diff --git a/src/arena.c b/src/arena.c
index abc8b7ec..57ef5f60 100644
--- a/src/arena.c
+++ b/src/arena.c
@@ -60,11 +60,11 @@ typedef struct mi_arena_s {
   size_t   field_count;                   // number of bitmap fields (where `field_count * MI_BITMAP_FIELD_BITS >= block_count`)
   int      numa_node;                     // associated NUMA node
   bool     is_zero_init;                  // is the arena zero initialized?
-  bool     is_committed;                  // is the memory fully committed? (if so, block_committed == NULL)
+  bool     allow_decommit;                // is decommit allowed? if true, is_large should be false and blocks_committed != NULL
   bool     is_large;                      // large- or huge OS pages (always committed)
   _Atomic(uintptr_t) search_idx;          // optimization to start the search for free blocks
   mi_bitmap_field_t* blocks_dirty;        // are the blocks potentially non-zero?
-  mi_bitmap_field_t* blocks_committed;    // if `!is_committed`, are the blocks committed?
+  mi_bitmap_field_t* blocks_committed;    // are the blocks committed? (can be NULL for memory that cannot be decommitted)
   mi_bitmap_field_t  blocks_inuse[1];     // in-place bitmap of in-use blocks (of size `field_count`)
 } mi_arena_t;
 
@@ -127,8 +127,8 @@ static mi_decl_noinline void* mi_arena_alloc_from(mi_arena_t* arena, size_t aren
   *memid     = mi_arena_id_create(arena_index, bitmap_index);
   *is_zero   = _mi_bitmap_claim_across(arena->blocks_dirty, arena->field_count, needed_bcount, bitmap_index, NULL);
   *large     = arena->is_large;
-  *is_pinned = (arena->is_large || arena->is_committed);
-  if (arena->is_committed) {
+  *is_pinned = (arena->is_large || !arena->allow_decommit);
+  if (arena->blocks_committed == NULL) {
     // always committed
     *commit = true;
   }
@@ -260,7 +260,7 @@ void _mi_arena_free(void* p, size_t size, size_t memid, bool all_committed, mi_o
       return;
     }
     // potentially decommit
-    if (arena->is_committed) {
+    if (!arena->allow_decommit || arena->blocks_committed == NULL) {
       mi_assert_internal(all_committed); // note: may be not true as we may "pretend" to be not committed (in segment.c)
     }
     else {
@@ -315,12 +315,16 @@ bool mi_manage_os_memory(void* start, size_t size, bool is_committed, bool is_la
   arena->numa_node    = numa_node; // TODO: or get the current numa node if -1? (now it allows anyone to allocate on -1)
   arena->is_large     = is_large;
   arena->is_zero_init = is_zero;
-  arena->is_committed = is_committed;
+  arena->allow_decommit = !is_large && !is_committed; // only allow decommit for initially uncommitted memory
   arena->search_idx   = 0;
   arena->blocks_dirty = &arena->blocks_inuse[fields]; // just after inuse bitmap
-  arena->blocks_committed = (is_committed ? NULL : &arena->blocks_inuse[2*fields]); // just after dirty bitmap
+  arena->blocks_committed = (!arena->allow_decommit ? NULL : &arena->blocks_inuse[2*fields]); // just after dirty bitmap
   // the bitmaps are already zero initialized due to os_alloc
-  // just claim leftover blocks if needed
+  // initialize committed bitmap?
+  if (arena->blocks_committed != NULL && is_committed) {
+    memset(arena->blocks_committed, 0xFF, fields*sizeof(mi_bitmap_field_t));
+  }
+  // and claim leftover blocks if needed (so we never allocate there)
   ptrdiff_t post = (fields * MI_BITMAP_FIELD_BITS) - bcount;
   mi_assert_internal(post >= 0);
   if (post > 0) {
diff --git a/src/init.c b/src/init.c
index 76c20842..e957559e 100644
--- a/src/init.c
+++ b/src/init.c
@@ -526,7 +526,7 @@ void mi_process_init(void) mi_attr_noexcept {
   } 
   if (mi_option_is_enabled(mi_option_reserve_os_memory)) {
     long ksize = mi_option_get(mi_option_reserve_os_memory);
-    if (ksize > 0) mi_reserve_os_memory((size_t)ksize*KiB, true, true);
+    if (ksize > 0) mi_reserve_os_memory((size_t)ksize*KiB, true /* commit? */, true /* allow large pages? */);
   }
 }