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Mon, 10 Aug 2026 16:44:59 -0700 (PDT) Date: Mon, 10 Aug 2026 16:44:58 -0700 In-Reply-To: <20260728121138.1103610-8-tarunsahu@google.com> Mime-Version: 1.0 References: <20260728121138.1103610-1-tarunsahu@google.com> <20260728121138.1103610-8-tarunsahu@google.com> Message-ID: Subject: Re: [PATCH v4 07/11] KVM: guest_memfd: Add support for freezing mappings From: Sean Christopherson To: Tarun Sahu Cc: ackerleytng@google.com, fuad.tabba@linux.dev, Andrew Morton , dmatlack@google.com, Shuah Khan , Jonathan Corbet , david@redhat.com, Pasha Tatashin , Pratyush Yadav , sagis@google.com, Paolo Bonzini , Mike Rapoport , Alexander Graf , linux-kselftest@vger.kernel.org, andre.przywara@arm.com, michael.roth@amd.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, will@kernel.org, vannapurve@google.com, maz@kernel.org, fvdl@google.com, kvm@vger.kernel.org, oliver.upton@linux.dev, kvmarm@lists.linux.dev, alexandru.elisei@arm.com, skhawaja@google.com, aneesh.kumar@kernel.org, linux-doc@vger.kernel.org, David Hildenbrand , yan.y.zhao@intel.com, kexec@lists.infradead.org, suzuki.poulose@arm.com Content-Type: text/plain; charset="us-ascii" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260810_164501_072871_F225692E X-CRM114-Status: GOOD ( 12.39 ) X-BeenThere: kexec@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "kexec" Errors-To: kexec-bounces+kexec=archiver.kernel.org@lists.infradead.org On Tue, Jul 28, 2026, Tarun Sahu wrote: > +/** > + * kvm_gmem_freeze - Freeze or unfreeze a guest_memfd inode mapping. > + * @inode: The guest_memfd inode. > + * @freeze: True to freeze, false to unfreeze. > + * > + * This API is used strictly during the live update / preservation transition > + * window to prevent host userspace and guest-side faults from making any > + * mapping modifications (such as fallocate or page fault allocation) > + * to the guest_memfd page cache. > + * > + * Synchronization Strategy (Sleepable RCU): > + * To avoid high-contention VFS locks (like inode_lock or > + * filemap_invalidate_lock) on the vCPU page fault hot paths, this subsystem > + * implements a lightweight, system-wide Sleepable RCU (SRCU) mechanism > + * (`kvm_gmem_freeze_srcu`): > + * > + * Global vs. Per-Inode SRCU > + * ====================== > + * A single system-wide global static `srcu_struct` is used instead of a > + * per-inode SRCU structure to completely prevent unprivileged users from > + * exhausting the host's per-CPU memory allocator. This argument doesn't hold up given that each kvm structure has two SRCU structs.