diff --git a/src/arena.c b/src/arena.c
index 7f472606..2fafd0bf 100644
--- a/src/arena.c
+++ b/src/arena.c
@@ -237,7 +237,9 @@ static mi_decl_noinline void* mi_arena_try_alloc_at(mi_arena_t* arena, size_t ar
   }
 
   // set the dirty bits (todo: no need for an atomic op here?)
-  memid->was_zero = _mi_bitmap_claim_across(arena->blocks_dirty, arena->field_count, needed_bcount, bitmap_index, NULL);
+  if (arena->memid.was_zero && arena->blocks_dirty != NULL) {
+    memid->was_zero = _mi_bitmap_claim_across(arena->blocks_dirty, arena->field_count, needed_bcount, bitmap_index, NULL);
+  }
 
   // set commit state
   if (arena->blocks_committed == NULL) {
@@ -373,7 +375,7 @@ void* _mi_arena_alloc_aligned(size_t size, size_t alignment, size_t align_offset
   // try to allocate in an arena if the alignment is small enough and the object is not too small (as for heap meta data)
   if (size >= MI_ARENA_MIN_OBJ_SIZE && alignment <= MI_SEGMENT_ALIGN && align_offset == 0) {
     void* p = mi_arena_try_alloc(numa_node, size, alignment, commit, allow_large, req_arena_id, memid, tld);
-    if (p != NULL) return p;
+    if (p != NULL) return p;    
 
     // otherwise, try to first eagerly reserve a new arena 
     if (req_arena_id == _mi_arena_id_none()) {
@@ -399,7 +401,7 @@ void* _mi_arena_alloc_aligned(size_t size, size_t alignment, size_t align_offset
   }
   else {
     return _mi_os_alloc_aligned(size, alignment, commit, allow_large, memid, tld->stats);
-  }
+  }  
 }
 
 void* _mi_arena_alloc(size_t size, bool commit, bool allow_large, mi_arena_id_t req_arena_id, mi_memid_t* memid, mi_os_tld_t* tld)
diff --git a/src/os.c b/src/os.c
index 88204a9d..48d25446 100644
--- a/src/os.c
+++ b/src/os.c
@@ -144,6 +144,7 @@ static void mi_os_prim_free(void* addr, size_t size, bool still_committed, mi_st
   mi_assert_internal((size % _mi_os_page_size()) == 0);
   if (addr == NULL || size == 0) return; // || _mi_os_is_huge_reserved(addr)
   int err = _mi_prim_free(addr, size);
+  mi_track_mem_noaccess(addr,size);
   if (err != 0) {
     _mi_warning_message("unable to free OS memory (error: %d (0x%x), size: 0x%zx bytes, address: %p)\n", err, err, size, addr);
   }
@@ -207,7 +208,12 @@ static void* mi_os_prim_alloc(size_t size, size_t try_alignment, bool commit, bo
     _mi_stat_increase(&stats->reserved, size);
     if (commit) { 
       _mi_stat_increase(&stats->committed, size); 
-      mi_track_mem_defined(p,size); // seems needed for asan (or `mimalloc-test-api` fails)
+      // seems needed for asan (or `mimalloc-test-api` fails)
+      if (*is_zero) { mi_track_mem_defined(p,size); }   
+               else { mi_track_mem_undefined(p,size); }
+    }
+    else {
+      mi_track_mem_noaccess(p,size);
     }
   }
   return p;
@@ -272,7 +278,7 @@ static void* mi_os_prim_alloc_aligned(size_t size, size_t alignment, bool commit
       if (post_size > 0) { mi_os_prim_free((uint8_t*)aligned_p + mid_size, post_size, commit, stats); }
       // we can return the aligned pointer on `mmap` (and sbrk) systems
       p = aligned_p;
-      *base = aligned_p; // since we freed the pre part, `*base == p`.
+      *base = aligned_p; // since we freed the pre part, `*base == p`.      
     }
   }
 
@@ -406,7 +412,10 @@ bool _mi_os_commit(void* addr, size_t size, bool* is_zero, mi_stats_t* tld_stats
   if (os_is_zero && is_zero != NULL) { 
     *is_zero = true;
     mi_assert_expensive(mi_mem_is_zero(start, csize));
-  } 
+  }
+  // note: the following seems required for asan (otherwise `mimalloc-test-stress` fails)
+  if (os_is_zero) { mi_track_mem_defined(start,csize); }
+             else { mi_track_mem_undefined(start,csize); }
   return true;
 }
 
