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[2003:cb:c707:7600:5c18:5a7d:c5b7:e7a9]) by smtp.gmail.com with ESMTPSA id v6-20020a5d59c6000000b0033b47ee01f1sm10751259wry.49.2024.02.26.23.40.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 26 Feb 2024 23:40:25 -0800 (PST) Message-ID: <4a0fd313-4423-496d-aec1-d25707a89288@redhat.com> Date: Tue, 27 Feb 2024 08:40:23 +0100 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v1 03/26] KVM: Add restricted support for mapping guestmem by the host Content-Language: en-US To: Fuad Tabba Cc: kvm@vger.kernel.org, kvmarm@lists.linux.dev, pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, seanjc@google.com, viro@zeniv.linux.org.uk, brauner@kernel.org, willy@infradead.org, akpm@linux-foundation.org, xiaoyao.li@intel.com, yilun.xu@intel.com, chao.p.peng@linux.intel.com, jarkko@kernel.org, amoorthy@google.com, dmatlack@google.com, yu.c.zhang@linux.intel.com, isaku.yamahata@intel.com, mic@digikod.net, vbabka@suse.cz, vannapurve@google.com, ackerleytng@google.com, mail@maciej.szmigiero.name, michael.roth@amd.com, wei.w.wang@intel.com, liam.merwick@oracle.com, isaku.yamahata@gmail.com, kirill.shutemov@linux.intel.com, suzuki.poulose@arm.com, steven.price@arm.com, quic_eberman@quicinc.com, quic_mnalajal@quicinc.com, quic_tsoni@quicinc.com, quic_svaddagi@quicinc.com, quic_cvanscha@quicinc.com, quic_pderrin@quicinc.com, quic_pheragu@quicinc.com, catalin.marinas@arm.com, james.morse@arm.com, yuzenghui@huawei.com, oliver.upton@linux.dev, maz@kernel.org, will@kernel.org, qperret@google.com, keirf@google.com References: <20240222161047.402609-1-tabba@google.com> <20240222161047.402609-4-tabba@google.com> <86461043-fa5b-405d-bd2e-dc1aba9977c5@redhat.com> <83d6edb8-bfd0-4233-a4cf-b573fa62c8d9@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 26.02.24 18:30, Fuad Tabba wrote: > Hi David, > > Thank you very much for reviewing this. > > On Mon, Feb 26, 2024 at 9:58 AM David Hildenbrand wrote: >> >> On 26.02.24 09:58, Fuad Tabba wrote: >>> Hi David, >>> >>> On Thu, Feb 22, 2024 at 4:28 PM David Hildenbrand wrote: >>>> >>>>> +static vm_fault_t kvm_gmem_fault(struct vm_fault *vmf) >>>>> +{ >>>>> + struct folio *folio; >>>>> + >>>>> + folio = kvm_gmem_get_folio(file_inode(vmf->vma->vm_file), vmf->pgoff); >>>>> + if (!folio) >>>>> + return VM_FAULT_SIGBUS; >>>>> + >>>>> + /* >>>>> + * Check if the page is allowed to be faulted to the host, with the >>>>> + * folio lock held to ensure that the check and incrementing the page >>>>> + * count are protected by the same folio lock. >>>>> + */ >>>>> + if (!kvm_gmem_isfaultable(vmf)) { >>>>> + folio_unlock(folio); >>>>> + return VM_FAULT_SIGBUS; >>>>> + } >>>>> + >>>>> + vmf->page = folio_file_page(folio, vmf->pgoff); >>>> >>>> We won't currently get hugetlb (or even THP) here. It mimics what shmem >>>> would do. >>> >>> At the moment there isn't hugetlb support in guest_memfd(), and >>> neither in pKVM. Although we do plan on supporting it. >>> >>>> finish_fault->set_pte_range() will call folio_add_file_rmap_ptes(), >>>> getting the rmap involved. >>>> >>>> Do we have some tests in place that make sure that >>>> fallocate(FALLOC_FL_PUNCH_HOLE|FALLOC_FL_KEEP_SIZE) will properly unmap >>>> the page again (IOW, that the rmap does indeed work?). >>> >>> I'm not sure if you mean kernel tests, or if I've tested it. There are >>> guest_memfd() tests for >>> fallocate(FALLOC_FL_PUNCH_HOLE|FALLOC_FL_KEEP_SIZE) , which I have >>> run. I've also tested it manually with sample programs, and it behaves >>> as expected. >> >> Can you point me at the existing tests? I'm interested in >> mmap()-specific guest_memfd tests. >> >> A test that would make sense to me: >> >> 1) Create guest_memfd() and size it to contain at least one page. >> >> 2) mmap() it >> >> 3) Write some pattern (e.g., all 1's) to the first page using the mmap >> >> 4) fallocate(FALLOC_FL_PUNCH_HOLE|FALLOC_FL_KEEP_SIZE) the first page >> >> 5) Make sure reading from the first page using the mmap reads all 0's >> >> IOW, during 4) we properly unmapped (via rmap) and discarded the page, >> such that 5) will populate a fresh page in the page cache filled with >> 0's and map that one. > > The existing tests don't test mmap (or rather, they test the inability > to mmap). They do test FALLOC_FL_PUNCH_HOLE. [1] > > The tests for mmap() are ones that I wrote myself. I will write a test > like the one you mentioned, and send it with V2, or earlier if you > prefer. Thanks, no need to rush. As long as we have some simple test for that scenario at some point, all good! -- Cheers, David / dhildenb