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Wed, 28 Jan 2026 09:07:05 -0800 (PST) Received: from 176938342045 named unknown by gmailapi.google.com with HTTPREST; Wed, 28 Jan 2026 09:07:04 -0800 Received: from 176938342045 named unknown by gmailapi.google.com with HTTPREST; Wed, 28 Jan 2026 09:07:04 -0800 From: Ackerley Tng In-Reply-To: References: <638600e19c6e23959bad60cf61582f387dff6445.1760731772.git.ackerleytng@google.com> MIME-Version: 1.0 Date: Wed, 28 Jan 2026 09:07:04 -0800 X-Gm-Features: AZwV_QhaOE_wJlLL_hX7tpT0otF_GVqIj23tN-vGGdhqYGAPjKuSGPxvod54AXo Message-ID: Subject: Re: [RFC PATCH v1 01/37] KVM: guest_memfd: Introduce per-gmem attributes, use to guard user mappings To: Binbin Wu Cc: cgroups@vger.kernel.org, kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, x86@kernel.org, akpm@linux-foundation.org, bp@alien8.de, brauner@kernel.org, chao.p.peng@intel.com, chenhuacai@kernel.org, corbet@lwn.net, dave.hansen@intel.com, dave.hansen@linux.intel.com, david@redhat.com, dmatlack@google.com, erdemaktas@google.com, fan.du@intel.com, fvdl@google.com, haibo1.xu@intel.com, hannes@cmpxchg.org, hch@infradead.org, hpa@zytor.com, hughd@google.com, ira.weiny@intel.com, isaku.yamahata@intel.com, jack@suse.cz, james.morse@arm.com, jarkko@kernel.org, jgg@ziepe.ca, jgowans@amazon.com, jhubbard@nvidia.com, jroedel@suse.de, jthoughton@google.com, jun.miao@intel.com, kai.huang@intel.com, keirf@google.com, kent.overstreet@linux.dev, liam.merwick@oracle.com, maciej.wieczor-retman@intel.com, mail@maciej.szmigiero.name, maobibo@loongson.cn, mathieu.desnoyers@efficios.com, maz@kernel.org, mhiramat@kernel.org, mhocko@kernel.org, mic@digikod.net, michael.roth@amd.com, mingo@redhat.com, mlevitsk@redhat.com, mpe@ellerman.id.au, muchun.song@linux.dev, nikunj@amd.com, nsaenz@amazon.es, oliver.upton@linux.dev, palmer@dabbelt.com, pankaj.gupta@amd.com, paul.walmsley@sifive.com, pbonzini@redhat.com, peterx@redhat.com, pgonda@google.com, prsampat@amd.com, pvorel@suse.cz, qperret@google.com, richard.weiyang@gmail.com, rick.p.edgecombe@intel.com, rientjes@google.com, rostedt@goodmis.org, roypat@amazon.co.uk, rppt@kernel.org, seanjc@google.com, shakeel.butt@linux.dev, shuah@kernel.org, steven.price@arm.com, steven.sistare@oracle.com, suzuki.poulose@arm.com, tabba@google.com, tglx@linutronix.de, thomas.lendacky@amd.com, vannapurve@google.com, vbabka@suse.cz, viro@zeniv.linux.org.uk, vkuznets@redhat.com, wei.w.wang@intel.com, will@kernel.org, willy@infradead.org, wyihan@google.com, xiaoyao.li@intel.com, yan.y.zhao@intel.com, yilun.xu@intel.com, yuzenghui@huawei.com, zhiquan1.li@intel.