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Peter Anvin" , Wanpeng Li , Alexander Duyck , "K. Y. Srinivasan" , Thomas Gleixner , Kees Cook , devel@driverdev.osuosl.org, Stefano Stabellini , Stephen Hemminger , "Aneesh Kumar K.V" , Joerg Roedel , X86 ML , YueHaibing , "Matthew Wilcox \(Oracle\)" , Mike Rapoport , Peter Zijlstra , Ingo Molnar , Vlastimil Babka , Anthony Yznaga , Oscar Salvador , "Isaac J. Manjarres" , Matt Sickler , Juergen Gross , Anshuman Khandual , Haiyang Zhang , Sasha Levin , kvm-ppc@vger.kernel.org, Qian Cai , Alex Williamson , Mike Rapoport , Borislav Petkov , Nicholas Piggin , Andy Lutomirski , xen-devel , Boris Ostrovsky , Vitaly Kuznetsov , Allison Randal , Jim Mattson , Mel Gorman , Adam Borowski , Cornelia Huck , Pavel Tatashin , Linux Kernel Mailing List , Sean Christopherson , Johannes Weiner , Paolo Bonzini , Andrew Morton , linuxppc-dev Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Tue, Nov 5, 2019 at 12:31 PM David Hildenbrand wrote: > > >>> I think I know what's going wrong: > >>> > >>> Pages that are pinned via gfn_to_pfn() and friends take a references, > >>> however are often released via > >>> kvm_release_pfn_clean()/kvm_release_pfn_dirty()/kvm_release_page_clean()... > >>> > >>> > >>> E.g., in arch/x86/kvm/x86.c:reexecute_instruction() > >>> > >>> ... > >>> pfn = gfn_to_pfn(vcpu->kvm, gpa_to_gfn(gpa)); > >>> ... > >>> kvm_release_pfn_clean(pfn); > >>> > >>> > >>> > >>> void kvm_release_pfn_clean(kvm_pfn_t pfn) > >>> { > >>> if (!is_error_noslot_pfn(pfn) && !kvm_is_reserved_pfn(pfn)) > >>> put_page(pfn_to_page(pfn)); > >>> } > >>> > >>> This function makes perfect sense as the counterpart for kvm_get_pfn(): > >>> > >>> void kvm_get_pfn(kvm_pfn_t pfn) > >>> { > >>> if (!kvm_is_reserved_pfn(pfn)) > >>> get_page(pfn_to_page(pfn)); > >>> } > >>> > >>> > >>> As all ZONE_DEVICE pages are currently reserved, pages pinned via > >>> gfn_to_pfn() and friends will often not see a put_page() AFAIKS. > > > > Assuming gup() takes a reference for ZONE_DEVICE pages, yes, this is a > > KVM bug. > > Yes, it does take a reference AFAIKs. E.g., > > mm/gup.c:gup_pte_range(): > ... > if (pte_devmap(pte)) { > if (unlikely(flags & FOLL_LONGTERM)) > goto pte_unmap; > > pgmap = get_dev_pagemap(pte_pfn(pte), pgmap); > if (unlikely(!pgmap)) { > undo_dev_pagemap(nr, nr_start, pages); > goto pte_unmap; > } > } else if (pte_special(pte)) > goto pte_unmap; > > VM_BUG_ON(!pfn_valid(pte_pfn(pte))); > page = pte_page(pte); > > head = try_get_compound_head(page, 1); > > try_get_compound_head() will increment the reference count. > > > > >>> Now, my patch does not change that, the result of > >>> kvm_is_reserved_pfn(pfn) will be unchanged. A proper fix for that would > >>> probably be > >>> > >>> a) To drop the reference to ZONE_DEVICE pages in gfn_to_pfn() and > >>> friends, after you successfully pinned the pages. (not sure if that's > >>> the right thing to do but you're the expert) > >>> > >>> b) To not use kvm_release_pfn_clean() and friends on pages that were > >>> definitely pinned. > > > > This is already KVM's intent, i.e. the purpose of the PageReserved() check > > is simply to avoid putting a non-existent reference. The problem is that > > KVM assumes pages with PG_reserved set are never pinned, which AFAICT was > > true when the code was first added. > > > >> (talking to myself, sorry) > >> > >> Thinking again, dropping this patch from this series could effectively also > >> fix that issue. E.g., kvm_release_pfn_clean() and friends would always do a > >> put_page() if "pfn_valid() and !PageReserved()", so after patch 9 also on > >> ZONDE_DEVICE pages. > > > > Yeah, this appears to be the correct fix. > > > >> But it would have side effects that might not be desired. E.g.,: > >> > >> 1. kvm_pfn_to_page() would also return ZONE_DEVICE pages (might even be the > >> right thing to do). > > > > This should be ok, at least on x86. There are only three users of > > kvm_pfn_to_page(). Two of those are on allocations that are controlled by > > KVM and are guaranteed to be vanilla MAP_ANONYMOUS. The third is on guest > > memory when running a nested guest, and in that case supporting ZONE_DEVICE > > memory is desirable, i.e. KVM should play nice with a guest that is backed > > by ZONE_DEVICE memory. > > > >> 2. kvm_set_pfn_dirty() would also set ZONE_DEVICE pages dirty (might be > >> okay) > > > > This is ok from a KVM perspective. > > What about > > void kvm_get_pfn(kvm_pfn_t pfn) > { > if (!kvm_is_reserved_pfn(pfn)) > get_page(pfn_to_page(pfn)); > } > > Is a pure get_page() sufficient in case of ZONE_DEVICE? > (asking because of the live references obtained via > get_dev_pagemap(pte_pfn(pte), pgmap) in mm/gup.c:gup_pte_range() > somewhat confuse me :) ) It is not sufficient. PTE_DEVMAP is there to tell the gup path "be careful, this pfn has device-lifetime, make sure the device is pinned and not actively in the process of dying before pinning any pages associated with this device". However, if kvm_get_pfn() is honoring kvm_is_reserved_pfn() that returns true for ZONE_DEVICE, I'm missing how it is messing up the reference counts. > > The scarier code (for me) is transparent_hugepage_adjust() and > > kvm_mmu_zap_collapsible_spte(), as I don't at all understand the > > interaction between THP and _PAGE_DEVMAP. > > The x86 KVM MMU code is one of the ugliest code I know (sorry, but it > had to be said :/ ). Luckily, this should be independent of the > PG_reserved thingy AFAIKs. Both transparent_hugepage_adjust() and kvm_mmu_zap_collapsible_spte() are honoring kvm_is_reserved_pfn(), so again I'm missing where the page count gets mismanaged and leads to the reported hang.