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
--
Best regards,
Dmitry
