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b=ZKwXk10t3KvnpEDSJQebHfUK+3bQguAKrVSqADBK9iE76YbYNwZ3dt9/Z2YKsClBT pAcdPluWKqTcp3tgG9FsZRFIUIfinnubH9tqyFCvM6WMXXE/XPJyXpMMZrsaZblncq DmayVqJwfFbGpYEAfNUd7BZ4pOGD7v74lth2RwPcWKTxdmDjyXT8v/BYAFbfZ6CDH0 sGihCC9IxBd22g5DMKnaqH34bYCNJXeShs4SP6TbHGZssSVQGZCPcMH1bLQDiPC+in TO1W6JUZb0PqCXP+Na94lG8jsCKyjjUHGr3NNxirNMWyzFWitM+jHdnBDxFh3GzmEb zH0SqpwRo19Pw== Message-ID: <9b4b9f48-595d-46c9-8f77-fc021ac2619c@kernel.org> Date: Fri, 6 Mar 2026 16:16:18 +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> <0c0b911c-cda2-44a4-897e-361e02be7da5@kernel.org> <936fa782-d937-4b14-b92d-cc8707336e5e@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-Rspam-User: X-Stat-Signature: qsyd4m3s7hh64ferqpyupt6x16a4dppn X-Rspamd-Queue-Id: EFAAD1C0002 X-Rspamd-Server: rspam03 X-HE-Tag: 1772810209-305193 X-HE-Meta: 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 8D4FexOJ R6b4/rHTeqa5RFgwJd1RHU4ajgjMQIsRy8Mp7spsZGrVO3dZxtD9GuMbNlK9phSC0QIU7RaMrPo+JatsvUHd56H64m+8TVHrP8SZ1SZ/Gmv63oj64DPfPoYKcUDFnEa14IqvHpsTbYT/ItWQI4Gmijxt4zVx/qQethXtDb7gOnmMdYiKUtMqCBKHp2CN6m3mfqfDQanC+1eBc/yJavrrb5QEG0/Z1PbNeKMgBaItYNuQL+c2XC8Y5o/VniRnNRtdI/ui8sg0mwWW+B3PdlzRJbRzaOW5LnZbWTNpWlUHc/9pTvv3Hu8hgqgmR0rvdwqp2zmaD1JCZTFa2ONhfD8YBpukmvYPbqr5pFWuzJGUZrnYbWoRVuW32Cow+7cjfrxIMliF4tqy8y6ohPxO98ZXf6UhWMu3DSHm7fbpeU8IqyhQajvsMYmgPAcC8tDADuPpdcKYU Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/6/26 15:49, Nikita Kalyazin wrote: > > > On 06/03/2026 14:22, David Hildenbrand (Arm) wrote: >> [...] >> >>> >>> 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. > > My only concern about that is the assumptions we make in KVM may not > apply to the general case and the WARN_ON_ONCE may become too > restrictive compared to proper error handling in some (rare) cases.  For > example, is it possible for the folio to migrate in between? Not without migration support. But then, migration would have to make sure that the destination folio has the direct map removed. So I wouldn't worry about that. Once you return an error from folio_restore_direct_map(), you better document when/why it happens and how the caller should handle it. There is nothing existing callers could really do, so it's an implementation detail of the folio_zap_direct_map()/folio_restore_direct_map() functions to make sure it keeps working. If it ever fails, we'll have to figure out how to keep the interface working. You can document any restrictions regarding when folio_zap_direct_map()/folio_restore_direct_map() is allowed to be used in the kerneldoc. -- Cheers, David