Re: [PATCH v9 07/41] KVM: guest_memfd: Wire up core private/shared attribute interfaces

[Sean Christopherson <seanjc@google.com>]

On Fri, Jul 31, 2026, Xiaoyao Li wrote:
> On 7/31/2026 4:42 AM, Ackerley Tng wrote:
> > Xiaoyao Li <xiaoyao.li@intel.com> writes:
> > 
> > > On 7/29/2026 8:35 AM, Ackerley Tng via B4 Relay wrote:
> > > > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> > > > index 33c9830190e2e..89cf922232920 100644
> > > > --- a/virt/kvm/guest_memfd.c
> > > > +++ b/virt/kvm/guest_memfd.c
> > > > @@ -893,6 +893,27 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> > > >    EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_gmem_get_pfn);
> > > > 
> > > >    #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_POPULATE
> > > > +static bool kvm_range_is_private(struct file *file, pgoff_t index,
> > > > +				 size_t nr_pages, struct kvm *kvm, gfn_t gfn)
> > > > +{
> > > > +	struct inode *inode = file_inode(file);
> > > > +	pgoff_t last = index + nr_pages - 1;
> > > > +	struct maple_tree *mt;
> > > > +	void *entry;
> > > > +
> > > > +	if (!gmem_in_place_conversion)
> > > > +		return kvm_range_has_vm_memory_attributes(kvm, gfn, gfn + nr_pages,
> > > > +							  KVM_MEMORY_ATTRIBUTE_PRIVATE,
> > > > +							  KVM_MEMORY_ATTRIBUTE_PRIVATE);
> > > > +
> > > > +	mt = &GMEM_I(inode)->attributes;
> > > > +	mt_for_each(mt, entry, index, last) {
> > > > +		if (kvm_gmem_interpret_entry(inode, entry) !=
> > > > +		    KVM_MEMORY_ATTRIBUTE_PRIVATE)
> > > > +			return false;
> > > > +	}
> > > > +	return true;
> > > > +}
> > > > 
> > > >    static long __kvm_gmem_populate(struct kvm *kvm, struct kvm_memory_slot *slot,
> > > >    				struct file *file, gfn_t gfn, struct page *src_page,
> > > > @@ -913,9 +934,7 @@ static long __kvm_gmem_populate(struct kvm *kvm, struct kvm_memory_slot *slot,
> > > > 
> > > >    	folio_unlock(folio);
> > > > 
> > > > -	if (!kvm_range_has_vm_memory_attributes(kvm, gfn, gfn + 1,
> > > > -						KVM_MEMORY_ATTRIBUTE_PRIVATE,
> > > > -						KVM_MEMORY_ATTRIBUTE_PRIVATE)) {
> > > > +	if (!kvm_range_is_private(file, index, 1, kvm, gfn)) {
> > > 
> > > It's checking if a single gfn is private.
> > > 
> > 
> > But with huge page support we'd need to check a range of gfns... I guess
> > there's the argument of not keeping complexity for the future, but in
> > this case it's undoing functionality for this series and probably adding
> > it back later.
> 
> let's leave the work for huge page support in the future.

It's not just for hugepage support, the core functionality is used in this series
by __kvm_gmem_set_attributes() in:

  KVM: guest_memfd: Return early if range already has requested attributes

> > > We can just use kvm_mem_is_private()? And it seems can be a separate patch.
> > > 
> > 
> > Do you mean that this refactoring could be a separate patch? I could
> > refactor out kvm_range_is_private() in a separate patch, then put in the
> > mt_for_each() check in this patch together as part of all the rest of
> > the "wiring".
> > 
> > I considered that but it seemed like the refactoring was too small to
> > separate out into another patch when the mt_for_each() part is going to
> > be in this patch anyway.
> > 
> > Or, I could add an earlier patch to first replace the call to
> > kvm_range_has_vm_memory_attributes() with a call to check just 1 gfn,
> > then this wiring patch would be simpler.
> > 
> 
> This is what I meant. An earlier patch to just replace
> kvm_range_has_vm_memory_attributes() with kvm_mem_is_private(). Then we can
> drop this 'big' diff in this patch.
> 
> If kvm_range_is_private() is useful/required by huge page support, then let
> the huge page series to introduce it. It has nothing to do with the in-place
> conversion series.

If it weren't for the fact that the range-based search is used later in this
series, I would 100% agree with Xiaoyao.  But since the core logic is used and
needed elsewhere, and because kvm_range_has_vm_memory_attributes() takes a range,
my vote is to provide the plumbing now, even though a small portion of it isn't
strictly necessary.