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[35.203.150.192]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2ea7b34dadbsm1687675ad.26.2026.10.22.10.57.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 22 Oct 2026 10:57:18 -0700 (PDT) Date: Thu, 22 Oct 2026 17:57:15 +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: kvmarm@lists.linux.dev 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 09:14 PM, Pasha Tatashin wrote: > On Wed, Oct 21, 2026 at 7:02 PM David Matlack wrote: > > > > 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. > > It is not the kernel deciding on its own which handler to use; it is > the system admin (or host orchestrator) that configures LUO before the > LU process starts, because the sysadmin knows what kernel we are > rebooting into and the LUO specs of that target kernel. Meanwhile, the > process calling PRESERVE_FD (e.g., a VMM) only knows about its own FDs > and should not have to track target kernel serialization versions. > > Also, the PRESERVE_FD and RETRIEVE_FD arguments are uABI tied to the > FD type, not to a specific internal handler version. They are stable, > and we can expand them the same way any ioctl argument is expanded > (via size and flags), without ever shrinking or breaking them. We > should not play the game of providing completely different uABI > argument structs for different versions of a file handler. If a caller > passes a new feature in the uABI args that the currently configured > file handler version does not support, PRESERVE_FD simply returns > -EOPNOTSUPP, just like any other extensible ioctl. If we require that PRESERVE_FD and RETRIEVE_FD uAPI is stable across different file handlers for the same file type, then my concern is resolved. > More generally, we are rebooting the host into a single target kernel, > so compatibility is a session-level property, not a per-FD property. > If we need an upfront global ioctl anyway, configuring the target > compatibility upfront in one place is much cleaner than exposing FLBs > to userspace and splitting version selection between a global ioctl > and per-FD PRESERVE_FD calls. That's a good point. Ok let's not do per-FD FH selection.