@@ -428,6 +437,7 @@ static bool mi_os_decommit_ex(void* addr, size_t size, bool* needs_recommit, mi_
     _mi_warning_message("cannot decommit OS memory (error: %d (0x%x), address: %p, size: 0x%zx bytes)\n", err, err, start, csize);
   }
   mi_assert_internal(err == 0);
+  mi_track_mem_noaccess(start,csize);
   return (err == 0);
 }
 
@@ -457,9 +467,7 @@ bool _mi_os_reset(void* addr, size_t size, mi_stats_t* stats) {
   if (err != 0) {
     _mi_warning_message("cannot reset OS memory (error: %d (0x%x), address: %p, size: 0x%zx bytes)\n", err, err, start, csize);
   }
-  else {
-    mi_track_mem_undefined(start, csize);
-  }
+  mi_track_mem_undefined(start,csize);
   return (err == 0);
 }
 
@@ -508,6 +516,8 @@ static  bool mi_os_protectx(void* addr, size_t size, bool protect) {
   if (err != 0) {
     _mi_warning_message("cannot %s OS memory (error: %d (0x%x), address: %p, size: 0x%zx bytes)\n", (protect ? "protect" : "unprotect"), err, err, start, csize);
   }
+  if (protect) { mi_track_mem_noaccess(start,csize); }
+          else { mi_track_mem_undefined(start,csize); }
   return (err == 0);
 }
 
@@ -630,6 +640,8 @@ void* _mi_os_alloc_huge_os_pages(size_t pages, int numa_node, mi_msecs_t max_mse
     *memid = _mi_memid_create_os(true /* is committed */, all_zero, true /* is_large */);
     memid->memkind = MI_MEM_OS_HUGE;
     mi_assert(memid->is_pinned);
+    if (all_zero) { mi_track_mem_defined(start,size); }
+             else { mi_track_mem_undefined(start,size); }
   }
   return (page == 0 ? NULL : start);
 }
diff --git a/src/prim/unix/prim.c b/src/prim/unix/prim.c
index 5da5f96b..71f16915 100644
--- a/src/prim/unix/prim.c
+++ b/src/prim/unix/prim.c
@@ -190,7 +190,11 @@ static void* unix_mmap_prim(void* addr, size_t size, size_t try_alignment, int p
     if (hint != NULL) {
       p = mmap(hint, size, protect_flags, flags, fd, 0);
       if (p==MAP_FAILED || !_mi_is_aligned(p,try_alignment)) { 
+        #if MI_TRACK_ENABLED  // asan sometimes does not instrument errno correctly?
+        int err = 0;
+        #else
         int err = errno;
+        #endif
         _mi_warning_message("unable to directly request hinted aligned OS memory (error: %d (0x%x), size: 0x%zx bytes, alignment: 0x%zx, hint address: %p)\n", err, err, size, try_alignment, hint);
       }
       if (p!=MAP_FAILED) return p;
diff --git a/src/segment.c b/src/segment.c
index a08fd3de..442b187a 100644
--- a/src/segment.c
+++ b/src/segment.c
@@ -878,12 +878,17 @@ static mi_segment_t* mi_segment_alloc(size_t required, size_t page_alignment, mi
   if (segment == NULL) return NULL;
   
   // zero the segment info? -- not always needed as it may be zero initialized from the OS 
+  ptrdiff_t ofs    = offsetof(mi_segment_t, next);
+  size_t    prefix = offsetof(mi_segment_t, slices) - ofs;
+  size_t    zsize  = prefix + (sizeof(mi_slice_t) * (segment_slices + 1)); // one more  
   if (!segment->memid.was_zero) {
-    ptrdiff_t ofs    = offsetof(mi_segment_t, next);
-    size_t    prefix = offsetof(mi_segment_t, slices) - ofs;
-    size_t    zsize  = prefix + (sizeof(mi_slice_t) * (segment_slices + 1)); // one more
     _mi_memzero((uint8_t*)segment + ofs, zsize);
   }
+  else {
+    mi_track_mem_defined((uint8_t*)segment + ofs,zsize);
+    mi_assert(mi_mem_is_zero((uint8_t*)segment + ofs, zsize));
+  }
+
   
   // initialize the rest of the segment info
   const size_t slice_entries = (segment_slices > MI_SLICES_PER_SEGMENT ? MI_SLICES_PER_SEGMENT : segment_slices);