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AJvYcCX80/7I16FPqJEptkjqgpRrSaUUPPd8hCZ21zSMc5fvp9cM0ZNP3OxtJfo+SdA3Yq/aWCCqA9WkmA==@kvack.org X-Gm-Message-State: AOJu0YzaYQo9s7nlGBHSyH7rSj/uyRZ9LKjbvhpYtsBkjW10uI1nB79t Nm09o4dQb6LJDNi7N6brtlUrNwT28gqa+4Z5HmxTWRQsYCSIvtIB1o24oRtAstI51+KcgsS5p6G 7+fDnTk4Yypd6xhqLjUNDekvxhybF855kmfVe X-Gm-Gg: ASbGncti//5aQ/izlyfJD0A7txZzDuqbWM9OPOnDX20KzPJOCHEPbkDnIdOMLGufeOb tK5T/XattSepvO+RVToJR/KgvvZFp8aJY/oP4aV1vUCsvvezfFKeDVpbVY3WG34nwZ83L1mS6iQ 5BcXzpYD/kKgnvaVbvuXUcUB9+BVf+8w== X-Google-Smtp-Source: AGHT+IE26OeD9m5IOF6U9oJIPVdqv6JXeBCc5CiNKTmlzHx15ZKQXbrAbjXcGlTg8m1ENRsNRij4hhyZetgnn+exia0= X-Received: by 2002:a05:6512:1044:b0:53d:f0a2:1fe3 with SMTP id 2adb3069b0e04-54400220e0dmr654101e87.2.1738777348802; Wed, 05 Feb 2025 09:42:28 -0800 (PST) MIME-Version: 1.0 References: <20250117163001.2326672-1-tabba@google.com> <20250117163001.2326672-7-tabba@google.com> In-Reply-To: From: Vishal Annapurve Date: Wed, 5 Feb 2025 09:42:17 -0800 X-Gm-Features: AWEUYZmJ7u229YhlH0rclHuieGR4uFgxbfBd2XaG6hmgURrEu_RxRmpMubZ2jQk Message-ID: Subject: Re: [RFC PATCH v5 06/15] KVM: guest_memfd: Handle final folio_put() of guestmem pages To: Fuad Tabba Cc: kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org, pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, seanjc@google.com, viro@zeniv.linux.org.uk, brauner@kernel.org, willy@infradead.org, akpm@linux-foundation.org, xiaoyao.li@intel.com, yilun.xu@intel.com, chao.p.peng@linux.intel.com, jarkko@kernel.org, amoorthy@google.com, dmatlack@google.com, yu.c.zhang@linux.intel.com, isaku.yamahata@intel.com, mic@digikod.net, vbabka@suse.cz, ackerleytng@google.com, mail@maciej.szmigiero.name, david@redhat.com, michael.roth@amd.com, wei.w.wang@intel.com, liam.merwick@oracle.com, isaku.yamahata@gmail.com, kirill.shutemov@linux.intel.com, suzuki.poulose@arm.com, steven.price@arm.com, quic_eberman@quicinc.com, quic_mnalajal@quicinc.com, quic_tsoni@quicinc.com, quic_svaddagi@quicinc.com, quic_cvanscha@quicinc.com, quic_pderrin@quicinc.com, quic_pheragu@quicinc.com, catalin.marinas@arm.com, james.morse@arm.com, yuzenghui@huawei.com, oliver.upton@linux.dev, maz@kernel.org, will@kernel.org, qperret@google.com, keirf@google.com, roypat@amazon.co.uk, shuah@kernel.org, hch@infradead.org, jgg@nvidia.com, rientjes@google.com, jhubbard@nvidia.com, fvdl@google.com, hughd@google.com, jthoughton@google.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: B855A180008 X-Stat-Signature: 7eo6znadrubxuuccd7h9ygxmmgxigenm X-HE-Tag: 1738777350-649341 X-HE-Meta: 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 ir6Avgva 3StI1I74+mT3dfDIzIpEEA00afb7jbP59fpJcmXssce8bm+qO+ubh0761RXsMCCUeLRQjv3mdeHJAPVk8GdPMdVB5FKJvRYt/pJ7Eg16XUbMICiKK1PNThoIAyPBSp1FLTXP7MdE/lXOwQFDRsyw4dw3QPw4nIXYqrwSEuas7bkckHundl1iZuKea9OOxiKq14smH3v3WFcJNaVYoWXwGpdZu2m8Ql3CqEhWHqS4/A7JIGjRQDZaVfPnBwZPsO4M7tOaIm2a8c1RMCUVNGoRbVJ+Ej7z6rsKsfmd0ESRt1+lIAXDA4juyoSmWjBHbRNCXjk4YHqeXl2qvLMd07YouLW6KZTvkZ1JHTQ+MLC2vwN48nVXU3ZEkP3jZFuLUM9WDBdylKHVprx1fnrs= X-Bogosity: