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Fri, 10 Apr 2026 15:30:39 +0000 Message-ID: <45807109-570d-4681-bbd0-7a1649f515d9@amazon.com> Date: Fri, 10 Apr 2026 16:30:36 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v11 10/16] KVM: guest_memfd: Add flag to remove from direct map To: Ackerley Tng , "Kalyazin, Nikita" , "kvm@vger.kernel.org" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "kvmarm@lists.linux.dev" , "linux-fsdevel@vger.kernel.org" , "linux-mm@kvack.org" , "bpf@vger.kernel.org" , "linux-kselftest@vger.kernel.org" , "kernel@xen0n.name" , "linux-riscv@lists.infradead.org" , "linux-s390@vger.kernel.org" , "loongarch@lists.linux.dev" , "linux-pm@vger.kernel.org" CC: "pbonzini@redhat.com" , "corbet@lwn.net" , "maz@kernel.org" , "oupton@kernel.org" , "joey.gouly@arm.com" , "suzuki.poulose@arm.com" , "yuzenghui@huawei.com" , "catalin.marinas@arm.com" , "will@kernel.org" , "seanjc@google.com" , "tglx@kernel.org" , "mingo@redhat.com" , "bp@alien8.de" , "dave.hansen@linux.intel.com" , "x86@kernel.org" , "hpa@zytor.com" , "luto@kernel.org" , "peterz@infradead.org" , "willy@infradead.org" , "akpm@linux-foundation.org" , "david@kernel.org" , "lorenzo.stoakes@oracle.com" , "vbabka@kernel.org" , "rppt@kernel.org" , "surenb@google.com" , "mhocko@suse.com" , "ast@kernel.org" , "daniel@iogearbox.net" , "andrii@kernel.org" , "martin.lau@linux.dev" , "eddyz87@gmail.com" , "song@kernel.org" , "yonghong.song@linux.dev" , "john.fastabend@gmail.com" , "kpsingh@kernel.org" , "sdf@fomichev.me" , "haoluo@google.com" , "jolsa@kernel.org" , "jgg@ziepe.ca" , "jhubbard@nvidia.com" , "peterx@redhat.com" , "jannh@google.com" , "pfalcato@suse.de" , "skhan@linuxfoundation.org" , "riel@surriel.com" , "ryan.roberts@arm.com" , "jgross@suse.com" , "yu-cheng.yu@intel.com" , "kas@kernel.org" , "coxu@redhat.com" , "kevin.brodsky@arm.com" , "yosry@kernel.org" , "ajones@ventanamicro.com" , "maobibo@loongson.cn" , "tabba@google.com" , "prsampat@amd.com" , "wu.fei9@sanechips.com.cn" , "mlevitsk@redhat.com" , "jmattson@google.com" , "jthoughton@google.com" , "agordeev@linux.ibm.com" , "alex@ghiti.fr" , "aou@eecs.berkeley.edu" , "borntraeger@linux.ibm.com" , "chenhuacai@kernel.org" , "dev.jain@arm.com" , "gor@linux.ibm.com" , "hca@linux.ibm.com" , "palmer@dabbelt.com" , "pjw@kernel.org" , "shijie@os.amperecomputing.com" , "svens@linux.ibm.com" , "thuth@redhat.com" , "wyihan@google.com" , "yang@os.amperecomputing.com" , "Jonathan.Cameron@huawei.com" , "Liam.Howlett@oracle.com" , "urezki@gmail.com" , "zhengqi.arch@bytedance.com" , "gerald.schaefer@linux.ibm.com" , "jiayuan.chen@shopee.com" , "lenb@kernel.org" , "osalvador@suse.de" , "pavel@kernel.org" , "rafael@kernel.org" , "vannapurve@google.com" , "jackmanb@google.com" , "aneesh.kumar@kernel.org" , "patrick.roy@linux.dev" , "Thomson, Jack" , "Itazuri, Takahiro" , "Manwaring, Derek" References: <20260317141031.514-1-kalyazin@amazon.com> <20260317141031.514-11-kalyazin@amazon.com> Content-Language: en-US From: Nikita Kalyazin Autocrypt: addr=kalyazin@amazon.com; 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charset="us-ascii"; Format="flowed" Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org On 23/03/2026 21:15, Ackerley Tng wrote: > "Kalyazin, Nikita" writes: > >> >> [...snip...] >> >> static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) >> { >> struct inode *inode = file_inode(vmf->vma->vm_file); >> struct folio *folio; >> vm_fault_t