Re: [PATCH v4 07/11] KVM: guest_memfd: Add support for freezing mappings

[Sean Christopherson <seanjc@google.com>]

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.