MDEV-33279 Disable transparent huge pages after page buffers has been…

… allocatedDisable transparent huge pages (THP)

The reason for disabling transparent huge pages (THP) is that they
do not work well with MariaDB (or other databases, see links in
MDEV-33279). The effect of using THP are that MariaDB will use much more
(10x) more memory and will no be able to release memory back to the system.

Disabling THP is done after all storage engines are started, to allow
buffer pools and keybuffers (big allocations) to be allocated as huge
pages.

sql/mysqld.cc

@@ -5451,6 +5451,15 @@ static int init_server_components()
 #else
   locked_in_memory= 0;
 #endif
+#ifdef PR_SET_THP_DISABLE
+  /*
+    Engine page buffers are now allocated.
+    Disable transparent huge pages for all
+    future allocations as these causes memory
+    leaks.
+  */
+  prctl(PR_SET_THP_DISABLE, 1, 0, 0, 0);
+#endif
 
   ft_init_stopwords();