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[34.127.75.64]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3ada4896c4dsm1314919a91.7.2026.10.21.16.02.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 21 Oct 2026 16:02:18 -0700 (PDT) Date: Wed, 21 Oct 2026 23:02:14 +0000 From: David Matlack To: Pasha Tatashin Cc: Tarun Sahu , Andrew Morton , joro@8bytes.org, Mike Rapoport , Pratyush Yadav , maz@kernel.org, arnd@arndb.de, jgg@nvidia.com, robin.murphy@arm.com, loganodell@google.com, oupton@kernel.org, kevin.tian@intel.com, alex@shazbot.org, dwmw2@infradead.org, pbonzini@redhat.com, seanjc@google.com, will@kernel.org, Alexander Graf , Bjorn Helgaas , baolu.lu@linux.intel.com, linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org, sagis@google.com, linux-mm@kvack.org, kvmarm@lists.linux.dev, iommu@lists.linux.dev, linux-pci@vger.kernel.org, kexec@lists.infradead.org, skhawaja@google.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH v2 2/4] liveupdate: Select file handler by name in the preserve ioctl Message-ID: References: <20261021152527.375070-1-tarunsahu@google.com> <20261021152527.375070-3-tarunsahu@google.com> X-Mailing-List: kvm@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On 2026-10-21 01:17 PM, Pasha Tatashin wrote: > On Wed, Oct 21, 2026 at 11:25 AM Tarun Sahu wrote: > > > > Currently luo_preserve_file() iterates over luo_file_handler_list and > > picks the first registered handler whose .can_preserve() callback > > returns true. When a subsystem introduces a new, backwards-incompatible > > file handler alongside an existing one for the same file type, userspace > > has no way to choose which handler preserves a file descriptor, nor can > > the kernel keep the existing rollback-safe handler as the default while > > allowing callers to opt into the new handler. > > That is not how it will work. If we are in the .can_preserve() call, > it means the user has already decided which file handlers to use. We > need to do that before any preservation happens; otherwise, we could > run into conflicts on FLBs i.e. FH of different version require > different FLBs. > > Therefore, there should be a luoctl call (i.e via a new ioctl to set > compatability) that provides the kernel with the specs file of the > target kernel we are updating to, allowing the kernel to select the > compatible file handlers. Since .can_preserve() is called when the > user is already preserving files, the specification must have already > been supplied, or the kernel should fall back to using the default > (i.e., latest) file handlers. I don't think that is the right direction. IMO the kernel should not be choosing which file handler to use, userspace should be. If userspace gets it wrong and chooses a file handler the next kernel does not support, that's on them. The problem with the kernel making a decision about file handlers is then userspace does not know what arguments to pass in. We are adding custom per-file handler arguments to PRESERVE_FD and RETRIEVE_FD. If the kernel is picking which file handler to use, say for a memfd, then userspace doesn't know which arguments to pass in. Sure you could add some method where userspace could query which file handlers are active and decide arguments based on that. But it feels backwards. I think a much cleaner solution is that userspace chooses file handler during PRESERVE_FD. I would even require it for RETRIEVE_FD so that we are guaranteed that the args the user is passing in are being sent to the right file handler. Passing to RETRIEVE_FD also opens the door to transforming across Live Update (similar to how AWS did LU from Xen to KVM as discussed at LPC). Regarding FLB conflicts, today we don't support multiple FLBs for the same global state, so there is nothing to conflict. Selecting a file handler during PRESERVE_FD doesn't change that. If we ever need to support multiple FLBs for the same global state (e.g. an incompatible change to the PCI FLB), we can add an ioctl that lets userspace select which FLB to use before preserving any files. After that, only file handlers registered with the selected FLB can be used to preserve files. PRESERVE_FD with any other file handler fails. That keeps the FLB decision up front and global, like you want, while still letting userspace pick the file handler for each fd. I'd also expect this to be rare, since FLB serialized state can use the same feature flags as everything else in this series.