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[34.90.227.64]) by smtp.gmail.com with ESMTPSA id tk7-20020a170907c28700b00a43aa27b5f2sm2003321ejc.94.2024.02.28.10.51.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 28 Feb 2024 10:51:25 -0800 (PST) Date: Wed, 28 Feb 2024 18:51:22 +0000 From: Quentin Perret To: David Hildenbrand , Matthew Wilcox , Fuad Tabba , kvm@vger.kernel.org, kvmarm@lists.linux.dev, 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, 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, vannapurve@google.com, ackerleytng@google.com, mail@maciej.szmigiero.name, 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_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, keirf@google.com, linux-mm@kvack.org Subject: Re: Re: folio_mmapped Message-ID: References: <40a8fb34-868f-4e19-9f98-7516948fc740@redhat.com> <20240226105258596-0800.eberman@hu-eberman-lv.qualcomm.com> <925f8f5d-c356-4c20-a6a5-dd7efde5ee86@redhat.com> <755911e5-8d4a-4e24-89c7-a087a26ec5f6@redhat.com> <99a94a42-2781-4d48-8b8c-004e95db6bb5@redhat.com> <20240228103842643-0800.eberman@hu-eberman-lv.qualcomm.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240228103842643-0800.eberman@hu-eberman-lv.qualcomm.com> On Wednesday 28 Feb 2024 at 10:43:27 (-0800), Elliot Berman wrote: > On Wed, Feb 28, 2024 at 01:34:15PM +0000, Quentin Perret wrote: > > Alternatively, the shared->private conversion happens in the KVM vcpu > > run loop, so we'd be in a good position to exit the VCPU_RUN ioctl with a > > new exit reason saying "can't donate that page while it's shared" and > > have userspace use MADVISE_DONTNEED or munmap, or whatever on the back > > of that. But I tend to prefer the rmap option if it's workable as that > > avoids adding new KVM userspace ABI. > > > > You'll still probably need the new exit reason saying "can't donate that > page while it's shared" if the refcount tests fail. Can use David's > iouring as example of some other part of the kernel has a reference to > the page. I can't think of anything to do other than exiting to > userspace because we don't know how to drop that extra ref. Ack, I realized that later on. I guess there may be cases where userspace doesn't know how to drop that pin, but that's not the kernel's fault and it can't do anything about it, so a userspace exit is our best chance... Thanks, Quentin