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b=mXwWjpOKxSAKz1T2nD/QvcSF/IG7b06S+UBwPPRdQJACSatNJTMLGg5HzkdFLHGSX DY1n5D1hIaia3+msWWDcPO3u1EpX5i0VmhzF4/w1jeQ9+TISmBhx+YRDVIF7xF3fSi m/lY8JUvsy49ZASPceDZVQZgFdptQMg/ivlGyj4hyvRLZJnw5iwvAqea4xBrVUgYwN SHnl5nOBdYteL668eMquMzfsSRzPPoDETBYuTmPr9SHSwD33ssw47rA3ZAaMpTK4YG ZeK+kr0Ji8srwKL8IVKdM+FFFytCVbWpBOR1beq/KRuFI/mF4rCVvAqouJDnyITPfp h6L7Pkhd32IDA== Message-ID: <0c0b911c-cda2-44a4-897e-361e02be7da5@kernel.org> Date: Fri, 6 Mar 2026 15:22:19 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v10 09/15] KVM: guest_memfd: Add flag to remove from direct map To: kalyazin@amazon.com, "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" 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" , "lorenzo.stoakes@oracle.com" , "vbabka@suse.cz" , "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" , "shuah@kernel.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" , "ackerleytng@google.com" , "maobibo@loongson.cn" , "prsampat@amd.com" , "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" , "Cali, Marco" References: <20260126164445.11867-1-kalyazin@amazon.com> <20260126164445.11867-10-kalyazin@amazon.com> <13ed00e1-f0db-4326-a800-2ba306833921@kernel.org> <690c22f9-b71a-4f14-9857-008c7c858373@amazon.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 042EE140006 X-Rspamd-Server: rspam07 X-Stat-Signature: ooz7zaqkcrjq6rbi3j11j6pr51nokdfa X-Rspam-User: X-HE-Tag: 1772806969-228453 X-HE-Meta: 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 J97dvJfW PROV/dC6EvfLMb7jagTS6ZjtQG31VrBpuz0YEzjqT9qnSBJOW10ebhaTn8+2oolyFk2yRBYdUbqV8Xdud+ylFTMhhFErCNqa40sHFbGvMY0axHUjXIA/aZnWaKFgZ/Z60gn5KtEaphE4O9g+qxlHa70v9ywiS6eIARWTkh+C/W3G1r02nrLReJn4JFhLdJJCzZMjjSTfVkdIlgHZzzbcF21iREFxFE8TRFYuHXky34CtTc84yu5V96gOlfLHkXyf6MBr6mqhP8BSpifwIwiIMBTlG9hH06OzF6aeJT/CdWe8rBD5Xe3xvVuxDNceDmfa/zh9huGC5ceXse7EgAqat18TFMgIBgyQ8SvqBz1CDeRtPDe6MPZKr0ZM4nUzV3/QL9Rxs8T9NIjs9GrFajoJuXOJc82oVCuv2+27w8CIdhn7v6+kMNt+h4L4m26YstaJW35V/BNGVmiRlJEMzXYDUkwsLOQpOZeRKexgdNjY1zMtM57U= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: [...] >>> +     /* >>> +      * Direct map restoration cannot fail, as the only error condition >>> +      * for direct map manipulation is failure to allocate page tables >>> +      * when splitting huge pages, but this split would have already >>> +      * happened in folio_zap_direct_map() in >>> kvm_gmem_folio_zap_direct_map(). >>> +      * Note that the splitting occurs always because guest_memfd >>> +      * currently supports only base pages. >>> +      * Thus folio_restore_direct_map() here only updates prot bits. >>> +      */ >>> +     WARN_ON_ONCE(folio_restore_direct_map(folio)); >> >> Which raised the question: why should this function then even return an >> error? > > Dave pointed earlier that the failures were possible [1].  Do you think > we can document it better? I'm fine with checking that somewhere (to catch any future problems). Why not do the WARN_ON_ONCE() in folio_restore_direct_map()? Then, carefully document (in the new kerneldoc for folio_restore_direct_map() etc) that folio_restore_direct_map() is only allowed after a prior successful folio_zap_direct_map(), and add a helpful comment above the WARN_ON_ONCE() in folio_restore_direct_map() that we don't expect errors etc. [...] >>> -     if (!is_prepared) >>> +     if (!is_prepared) { >>>                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; >>> +     } >> >> >> It's a bit nasty that we have two different places where we have to call >> this. Smells error prone. > > We will actually have 2 more: for the write() syscall and UFFDIO_COPY, > and 0 once we have [2] > > [2] https://lore.kernel.org/linux-mm/20260225-page_alloc-unmapped-v1-0- > e8808a03cd66@google.com/ > >> >> I was wondering why kvm_gmem_get_folio() cannot handle that? > > Most of the call sites follow the pattern alloc -> write -> zap so > they'll need direct map for some time after the allocation. > Okay. Nasty. :) -- Cheers, David