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[2003:d8:2f19:9c00:568:7df7:e1:293d]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-452730d15f4sm15690935e9.40.2025.06.06.02.55.24 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 06 Jun 2025 02:55:25 -0700 (PDT) Message-ID: <6cf86edb-1e7e-4b44-93d0-f03f9523c24a@redhat.com> Date: Fri, 6 Jun 2025 11:55:23 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v11 08/18] KVM: guest_memfd: Allow host to map guest_memfd pages To: Fuad Tabba Cc: kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.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, 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, roypat@amazon.co.uk, shuah@kernel.org, hch@infradead.org, jgg@nvidia.com, rientjes@google.com, jhubbard@nvidia.com, fvdl@google.com, hughd@google.com, jthoughton@google.com, peterx@redhat.com, pankaj.gupta@amd.com, ira.weiny@intel.com References: <20250605153800.557144-1-tabba@google.com> <20250605153800.557144-9-tabba@google.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 1379B1C0002 X-Stat-Signature: 91n3qsrhs4kuxyu7o8s7o7ccdmuyaqjf X-Rspam-User: X-HE-Tag: 1749203730-421943 X-HE-Meta: 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 bYwOPcYA zEdtVM4X9UaF3lC0sNmX4EXyFoyo/hLFLgl6nIYYpqzdnrRH2y3Fx753CRjOdKQDGLBaj3QYpr4RN3+qVrY1a/v3FFun33fxGie7xTRMUhAdJmxciA4Y0Vy6t8u8PRvb69NJs4ODiUz+wzOE/cx3UY03EEOehkDksnGVUzf9MtfIRm9J/IobA3fSI52Tkxn0/Ma9N X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 06.06.25 11:30, Fuad Tabba wrote: > Hi David, > > On Fri, 6 Jun 2025 at 10:12, David Hildenbrand wrote: >> >> On 05.06.25 17:37, Fuad Tabba wrote: >>> This patch enables support for shared memory in guest_memfd, including >>> mapping that memory from host userspace. >>> >>> This functionality is gated by the KVM_GMEM_SHARED_MEM Kconfig option, >>> and enabled for a given instance by the GUEST_MEMFD_FLAG_SUPPORT_SHARED >>> flag at creation time. >>> >>> Co-developed-by: Ackerley Tng >>> Signed-off-by: Ackerley Tng >>> Signed-off-by: Fuad Tabba >>> --- >> >> [...] >> >>> +static bool kvm_gmem_supports_shared(struct inode *inode) >>> +{ >>> + u64 flags; >>> + >>> + if (!IS_ENABLED(CONFIG_KVM_GMEM_SHARED_MEM)) >>> + return false; >>> + >>> + flags = (u64)inode->i_private; >> >> Can probably do above >> >> const u64 flags = (u64)inode->i_private; >> > > Ack. > >>> + >>> + return flags & GUEST_MEMFD_FLAG_SUPPORT_SHARED; >>> +} >>> + >>> +static vm_fault_t kvm_gmem_fault_shared(struct vm_fault *vmf) >>> +{ >>> + struct inode *inode = file_inode(vmf->vma->vm_file); >>> + struct folio *folio; >>> + vm_fault_t ret = VM_FAULT_LOCKED; >>> + >>> + if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode)) >>> + return VM_FAULT_SIGBUS; >>> + >>> + folio = kvm_gmem_get_folio(inode, vmf->pgoff); >>> + if (IS_ERR(folio)) { >>> + int err = PTR_ERR(folio); >>> + >>> + if (err == -EAGAIN) >>> + return VM_FAULT_RETRY; >>> + >>> + return vmf_error(err); >>> + } >>> + >>> + if (WARN_ON_ONCE(folio_test_large(folio))) { >>> + ret = VM_FAULT_SIGBUS; >>> + goto out_folio; >>> + } >>> + >>> + if (!folio_test_uptodate(folio)) { >>> + clear_highpage(folio_page(folio, 0)); >>> + kvm_gmem_mark_prepared(folio); >>> + } >>> + >>> + vmf->page = folio_file_page(folio, vmf->pgoff); >>> + >>> +out_folio: >>> + if (ret != VM_FAULT_LOCKED) { >>> + folio_unlock(folio); >>> + folio_put(folio); >>> + } >>> + >>> + return ret; >>> +} >>> + >>> +static const struct vm_operations_struct kvm_gmem_vm_ops = { >>> + .fault = kvm_gmem_fault_shared, >>> +}; >>> + >>> +static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma) >>> +{ >>> + if (!kvm_gmem_supports_shared(file_inode(file))) >>> + return -ENODEV; >>> + >>> + if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) != >>> + (VM_SHARED | VM_MAYSHARE)) { >>> + return -EINVAL; >>> + } >>> + >>> + vma->vm_ops = &kvm_gmem_vm_ops; >>> + >>> + return 0; >>> +} >>> + >>> static struct file_operations kvm_gmem_fops = { >>> + .mmap = kvm_gmem_mmap, >>> .open = generic_file_open, >>> .release = kvm_gmem_release, >>> .fallocate = kvm_gmem_fallocate, >>> @@ -428,6 +500,7 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags) >>> } >>> >>> file->f_flags |= O_LARGEFILE; >>> + allow_write_access(file); >> >> Why is that required? >> >> As the docs mention, it must be paired with a previous deny_write_access(). >> >> ... and I don't find similar usage anywhere else. > > This is to address Gavin's concern [*] regarding MADV_COLLAPSE, which > isn't an issue until hugepage support is enabled. Should we wait until > we have hugepage support? If we keep this, we *definitely* need a comment why we do something nobody else does. But I don't think allow_write_access() would ever be the way we want to fence off MADV_COLLAPSE. :) Maybe AS_INACCESSIBLE or sth. like that could fence it off in file_thp_enabled(). Fortunately, CONFIG_READ_ONLY_THP_FOR_FS might vanish at some point ... so I've been told. So if it's not done for secretmem for now or others, we also shouldn't be doing it for now I think. -- Cheers, David / dhildenb