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
