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Re: Instability issues with NetBSD-9, xen-4.11 and the xbdb backend driver

	hello Manuel.  So I'm still trying to get my head around how this code
actually works. However, I think there are multiple paths to
xbdback_send_reply() in error handling cases, which might be causing part
of the problem.

	These errors, are, of course, a symptom of the fact that I'm getting
data corruption on the requests coming from the domu into the dom0.  What
ever is corrupting the data, it seems to only happen when files on the domu
are very large.  After looking at  reems of output, I see a lot of messages

Nov 13 18:02:06 xen-hardconnect /netbsd: [ 2971.7600968] xbdback_evthandler domain 5: cont 0xdeadbeef

I don't think I should ever see a continuation with that address, ever!
Interestingly, I start seeing those messages long before anything
apparently goes wrong, suggesting the corruption begins long before
anything notices, which makes sense.
In looking at the code, however, I really don't understand how the
memory associated with the xbd_io portions is managed.  It looks like we
set the pointer to null in many cases without first freeing that memory.
How is it that this doesn't leak memory wildly?  
I'm sure I'm just not understanding the code yet, so any pointers or notes
would be helpful.

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