diff --git a/src/unix/mmap.c b/src/unix/mmap.c
index 24ef9db81..9bb3567dc 100644
--- a/src/unix/mmap.c
+++ b/src/unix/mmap.c
@@ -92,7 +92,7 @@ closure_func_basic(thunk, void, pending_fault_complete)
     }
 }
 
-static pending_fault new_pending_fault_locked(process p, context ctx, u64 addr)
+pending_fault new_pending_fault_locked(process p, context ctx, u64 addr)
 {
     pending_fault pf;
     list l;
@@ -362,7 +362,7 @@ static status demand_page_internal(process p, context ctx, u64 vaddr, vmap vm, p
             anonymous = false;
             break;
         default:
-            halt("%s: invalid vmap type %d, flags 0x%lx\n", func_ss, mmap_type, vm->flags);
+            return vm->fault(p, ctx, vaddr, vm, pf);
         }
     } else if (vm->flags & VMAP_FLAG_PROG) {
         pf_debug("   file-backed program page fault\n");
@@ -1392,7 +1392,9 @@ static sysreturn mmap(void *addr, u64 length, int prot, int flags, int fd, u64 o
     if (fixed)
         process_remove_range_locked(p, q, vmap_mmap_type != VMAP_MMAP_TYPE_CUSTOM);
     k.node.r = q;
-    vmap_assert(allocate_vmap_locked(p->vmaps, &k) != INVALID_ADDRESS);
+    vmap vm = allocate_vmap_locked(p->vmaps, &k);
+    vmap_assert(vm != INVALID_ADDRESS);
+    vm->fault = k.fault;
     vmap_unlock(p);
 
     if (vmap_mmap_type == VMAP_MMAP_TYPE_FILEBACKED && (vmflags & VMAP_FLAG_SHARED))
diff --git a/src/unix/unix_internal.h b/src/unix/unix_internal.h
index 973152664..32498e01d 100644
--- a/src/unix/unix_internal.h
+++ b/src/unix/unix_internal.h
@@ -267,6 +267,7 @@ typedef struct pending_fault {
     enum {
         PENDING_FAULT_ANONYMOUS,
         PENDING_FAULT_FILEBACKED,
+        PENDING_FAULT_CUSTOM,
     } type;
     union {
         struct {
@@ -277,12 +278,15 @@ typedef struct pending_fault {
             closure_struct(pagecache_page_handler, demand_file_page);
             void *page_kvirt;
         } filebacked;
+        void *custom;
     };
     struct list l_free;
     closure_struct(thunk, async_handler);
     closure_struct(thunk, complete);
 } *pending_fault;
 
+pending_fault new_pending_fault_locked(process p, context ctx, u64 addr);
+
 /* XXX probably should bite bullet and allocate these... */
 #define FRAME_MAX_PADDED ((FRAME_MAX + 15) & ~15)
 
@@ -433,6 +437,7 @@ typedef struct vmap {
         fdesc fd;
         u64 bss_offset;
     };
+    status (*fault)(process p, context ctx, u64 vaddr, struct vmap *vm, pending_fault *pf);
 } *vmap;
 
 #define ivmap(__f, __af, __o, __c, __fd) (struct vmap) {    \