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam12 X-Stat-Signature: cfad5yxz4dyewgkr9n7tb6c8fo5pohbr X-Rspamd-Queue-Id: 0F61314000B X-Rspam-User: X-HE-Tag: 1769620026-700184 X-HE-Meta: U2FsdGVkX1+3Q0TNpusmpCVFmFKTZM0lO6uyjaQUsmIyek9zzh6mUqHgtOuJJoLQcwHHLThppInaJhK+Z2VzSPiAahAJ7PBc8e8amk35lslLY1ofcAY6E/eg/frCV1RIyiNwVgFEFfRVxPzJ9xOLEcPRVfAoF2Orq1sFB7RALSx7T74+BzF3uFjcZNShy8MZZ2hfbYIT2YQ063PmQ7lPBkaH0t6GZaJ3UJ2a/fw7TsuRXZhx3ljiBaEHK8SzJZOR71tdVEQQcRdEad6yx5l/bugfegVvM2bfevO4ySnsyhSKhDUfFl5QevGHdYSc8FoUqWbncO49+opdCGEoW3EpyeO5KHw2KiCI3Kk5Xhc9peJnsu0EVls7GUInzlJQ0Rw3DXXPRgxZs+NJ1MzDCZhY7e9TWNDAVzyrdYVKpZP3JWDHWfxR5UzcbK/SW5WWfPmCEDIAWIKgSqGCqk0r/sHLdb8pDBdc+IVvRXDhAaQLEc1s4IR6HFZaGBuxyGcz/s5VuMYqc/M99a2U+K3g4BbwuFU21ScqE/Feb1FhOUU3FT2xSUI8/pNNNx15K6sNBafMiOhJwuj0MKDirSkZ81xiHkYQdc/0CnRxurQtOMIZ6ABVWHyJYgzE577v1HfQxLzTsEjNNn1iMOFb/yJmyZmEy4JcQMq/jEOFQocN9ds94bm4CSIdKvXeKtxpcuDO89Hh1SUjb0Rdql/5W+gcY/ukJe3NaP1GdL8FlMuWUNchJqbu4dWaldwPcCzedVdQ1IX0Axts012gfRFyY9zBSejiKzzyRBWApzgFM3jwFlAWonjdFBPdqh1GN+HXdDjxohCOTJaix0F0pJ+H4TaHzeLieTy1Ayr2vjmZThwSvgYO90PX0oR4yTFrql8tCxVouBbi2im1q/7titBLqQveZ2hhYWFAj45gA3kxSzLkzCMJJV7/fG6t9pA008BSozok92UUH1zFsRowCNpewEzGZvy iIvhf54R HA4xIP3YJTWCJu5YPhdMGBjpDBQzTR3tDp8+vcaALTZl2n3Pb0jAShwn946zIfhozs7w/ujgoLwyR0a7b0CMdN34c7SZ/S4PXIQcykKHXVwGHflIMKHtZeMDX29/6bsylUac7mA8/wRrUBNdwCNZkmLj5SWZRn9tinNnTtzQAntc9pomPCPPsvLDuMLsGNHolASTlL6At6mvaKfe8yrbS+8sadIhiqnf3T+bdHtYhhxWkhNjtntW7fuvafu/zqFtz2mMu/3/Yl2b2SoSCyOKJBxYgGUx7N1XezFJ0jJjO2zvhy6wLHjpqrgeeMH0yRJLUALHw+2zGhiXdyct86BMPmr0ohXZ2/gCvzTKyE1rciQu3Fw2yZzcCCdjz8bZ7NjUWrNbviLCqcPx9QxMSltRt/Cogrv2HmzWNzMfMqUysvVfyf8/D38p/sbbUpbOPbkr85MmMRmJbV+5xvUlUq/iDE7s0cxyaUpxKBjTa 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: Binbin Wu writes: > On 10/18/2025 4:11 AM, Ackerley Tng wrote: > [...] >> >> +static int kvm_gmem_init_inode(struct inode *inode, loff_t size, u64 fl= ags) >> +{ >> + struct gmem_inode *gi =3D GMEM_I(inode); >> + MA_STATE(mas, &gi->attributes, 0, (size >> PAGE_SHIFT) - 1); >> + u64 attrs; >> + int r; >> + >> + inode->i_op =3D &kvm_gmem_iops; >> + inode->i_mapping->a_ops =3D &kvm_gmem_aops; >> + inode->i_mode |=3D S_IFREG; >> + inode->i_size =3D size; >> + mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER); >> + mapping_set_inaccessible(inode->i_mapping); >> + /* Unmovable mappings are supposed to be marked unevictable as well. *= / > AS_UNMOVABLE has been removed and got merged into AS_INACCESSIBLE, not su= re if > it's better to use "Inaccessible" instead of "Unmovable" > Thanks, will update comment as follows: /* * guest_memfd memory is not migratable or swappable - set * inaccessible to gate off both. */ mapping_set_inaccessible(inode->i_mapping); WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping)); >> + WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping)); >> + >> + gi->flags =3D flags; >> + >> + mt_set_external_lock(&gi->attributes, >> + &inode->i_mapping->invalidate_lock); >> + >> + /* >> + * Store default attributes for the entire gmem instance. Ensuring eve= ry >> + * index is represented in the maple tree at all times simplifies the >> + * conversion and merging logic. >> + */ >> + attrs =3D gi->flags & GUEST_MEMFD_FLAG_INIT_SHARED ? 