diff --git a/src/context/memory.rs b/src/context/memory.rs
index 7dc985b433..e3bb7dd34c 100644
--- a/src/context/memory.rs
+++ b/src/context/memory.rs
@@ -1602,6 +1602,12 @@ impl Grant {
                     }
                     // Cannot be shared and CoW simultaneously.
                     Err(AddRefError::SharedToCow) => {
+                        // The call to cow() later implicitly removes one ref, so add it here
+                        // first, even if Shared.
+                        if src_page_info.add_ref(RefKind::Shared) == Err(AddRefError::RcOverflow) {
+                            return Err(Enomem);
+                        }
+
                         // TODO: Copy in place, or use a zeroed page?
                         let CowResult { new_frame, old_frame } = cow(src_frame, src_page_info, rk).map_err(|_| Enomem)?;
                         if let Some(old_frame) = old_frame {
diff --git a/src/memory/mod.rs b/src/memory/mod.rs
index b791e50f24..145aa40cf4 100644
--- a/src/memory/mod.rs
+++ b/src/memory/mod.rs
@@ -643,7 +643,7 @@ pub fn init_mm(allocator: BumpAllocator<RmmA>) {
         THE_ZEROED_FRAME.get().write(Some((the_frame, the_info)));
     }
 }
-#[derive(Debug)]
+#[derive(Debug, PartialEq)]
 pub enum AddRefError {
     CowToShared,
     SharedToCow,
diff --git a/syscall b/syscall
index 5b07568677..1583e94276 160000
--- a/syscall
+++ b/syscall
@@ -1 +1 @@
-Subproject commit 5b075686774ef88374d93834c54e339d456a8ec2
+Subproject commit 1583e94276dbd84f4e23f044accadef5bb28ad6b