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Thu, 22 Jan 2026 18:04:57 +0000 (UTC) X-Farcaster-Flow-ID: 3ec1e9f4-fd44-4c0b-b87e-d835b8518924 Received: from EX19D005EUB003.ant.amazon.com (10.252.51.31) by EX19MTAEUB001.ant.amazon.com (10.252.51.26) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.2562.35; Thu, 22 Jan 2026 18:04:57 +0000 Received: from [192.168.23.186] (10.106.82.17) by EX19D005EUB003.ant.amazon.com (10.252.51.31) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.2562.35; Thu, 22 Jan 2026 18:04:53 +0000 Message-ID: Date: Thu, 22 Jan 2026 18:04:51 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Reply-To: Subject: Re: [PATCH v9 07/13] 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" 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@linutronix.de" , "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" , "Liam.Howlett@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" , "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" , "Jonathan.Cameron@huawei.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" , "vannapurve@google.com" , "jackmanb@google.com" , "aneesh.kumar@kernel.org" , "patrick.roy@linux.dev" , "Thomson, Jack" , "Itazuri, Takahiro" , "Manwaring, Derek" , "Cali, Marco" References: <20260114134510.1835-1-kalyazin@amazon.com> <20260114134510.1835-8-kalyazin@amazon.com> Content-Language: en-US From: Nikita Kalyazin Autocrypt: addr=kalyazin@amazon.com; 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Here's what was also discussed at the guest_memfd biweekly on > 2026-01-22: > >> >> [...snip...] >> >>>> @@ -423,6 +464,12 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) >>>> kvm_gmem_mark_prepared(folio); >>>> } >>>> >>>> + err = kvm_gmem_folio_zap_direct_map(folio); >>> >>> Perhaps the check for gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP should >>> be done here before making the call to kvm_gmem_folio_zap_direct_map() >>> to make it more obvious that zapping is conditional. >> >> Makes sense to me. >> >>> >>> Perhaps also add a check for kvm_arch_gmem_supports_no_direct_map() so >>> this call can be completely removed by the compiler if it wasn't >>> compiled in. >> >> But if it is compiled in, we will be paying the cost of the call on >> every page fault? Eg on arm64, it will call the following: >> >> bool can_set_direct_map(void) >> { >> >> ... >> >> return rodata_full || debug_pagealloc_enabled() || >> arm64_kfence_can_set_direct_map() || is_realm_world(); >> } >> > > You're right that this could end up paying the cost on every page > fault. Please ignore this request! > >>> >>> The kvm_gmem_folio_no_direct_map() check should probably remain in >>> kvm_gmem_folio_zap_direct_map() since that's a "if already zapped, don't >>> zap again" check. >>> >>>> + if (err) { >>>> + ret = vmf_error(err); >>>> + goto out_folio; >>>> + } >>>> + >>>> vmf->page = folio_file_page(folio, vmf->pgoff); >>>> >>>> out_folio: >>>> @@ -533,6 +580,8 @@ static void kvm_gmem_free_folio(struct folio *folio) >>>> kvm_pfn_t pfn = page_to_pfn(page); >>>> int order = folio_order(folio); >>>> >>>> + kvm_gmem_folio_restore_direct_map(folio); >>>> + >>> >>> I can't decide if the kvm_gmem_folio_no_direct_map(folio) should be in >>> the caller or within kvm_gmem_folio_restore_direct_map(), since this >>> time it's a folio-specific property being checked. >> >> I'm tempted to keep it similar to the kvm_gmem_folio_zap_direct_map() >> case. How does the fact it's a folio-speicific property change your >> reasoning? >> > > This is good too: > > if (kvm_gmem_folio_no_direct_map(folio)) > kvm_gmem_folio_restore_direct_map(folio) It turns out we can't do that because folio->mapping is gone by the time filemap_free_folio() is called so we can't inspect the flags. Are you ok with only having this check when zapping (but not when restoring)? Do you think we should add a comment saying it's conditional here? > >>> >>> Perhaps also add a check for kvm_arch_gmem_supports_no_direct_map() so >>> this call can be completely removed by the compiler if it wasn't >>> compiled in. IIUC whether the check is added in the caller or within >>> kvm_gmem_folio_restore_direct_map() the call can still be elided. >> >> Same concern as the above about kvm_gmem_folio_zap_direct_map(), ie the >> performance of the case where kvm_arch_gmem_supports_no_direct_map() exists. >> > > Please ignore this request! > >>> >>>> kvm_arch_gmem_invalidate(pfn, pfn + (1ul << order)); >>>> } >>>> >>>> @@ -596,6 +645,9 @@ 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); >>>> @@ -807,6 +859,8 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot, >>>> if (!is_prepared) >>>> r = kvm_gmem_prepare_folio(kvm, slot, gfn, folio); >>>> >>>> + kvm_gmem_folio_zap_direct_map(folio); >>>> + >>> >>> Is there a reason why errors are not handled when faulting private memory? >> >> No, I can't see a reason. Will add a check, thanks. >> >>> >>>> folio_unlock(folio); >>>> >>>> if (!r) >>>> -- >>>> 2.50.1