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

[tarunsahu@google.com]

Sean Christopherson <seanjc@google.com> writes:

> 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.

Ohh okay. I was very skeptic that per-cpu structure are not counted in
cgroups and liveupdate is once in a while usecase So avoided making it
per-cpu. But otherwise I am fine with going with per-cpu. Will take care
of it in next revision.

Thanks
~Tarun