Ham, tests=bogofilter, spamicity=0.047168, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Feb 5, 2025 at 9:39=E2=80=AFAM Vishal Annapurve wrote: > > On Wed, Feb 5, 2025 at 2:07=E2=80=AFAM Fuad Tabba wrot= e: > > > > Hi Vishal, > > > > On Wed, 5 Feb 2025 at 00:42, Vishal Annapurve w= rote: > > > > > > On Fri, Jan 17, 2025 at 8:30=E2=80=AFAM Fuad Tabba = wrote: > > > > > > > > Before transitioning a guest_memfd folio to unshared, thereby > > > > disallowing access by the host and allowing the hypervisor to > > > > transition its view of the guest page as private, we need to be > > > > sure that the host doesn't have any references to the folio. > > > > > > > > This patch introduces a new type for guest_memfd folios, and uses > > > > that to register a callback that informs the guest_memfd > > > > subsystem when the last reference is dropped, therefore knowing > > > > that the host doesn't have any remaining references. > > > > > > > > Signed-off-by: Fuad Tabba > > > > --- > > > > The function kvm_slot_gmem_register_callback() isn't used in this > > > > series. It will be used later in code that performs unsharing of > > > > memory. I have tested it with pKVM, based on downstream code [*]. > > > > It's included in this RFC since it demonstrates the plan to > > > > handle unsharing of private folios. > > > > > > > > [*] https://android-kvm.googlesource.com/linux/+/refs/heads/tabba/g= uestmem-6.13-v5-pkvm > > > > > > Should the invocation of kvm_slot_gmem_register_callback() happen in > > > the same critical block as setting the guest memfd range mappability > > > to NONE, otherwise conversion/truncation could race with registration > > > of callback? > > > > I don't think it needs to, at least not as far potencial races are > > concerned. First because kvm_slot_gmem_register_callback() grabs the > > mapping's invalidate_lock as well as the folio lock, and > > gmem_clear_mappable() grabs the mapping lock and the folio lock if a > > folio has been allocated before. > > I was hinting towards such a scenario: > Core1 > Shared to private conversion > -> Results in mappability attributes > being set to NONE > ... > Trigger private to shared conversion/truncation for > ... > overlapping ranges > ... > kvm_slot_gmem_register_callback() on > the guest_memfd ranges converted > above (This will end up registering callback > for guest_memfd ranges which possibly don't > carry *_MAPPABILITY_NONE) > Sorry for the format mess above. I was hinting towards such a scenario: Core1- Shared to private conversion -> Results in mappability attributes being set to NONE ... Core2 Trigger private to shared conversion/truncation for overlapping ranges ... Core1 kvm_slot_gmem_register_callback() on the guest_memfd ranges converted above (This will end up registering callback for guest_memfd ranges which possibly don't carry *_MAPPABILITY_NONE) > > > > Second, __gmem_register_callback() checks before returning whether all > > references have been dropped, and adjusts the mappability/shareability > > if needed. > > > > Cheers, > > /fuad