diff --git a/src/backend/storage/buffer/localbuf.c b/src/backend/storage/buffer/localbuf.c
index 1be4f4f8da..1f02fed250 100644
--- a/src/backend/storage/buffer/localbuf.c
+++ b/src/backend/storage/buffer/localbuf.c
@@ -374,6 +374,9 @@ ExtendBufferedRelLocal(BufferManagerRelation bmr,
 		victim_buf_id = -buffers[i] - 1;
 		victim_buf_hdr = GetLocalBufferDescriptor(victim_buf_id);
 
+		/* in case we need to pin an existing buffer below */
+		ResourceOwnerEnlarge(CurrentResourceOwner);
+
 		InitBufferTag(&tag, &bmr.smgr->smgr_rlocator.locator, fork, first_block + i);
 
 		hresult = (LocalBufferLookupEnt *)
@@ -646,6 +649,8 @@ InitLocalBuffers(void)
  * XXX: We could have a slightly more efficient version of PinLocalBuffer()
  * that does not support adjusting the usagecount - but so far it does not
  * seem worth the trouble.
+ *
+ * Note that ResourceOwnerEnlarge() must have been done already.
  */
 bool
 PinLocalBuffer(BufferDesc *buf_hdr, bool adjust_usagecount)
diff --git a/src/backend/utils/resowner/resowner.c b/src/backend/utils/resowner/resowner.c
index 67cc332e80..aa199b23ff 100644
--- a/src/backend/utils/resowner/resowner.c
+++ b/src/backend/utils/resowner/resowner.c
@@ -548,8 +548,13 @@ ResourceOwnerRemember(ResourceOwner owner, Datum value, const ResourceOwnerDesc
 /*
  * Forget that an object is owned by a ResourceOwner
  *
- * Note: if same resource ID is associated with the ResourceOwner more than
+ * Note: If same resource ID is associated with the ResourceOwner more than
  * once, one instance is removed.
+ *
+ * Note: Forgetting a resource does not guarantee that there is room to
+ * remember a new resource.  One exception is when you forget the most
+ * recently remembered resource; that does make room for a new remember call.
+ * Some code callers rely on that exception.
  */
 void
 ResourceOwnerForget(ResourceOwner owner, Datum value, const ResourceOwnerDesc *kind)