RE: [PANIC, hyperv] BUG: unable to handle kernel paging request at ffff880077800004 (hv_ringbuffer_write)

[Dexuan Cui]

> -----Original Message-----
> From: Sitsofe Wheeler
> Sent: Wednesday, August 20, 2014 17:27 PM
> 
> While booting a Hyper-V 3.17.0-rc1 guest on a 2012 R2 host a BUG was
> triggered while registering hyperv_fb which in turn caused a panic.
> Various kernel debugging options (CONFIG_DEBUG_PAGEALLOC,
> CONFIG_SLUB_DEBUG=y...) were on at the time. This only seems to happen
> if the guest is being booted with only one CPU allocated to it.
 
Hi Sitsofe,
I can reproduce the exact issue with the same commit + your kconfig + UP
guest (SMP guest seems ok.)

> [    7.645526] hv_vmbus: registering driver hyperv_fb
> [    7.657553] BUG: unable to handle kernel paging request at
> ffff880077800004
> [    7.658224] IP: [<ffffffff8159a7ac>] hv_ringbuffer_write+0x7c/0x150
> [    7.658224] PGD 2da9067 PUD 2dac067 PMD 7fa27067 PTE
> 8000000077800060
> [    7.658224] Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
It seems 
hv_ringbuffer_write() -> 
    hv_get_ringbuffer_availbytes():
        reading rbi->ring_buffer->read_index causes a page fault.

It looks rbi->ring_buffer was unmapped somehow according to the
semantics of CONFIG_DEBUG_PAGEALLOC??? Or, was there a memory
corruption somewhere?

It looks the panic will disappear if the guest isn't configured with a 
"Network Adapter ".

Thanks,
-- Dexuan
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