0 : KVM_MEMORY_AT= TRIBUTE_PRIVATE; >> + >> + /* >> + * Acquire the invalidation lock purely to make lockdep happy. There >> + * should be no races at this time since the inode hasn't yet been ful= ly >> + * created. >> + */ >> + filemap_invalidate_lock(inode->i_mapping); >> + r =3D mas_store_gfp(&mas, xa_mk_value(attrs), GFP_KERNEL); >> + filemap_invalidate_unlock(inode->i_mapping); >> + >> + return r; >> +} >> + > [...] >> @@ -925,13 +986,39 @@ static struct inode *kvm_gmem_alloc_inode(struct s= uper_block *sb) >> >> mpol_shared_policy_init(&gi->policy, NULL); >> >> + /* >> + * Memory attributes are protected the filemap invalidation lock, but > =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 = =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0^ > =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 = =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 protected by Thanks! >> + * the lock structure isn't available at this time. Immediately mark >> + * maple tree as using external locking so that accessing the tree >> + * before its fully initialized results in NULL pointer dereferences >> + * and not more subtle bugs. >> + */ >> + mt_init_flags(&gi->attributes, MT_FLAGS_LOCK_EXTERN); >> + >> gi->flags =3D 0; >> return &gi->vfs_inode; >> } >> >> static void kvm_gmem_destroy_inode(struct inode *inode) >> { >> - mpol_free_shared_policy(&GMEM_I(inode)->policy); >> + struct gmem_inode *gi =3D GMEM_I(inode); >> + >> + mpol_free_shared_policy(&gi->policy); >> + >> + /* >> + * Note! Checking for an empty tree is functionally necessary to avoi= d >> + * explosions if the tree hasn't been initialized, i.e. if the inode i= s > > It makes sense to skip __mt_destroy() when mtree is empty. > But what explosions it could trigger if mtree is empty? > It seems __mt_destroy() can handle the case if the external lock is not s= et. > > Hope this updated comment clarify the explosion: /* * Note! Checking for an empty tree is functionally necessary * to avoid explosions if the tree hasn't been fully * initialized, i.e. if the inode is being destroyed before * guest_memfd can set the external lock, lockdep would find * that the tree's internal ma_lock was not held. */ >> + * being destroyed before guest_memfd can set the external lock. >> + */ >> + if (!mtree_empty(&gi->attributes)) { >> + /* >> + * Acquire the invalidation lock purely to make lockdep happy, >> + * the inode is unreachable at this point. >> + */ >> + filemap_invalidate_lock(inode->i_mapping); >> + __mt_destroy(&gi->attributes); >> + filemap_invalidate_unlock(inode->i_mapping); >> + } >> } >> >> static void kvm_gmem_free_inode(struct inode *inode) >> -- >> 2.51.0.858.gf9c4a03a3a-goog