Re: [PATCH v11 10/16] KVM: guest_memfd: Add flag to remove from direct map

[Ackerley Tng <ackerleytng@google.com>]

"David Hildenbrand (Arm)" <david@kernel.org> writes:

>
> [...snip...]
>
>> +static int kvm_gmem_folio_zap_direct_map(struct folio *folio)
>> +{
>> +	u64 gmem_flags = GMEM_I(folio_inode(folio))->flags;
>> +	int r = 0;
>> +
>> +	if (kvm_gmem_folio_no_direct_map(folio) || !(gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP))
>
> The function is only called when
>
> 	kvm_gmem_no_direct_map(folio_inode(folio))
>
> Does it really make sense to check for GUEST_MEMFD_FLAG_NO_DIRECT_MAP again?
>

Good point that GUEST_MEMFD_FLAG_NO_DIRECT_MAP was already checked in
the caller. I think we can drop this second check.

> If, at all, it should be a warning if GUEST_MEMFD_FLAG_NO_DIRECT_MAP is
> not set?
>
> Further, kvm_gmem_folio_zap_direct_map() uses the folio lock to
> synchronize, right? Might be worth pointing that out somehow (e.g.,
> lockdep check if possible).
>
>> +		goto out;
>> +
>> +	r = folio_zap_direct_map(folio);
>> +	if (!r)
>> +		folio->private = (void *)((u64)folio->private | KVM_GMEM_FOLIO_NO_DIRECT_MAP);
>> +
>> +out:
>> +	return r;
>> +}
>> +
>> +static void kvm_gmem_folio_restore_direct_map(struct folio *folio)
>> +{
>
> kvm_gmem_folio_zap_direct_map() is allowed to be called on folios that
> already have the directmap remove, kvm_gmem_folio_restore_direct_map()
> cannot be called if the directmap was already restored.
>

This inconsistency was probably introduced by my comments [1] (sorry!)

I think the inconsistency here is mostly because
kvm_gmem_folio_zap_direct_map() is called from two places but restore is
only called from one place :P

[1] https://lore.kernel.org/all/CAEvNRgEzVhEzr-3GWTsE7GSBsPdvVLq7WFEeLHzcmMe=R9S51w@mail.gmail.com/

> Should we make that more consistent?
>
>
> Hoping Sean can find some time to review
>
> --
> Cheers,
>
> David