KVM: guest_memfd: delay folio_mark_uptodate() until after successful preparation

The up-to-date flag as is now is not too useful; it tells guest_memfd not
to overwrite the contents of a folio, but it doesn't say that the page
is ready to be mapped into the guest.  For encrypted guests, mapping
a private page requires that the "preparation" phase has succeeded,
and at the same time the same page cannot be prepared twice.

So, ensure that folio_mark_uptodate() is only called on a prepared page.  If
kvm_gmem_prepare_folio() or the post_populate callback fail, the folio
will not be marked up-to-date; it's not a problem to call clear_highpage()
again on such a page prior to the next preparation attempt.
Reviewed-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/guest_memfd.c

@@ -73,8 +73,6 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index, bool
 
 		for (i = 0; i < nr_pages; i++)
 			clear_highpage(folio_page(folio, i));
-
-		folio_mark_uptodate(folio);
 	}
 
 	if (prepare) {
@@ -84,6 +82,8 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index, bool
 			folio_put(folio);
 			return ERR_PTR(r);
 		}
+
+		folio_mark_uptodate(folio);
 	}
 
 	/*
@@ -646,6 +646,8 @@ long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long
 
 		p = src ? src + i * PAGE_SIZE : NULL;
 		ret = post_populate(kvm, gfn, pfn, p, max_order, opaque);
+		if (!ret)
+			folio_mark_uptodate(folio);
 
 		folio_put(folio);
 		if (ret)