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? -- Best regards, Dmitry
