Added support to MEM_ROOT for write protected memory

This is useful for thing like Item_true and Item_false that we
allocated and initalize once and want to ensure that nothing can
change them

Main changes:
- Memory protection is achived by allocating memory with mmap() and
  protect it from write with mprotect()
- init_alloc_root(...,MY_ROOT_USE_MPROTECT) will create a
  memroot that one can later use with protect_root() to turn it
  read only or turn it back to read-write. All allocations to this
  memroot is done with mmap() to ensure page alligned allocations.
- alloc_root() code was rearranged to combine normal and valgrind code.
- init_alloc_root() now changes block size to be power of 2's, to get less
  memory fragmentation.
- Changed MEM_ROOT structure to make it smaller. Also renamed
  MEM_ROOT m_psi_key to psi_key.
- Moved MY_THREAD_SPECIFIC marker in MEM_ROOT from block size (old hack)
  to flags.
- Added global variable my_system_page_size. This is initialized at
  startup.

1 files changed, 1 insertions, 1 deletions

diff --git a/storage/myisammrg/ha_myisammrg.cc b/storage/myisammrg/ha_myisammrg.cc
index 70a4d6aa284..d37636abab7 100644
--- a/storage/myisammrg/ha_myisammrg.cc
+++ b/storage/myisammrg/ha_myisammrg.cc
@@ -121,7 +121,7 @@ ha_myisammrg::ha_myisammrg(handlerton *hton, TABLE_SHARE *table_arg)
   :handler(hton, table_arg), file(0), is_cloned(0)
 {
   init_sql_alloc(rg_key_memory_children, &children_mem_root,
-                 FN_REFLEN + ALLOC_ROOT_MIN_BLOCK_SIZE, 0, MYF(0));
+                 FN_REFLEN, 0, MYF(0));
 }