diff --git a/include/mimalloc/internal.h b/include/mimalloc/internal.h
index 535fe1fb..e43d4420 100644
--- a/include/mimalloc/internal.h
+++ b/include/mimalloc/internal.h
@@ -597,19 +597,6 @@ static inline mi_heap_t* mi_page_heap(const mi_page_t* page) {
   return page->heap;
 }
 
-static inline void mi_page_set_heap(mi_page_t* page, mi_heap_t* heap) {
-  if (heap != NULL) {
-    page->heap = heap;
-    page->heap_tag = heap->tag;
-    mi_atomic_store_release(&page->xthread_id, heap->tld->thread_id);
-  }
-  else {
-    page->heap = NULL;
-    mi_atomic_store_release(&page->xthread_id,0);
-  }
-}
-
-
 // Thread free flag helpers
 static inline mi_block_t* mi_tf_block(mi_thread_free_t tf) {
   return (mi_block_t*)(tf & ~1);
@@ -700,11 +687,11 @@ static inline bool mi_page_is_used_at_frac(const mi_page_t* page, uint16_t n) {
 
 static inline bool mi_page_is_abandoned(const mi_page_t* page) {
   // note: the xheap field of an abandoned heap is set to the subproc (for fast reclaim-on-free)
-  return (mi_page_xthread_id(page) <= MI_PAGE_IS_ABANDONED_MAPPED);
+  return (mi_page_thread_id(page) == 0);
 }
 
 static inline bool mi_page_is_abandoned_mapped(const mi_page_t* page) {
-  return (mi_page_xthread_id(page) == MI_PAGE_IS_ABANDONED_MAPPED);
+  return ((mi_page_xthread_id(page) & ~(MI_PAGE_IS_ABANDONED_MAPPED - 1)) == MI_PAGE_IS_ABANDONED_MAPPED);
 }
 
 static inline void mi_page_set_abandoned_mapped(mi_page_t* page) {
@@ -801,6 +788,21 @@ static inline void mi_page_set_has_aligned(mi_page_t* page, bool has_aligned) {
   mi_page_flags_set(page, has_aligned, MI_PAGE_HAS_ALIGNED);
 }
 
+static inline void mi_page_set_heap(mi_page_t* page, mi_heap_t* heap) {
+  mi_assert_internal(!mi_page_is_in_full(page));
+  const mi_page_flags_t flags = mi_page_flags(page);
+  const mi_threadid_t tid = (heap != NULL ? heap->tld->thread_id : 0) | flags;  // for MI_PAGE_HAS_ALIGNED
+  if (heap != NULL) {
+    page->heap = heap;
+    page->heap_tag = heap->tag;
+  }
+  else {
+    page->heap = NULL;
+  }
+  mi_atomic_store_release(&page->xthread_id, tid);
+}
+
+
 /* -------------------------------------------------------------------
   Guarded objects
 ------------------------------------------------------------------- */
diff --git a/include/mimalloc/types.h b/include/mimalloc/types.h
index 7e968e10..2a1702ff 100644
--- a/include/mimalloc/types.h
+++ b/include/mimalloc/types.h
@@ -248,7 +248,7 @@ typedef struct mi_block_s {
 // `is_abandoned_mapped` is true if the page is abandoned (thread_id==0) and it is in an arena so can be quickly found for reuse ("mapped")
 #define MI_PAGE_IN_FULL_QUEUE         MI_ZU(0x01)
 #define MI_PAGE_HAS_ALIGNED           MI_ZU(0x02)
-#define MI_PAGE_IS_ABANDONED_MAPPED   MI_ZU(0x04)
+#define MI_PAGE_IS_ABANDONED_MAPPED   MI_ZU(0x04)   // must be highest flag (see `internal.h:mi_page_is_abandoned_mapped`)
 #define MI_PAGE_FLAG_MASK             MI_ZU(0x07)
 typedef size_t mi_page_flags_t;
 
diff --git a/src/arena.c b/src/arena.c
index bcde865e..e111a417 100644
--- a/src/arena.c
+++ b/src/arena.c
@@ -1833,9 +1833,15 @@ mi_decl_export bool mi_arena_unload(mi_arena_id_t arena_id, void** base, size_t*
   // find accessed size
   size_t asize;
   // scan the commit map for the highest entry
+  // scan the commit map for the highest entry
   size_t idx;
-  if (mi_bitmap_bsr(arena->slices_committed, &idx)) {
-    asize = (idx + 1)* MI_ARENA_SLICE_SIZE;
+  //if (mi_bitmap_bsr(arena->slices_committed, &idx)) {
+  //  asize = (idx + 1)* MI_ARENA_SLICE_SIZE;
+  //}
+  if (mi_bitmap_bsr(arena->pages, &idx)) {
+    mi_page_t* page = (mi_page_t*)mi_arena_slice_start(arena, idx);
+    const size_t page_slice_count = page->memid.mem.arena.slice_count;
+    asize = mi_size_of_slices(idx + page_slice_count);
   }
   else {
     asize = mi_arena_info_slices(arena) * MI_ARENA_SLICE_SIZE;