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From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private
 memory
Message-ID: <179fdb45-d8a4-9567-edfe-2168794f599e@redhat.com>
Date:   Thu, 16 Sep 2021 11:24:05 +0200
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On 15/09/21 16:11, Kirill A. Shutemov wrote:
>> Would introducing memfd_unregister_guest() fix this?
> I considered this, but it get complex quickly.
> 
> At what point it gets called? On KVM memslot destroy?
> 
> What if multiple KVM slot share the same memfd? Add refcount into memfd on
> how many times the owner registered the memfd?

You will always have multiple KVM slots sharing the same memfd, because 
memslots are SRCU-protected.  So there will be multiple generations of 
memslots around and unregistering must be delayed to after 
synchronize_srcu (around the call to kvm_arch_commit_memory_region).

So KVM could just call memfd_{,un}register_guest as many times as it 
calls fdget/fput.  Looking at your test device, it would be like the 
following pseudo-patch:

	case GUEST_MEM_REGISTER: {
		struct fd memfd = fdget(arg);
		memfd_file = memfd.file;
		return memfd_register_guest(memfd_file->f_inode, file,
					    &guest_ops, &guest_mem_ops);
	}
	case GUEST_MEM_UNREGISTER: {
		if (!memfd_file)
			return -EINVAL;

+		memfd_unregister_guest(memfd_file->f_inode, file);
		fput(memfd_file);
		memfd_file = NULL;
		guest_mem_ops = NULL;
		return 0;

and shmem_unregister_guest would be something like

	struct shmem_inode_info *info = SHMEM_I(inode);

	if (WARN_ON_ONCE(info->guest_owner != owner))
		return;
	if (--info->guest_usage_count)
		return;
	info->guest_owner = NULL;
	info->guest_ops = NULL;

Paolo

> It would leave us in strange state: memfd refcount owners (struct KVM) and
> KVM memslot pins the struct file. Weird refcount exchnage program.