ret = VM_FAULT_LOCKED; >> + int err; >> >> if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode)) >> return VM_FAULT_SIGBUS; >> @@ -418,6 +454,14 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) >> folio_mark_uptodate(folio); >> } >> >> + if (kvm_gmem_no_direct_map(folio_inode(folio))) { >> + err = kvm_gmem_folio_zap_direct_map(folio); >> + if (err) { >> + ret = vmf_error(err); >> + goto out_folio; >> + } >> + } >> + >> vmf->page = folio_file_page(folio, vmf->pgoff); >> > > Sashiko pointed out that kvm_gmem_populate() might try and write to > direct-map-removed folios, but I think that's handled because populate > will first try and GUP folios, which is already blocked for > direct-map-removed folios. As far as I can see, it is a valid issue as populate only GUPs the source pages, not gmem. I think this is similar to what was discussed about TDX at some point and decided to exclude TDX support [1]. I followed the same path and excluded SEV-SNP in the patch 8 [2]. I kept your and David's "Reviewed-by:" for that patch, but please let me know if this makes you change your minds. [1] https://lore.kernel.org/kvm/aWpcDrGVLrZOqdcg@google.com [2] https://lore.kernel.org/kvm/20260410151746.61150-9-kalyazin@amazon.com > >> out_folio: >> @@ -528,6 +572,9 @@ static void kvm_gmem_free_folio(struct folio *folio) >> kvm_pfn_t pfn = page_to_pfn(page); >> int order = folio_order(folio); >> >> + if (kvm_gmem_folio_no_direct_map(folio)) >> + kvm_gmem_folio_restore_direct_map(folio); >> + >> kvm_arch_gmem_invalidate(pfn, pfn + (1ul << order)); >> } >> > > Sashiko says to invalidate then restore direct map, I think in this case > it doesn't matter since if the folio needed invalidation, it must be > private, and the host shouldn't be writing to the private pages anyway. > > One benefit of retaining this order (restore, invalidate) is that it > opens the invalidate hook to possibly do something regarding memory > contents? > > Or perhaps we should just take the suggestion (invalidate, restore) and > align that invalidate should not touch memory contents. > >> @@ -591,6 +638,9 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags) >> /* Unmovable mappings are supposed to be marked unevictable as well. */ >> WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping)); >> >> + if (flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP) >> + mapping_set_no_direct_map(inode->i_mapping); >> + >> GMEM_I(inode)->flags = flags; >> >> file = alloc_file_pseudo(inode, kvm_gmem_mnt, name, O_RDWR, &kvm_gmem_fops); >> @@ -803,13 +853,22 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot, >> } >> >> r = kvm_gmem_prepare_folio(kvm, slot, gfn, folio); >> + if (r) >> + goto out_unlock; >> >> + if (kvm_gmem_no_direct_map(folio_inode(folio))) { >> + r = kvm_gmem_folio_zap_direct_map(folio); >> + if (r) >> + goto out_unlock; >> + } >> + >> >> [...snip...] >> > > Preparing a folio used to involve zeroing, but that has since been > refactored out, so I believe zapping can come before preparing. > > Similar to the above point on invalidation: perhaps we should take the > suggestion to zap then prepare > > + And align that preparation should not touch memory contents > + Avoid needing to undo the preparation on zapping failure (.free_folio > is not called on folio_put(), it is only called folio on removal from > filemap). I reordered both, thanks. _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv