Re: [PATCH v2] powerpc/pci: unmap legacy INTx interrupts when a PHB is removed

["Cédric Le Goater" <clg@kaod.org>]

On 9/24/20 7:11 AM, Alexey Kardashevskiy wrote:
> 
> 
> On 23/09/2020 17:06, Cédric Le Goater wrote:
>> On 9/23/20 2:33 AM, Qian Cai wrote:
>>> On Fri, 2020-08-07 at 12:18 +0200, Cédric Le Goater wrote:
>>>> When a passthrough IO adapter is removed from a pseries machine using
>>>> hash MMU and the XIVE interrupt mode, the POWER hypervisor expects the
>>>> guest OS to clear all page table entries related to the adapter. If
>>>> some are still present, the RTAS call which isolates the PCI slot
>>>> returns error 9001 "valid outstanding translations" and the removal of
>>>> the IO adapter fails. This is because when the PHBs are scanned, Linux
>>>> maps automatically the INTx interrupts in the Linux interrupt number
>>>> space but these are never removed.
>>>>
>>>> To solve this problem, we introduce a PPC platform specific
>>>> pcibios_remove_bus() routine which clears all interrupt mappings when
>>>> the bus is removed. This also clears the associated page table entries
>>>> of the ESB pages when using XIVE.
>>>>
>>>> For this purpose, we record the logical interrupt numbers of the
>>>> mapped interrupt under the PHB structure and let pcibios_remove_bus()
>>>> do the clean up.
>>>>
>>>> Since some PCI adapters, like GPUs, use the "interrupt-map" property
>>>> to describe interrupt mappings other than the legacy INTx interrupts,
>>>> we can not restrict the size of the mapping array to PCI_NUM_INTX. The
>>>> number of interrupt mappings is computed from the "interrupt-map"
>>>> property and the mapping array is allocated accordingly.
>>>>
>>>> Cc: "Oliver O'Halloran" <oohall@gmail.com>
>>>> Cc: Alexey Kardashevskiy <aik@ozlabs.ru>
>>>> Signed-off-by: Cédric Le Goater <clg@kaod.org>
>>>
>>> Some syscall fuzzing will trigger this on POWER9 NV where the traces pointed to
>>> this patch.
>>>
>>> .config: https://gitlab.com/cailca/linux-mm/-/blob/master/powerpc.config
>>
>> OK. The patch is missing a NULL assignement after kfree() and that
>> might be the issue. 
>>
>> I did try PHB removal under PowerNV, so I would like to understand 
>> how we managed to remove twice the PCI bus and possibly reproduce. 
>> Any chance we could grab what the syscall fuzzer (syzkaller) did ? 
> 
> 
> 
> My guess would be it is doing this in parallel to provoke races.

Concurrency removal and rescan should be controlled by : 

   pci_stop_and_remove_bus_device_locked()
       pci_lock_rescan_remove() 

And, in the report, the stack traces are on the same CPU and PID. 



What I think is happening is that we did a couple of remove/rescan
of the same PHB. The problem is that ->irq_map is not reallocated
the second time because the PHB is re-scanned differently on the 
PowerNV platform. At the second remove, the ->irq_map being not NULL, 
we try to kfree it again and the allocator warns of a double free :/

This works fine on pseries/KVM because the PHB is never removed and 
on pseries/pHyp, pcibios_scan_phb() is called at PHB hotplug. But on 
PowerNV, pcibios_scan_phb() is only called at probe/boot time and 
not at hotplug time :/

I was a good thing to spot that before merge. But I need to revise 
that patch again.

Thanks,

C.