Patch from Nathan outlined with a crash the mishandling of the case where CMA is requested but not available.

opal/mca/btl/sm/btl_sm_component.c

@@ -903,6 +903,16 @@ mca_btl_sm_component_init(int *num_btls,
            so no problems with accidentally overwriting this set earlier */
         mca_btl_sm.super.btl_get = mca_btl_sm_get_sync;
     }
+#else
+    /* If the user explicitly asked for CMA and we can't provide itm
+     *   error */
+    if (mca_btl_sm_component.use_cma > 0) {
+        mca_btl_sm.super.btl_flags &= ~MCA_BTL_FLAGS_GET;
+        opal_show_help("help-mpi-btl-sm.txt",
+                       "CMA requested but not available",
+                       true, opal_process_info.nodename);
+        return NULL;
+    }
 #endif /* OPAL_BTL_SM_HAVE_CMA */
 
 #if OPAL_CUDA_SUPPORT

opal/mca/btl/sm/help-mpi-btl-sm.txt

@@ -90,6 +90,17 @@ mca_btl_sm_use_knem MCA parameter, but Knem support was either not
 compiled into this Open MPI installation, or Knem support was unable
 to be activated in this process.
 
+The shared memory BTL will now deactivate itself, likely resulting in
+lower performance for on-node communication.
+
+  Local host: %s
+#
+[CMA requested but not available]
+WARNING: Linux kernel CMA support was requested via the
+mca_btl_sm_use_cma MCA parameter, but CMA support was either not
+compiled into this Open MPI installation, or CMA support was unable
+to be activated in this process.
+
 The shared memory BTL will now deactivate itself, likely resulting in
 lower performance for on-node communication.