
> Does anyone have met such problems? Thanks.
> 
> Best regards,
> Jerry
> 
> 
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> Xen-devel@xxxxxxxxxxxxx 
> http://lists.xen.org/xen-devel 




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