Re: [PATCH v1] drm/ttm: Refcount allocated tail pages

[Dmitry Osipenko]

On 8/15/22 18:54, Dmitry Osipenko wrote:
> On 8/15/22 17:57, Dmitry Osipenko wrote:
>> On 8/15/22 16:53, Christian König wrote:
>>> Am 15.08.22 um 15:45 schrieb Dmitry Osipenko:
>>>> [SNIP]
>>>>> Well that comment sounds like KVM is doing the right thing, so I'm
>>>>> wondering what exactly is going on here.
>>>> KVM actually doesn't hold the page reference, it takes the temporal
>>>> reference during page fault and then drops the reference once page is
>>>> mapped, IIUC. Is it still illegal for TTM? Or there is a possibility for
>>>> a race condition here?
>>>>
>>>
>>> Well the question is why does KVM grab the page reference in the first
>>> place?
>>>
>>> If that is to prevent the mapping from changing then yes that's illegal
>>> and won't work. It can always happen that you grab the address, solve
>>> the fault and then immediately fault again because the address you just
>>> grabbed is invalidated.
>>>
>>> If it's for some other reason than we should probably investigate if we
>>> shouldn't stop doing this.
>>
>> CC: +Paolo Bonzini who introduced this code
>>
>> commit add6a0cd1c5ba51b201e1361b05a5df817083618
>> Author: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> Date:   Tue Jun 7 17:51:18 2016 +0200
>>
>>     KVM: MMU: try to fix up page faults before giving up
>>
>>     The vGPU folks would like to trap the first access to a BAR by setting
>>     vm_ops on the VMAs produced by mmap-ing a VFIO device.  The fault
>> handler
>>     then can use remap_pfn_range to place some non-reserved pages in the
>> VMA.
>>
>>     This kind of VM_PFNMAP mapping is not handled by KVM, but follow_pfn
>>     and fixup_user_fault together help supporting it.  The patch also
>> supports
>>     VM_MIXEDMAP vmas where the pfns are not reserved and thus subject to
>>     reference counting.
>>
>> @Paolo,
>> https://lore.kernel.org/dri-devel/73e5ed8d-0d25-7d44-8fa2-e1d61b1f5a04@xxxxxxx/T/#m7647ce5f8c4749599d2c6bc15a2b45f8d8cf8154
>>
> 
> If we need to bump the refcount only for VM_MIXEDMAP and not for
> VM_PFNMAP, then perhaps we could add a flag for that to the kvm_main
> code that will denote to kvm_release_page_clean whether it needs to put
> the page?

The other variant that kind of works is to mark TTM pages reserved using
SetPageReserved/ClearPageReserved, telling KVM not to mess with the page
struct. But the potential consequences of doing this are unclear to me.

Christian, do you think we can do it?

-- 
Best regards,
Dmitry