rename option pagemap_commit; always commit the page map on macos (for now)

include/mimalloc.h

@@ -398,8 +398,8 @@ typedef enum mi_option_e {
   mi_option_reclaim_on_free,            // allow to reclaim an abandoned segment on a free (=1)
   mi_option_full_page_retain,           // retain N full pages per size class (=2)
   mi_option_max_page_candidates,        // max candidate pages to consider for allocation (=4)
-  mi_option_max_vabits,                 // max virtual address bits to consider in user space (=48)
-  mi_option_debug_commit_full_pagemap,  // commit the full pagemap to catch invalid pointer uses (=0)
+  mi_option_max_vabits,                 // max user space virtual address bits to consider (=48)
+  mi_option_pagemap_commit,             // commit the full pagemap (to always catch invalid pointer uses) (=0)
   _mi_option_last,
   // legacy option names
   mi_option_large_os_pages = mi_option_allow_large_os_pages,

include/mimalloc/bits.h

@@ -120,7 +120,7 @@ typedef int32_t  mi_ssize_t;
 
 // use a flat page-map (or a 2-level one)
 #ifndef MI_PAGE_MAP_FLAT
-#if MI_MAX_VABITS <= 40
+#if MI_MAX_VABITS <= 40 && !defined(__APPLE__) 
 #define MI_PAGE_MAP_FLAT  1
 #else
 #define MI_PAGE_MAP_FLAT  0

include/mimalloc/internal.h

@@ -445,6 +445,7 @@ static inline mi_page_t* _mi_heap_get_free_small_page(mi_heap_t* heap, size_t si
 #if MI_PAGE_MAP_FLAT
 
 // flat page-map committed on demand
+// single indirection and low commit, but large initial virtual reserve (4 GiB with 48 bit virtual addresses)
 extern uint8_t* _mi_page_map;
 
 static inline size_t _mi_page_map_index(const void* p) {
@@ -471,6 +472,8 @@ static inline mi_page_t* _mi_unchecked_ptr_page(const void* p) {
 #else
 
 // 2-level page map:
+// double indirection but low commit and low virtual reserve.
+// 
 // The page-map is usually 4 MiB and points to sub maps of 64 KiB. 
 // The page-map is committed on-demand (in 64 KiB) parts (and sub-maps are committed on-demand as well)
 // One sub page-map = 64 KiB => covers 2^13 * 2^16 = 2^32 = 512 MiB address space