KVM: guest_memfd: Add helpers to query SHARED vs. PRIVATE for a given page

Add helpers to check if a given page in a guest_memfd instance is PRIVATE
versus SHARED, and use the "is shared" helper instead of an open-coded
equivalent in the user pagefault handler.  In addition to the immediate
usage, providing an "is private" helper will allow cleaning up the so
called prepare() code, and eventually will be heavily used once in-place
conversion support comes along.

No functional change intended.

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Reviewed-by: Fuad Tabba <fuad.tabba@linux.dev>
Link: https://patch.msgid.link/20260723210811.72720-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

virt/kvm/guest_memfd.c

@@ -60,6 +60,16 @@ static pgoff_t kvm_gmem_get_index(struct kvm_memory_slot *slot, gfn_t gfn)
 	return gfn - slot->base_gfn + slot->gmem.pgoff;
 }
 
+static bool kvm_gmem_is_private_mem(struct inode *inode, pgoff_t index)
+{
+	return !(GMEM_I(inode)->flags & GUEST_MEMFD_FLAG_INIT_SHARED);
+}
+
+static bool kvm_gmem_is_shared_mem(struct inode *inode, pgoff_t index)
+{
+	return !kvm_gmem_is_private_mem(inode, index);
+}
+
 static int __kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot,
 				    pgoff_t index, struct folio *folio)
 {
@@ -397,7 +407,7 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
 	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
 		return VM_FAULT_SIGBUS;
 
-	if (!(GMEM_I(inode)->flags & GUEST_MEMFD_FLAG_INIT_SHARED))
+	if (!kvm_gmem_is_shared_mem(inode, vmf->pgoff))
 		return VM_FAULT_SIGBUS;
 
 	folio = kvm_gmem_get_folio(inode, vmf->pgoff);