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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id n10sm954257ejk.86.2021.09.16.02.24.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 16 Sep 2021 02:24:07 -0700 (PDT) To: "Kirill A. Shutemov" , Chao Peng Cc: "Kirill A. Shutemov" , Andy Lutomirski , Sean Christopherson , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Borislav Petkov , Andrew Morton , Joerg Roedel , Andi Kleen , David Rientjes , Vlastimil Babka , Tom Lendacky , Thomas Gleixner , Peter Zijlstra , Ingo Molnar , Varad Gautam , Dario Faggioli , x86@kernel.org, linux-mm@kvack.org, linux-coco@lists.linux.dev, Kuppuswamy Sathyanarayanan , David Hildenbrand , Dave Hansen , Yu Zhang References: <20210824005248.200037-1-seanjc@google.com> <20210902184711.7v65p5lwhpr2pvk7@box.shutemov.name> <20210903191414.g7tfzsbzc7tpkx37@box.shutemov.name> <02806f62-8820-d5f9-779c-15c0e9cd0e85@kernel.org> <20210910171811.xl3lms6xoj3kx223@box.shutemov.name> <20210915195857.GA52522@chaop.bj.intel.com> <20210915141147.s4mgtcfv3ber5fnt@black.fi.intel.com> From: Paolo Bonzini Subject: Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private memory Message-ID: <179fdb45-d8a4-9567-edfe-2168794f599e@redhat.com> Date: Thu, 16 Sep 2021 11:24:05 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210915141147.s4mgtcfv3ber5fnt@black.fi.intel.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 15/09/21 16:11, Kirill A. Shutemov wrote: >> Would introducing memfd_unregister_guest() fix this? > I considered this, but it get complex quickly. > > At what point it gets called? On KVM memslot destroy? > > What if multiple KVM slot share the same memfd? Add refcount into memfd on > how many times the owner registered the memfd? You will always have multiple KVM slots sharing the same memfd, because memslots are SRCU-protected. So there will be multiple generations of memslots around and unregistering must be delayed to after synchronize_srcu (around the call to kvm_arch_commit_memory_region). So KVM could just call memfd_{,un}register_guest as many times as it calls fdget/fput. Looking at your test device, it would be like the following pseudo-patch: case GUEST_MEM_REGISTER: { struct fd memfd = fdget(arg); memfd_file = memfd.file; return memfd_register_guest(memfd_file->f_inode, file, &guest_ops, &guest_mem_ops); } case GUEST_MEM_UNREGISTER: { if (!memfd_file) return -EINVAL; + memfd_unregister_guest(memfd_file->f_inode, file); fput(memfd_file); memfd_file = NULL; guest_mem_ops = NULL; return 0; and shmem_unregister_guest would be something like struct shmem_inode_info *info = SHMEM_I(inode); if (WARN_ON_ONCE(info->guest_owner != owner)) return; if (--info->guest_usage_count) return; info->guest_owner = NULL; info->guest_ops = NULL; Paolo > It would leave us in strange state: memfd refcount owners (struct KVM) and > KVM memslot pins the struct file. Weird refcount exchnage program.