Fix/enhance THP integration.

Detect whether chunks start off as THP-capable by default (according to
the state of /sys/kernel/mm/transparent_hugepage/enabled), and use this
as the basis for whether to call pages_nohuge() once per chunk during
first purge of any of the chunk's page runs.

Add the --disable-thp configure option, as well as the the opt.thp
mallctl.

This resolves #541.

include/jemalloc/internal/arena.h

@@ -506,6 +506,7 @@ static const size_t	large_pad =
 #endif
     ;
 
+extern bool		opt_thp;
 extern purge_mode_t	opt_purge;
 extern const char	*purge_mode_names[];
 extern ssize_t		opt_lg_dirty_mult;

include/jemalloc/internal/jemalloc_internal.h.in

@@ -99,6 +99,13 @@ static const bool config_tcache =
     false
 #endif
     ;
+static const bool config_thp =
+#ifdef JEMALLOC_THP
+    true
+#else
+    false
+#endif
+    ;
 static const bool config_tls =
 #ifdef JEMALLOC_TLS
     true

include/jemalloc/internal/jemalloc_internal_defs.h.in

@@ -252,6 +252,12 @@
 /* Defined if madvise(2) is available. */
 #undef JEMALLOC_HAVE_MADVISE
 
+/*
+ * Defined if transparent huge pages are supported via the MADV_[NO]HUGEPAGE
+ * arguments to madvise(2).
+ */
+#undef JEMALLOC_HAVE_MADVISE_HUGE
+
 /*
  * Methods for purging unused pages differ between operating systems.
  *
@@ -264,10 +270,7 @@
 #undef JEMALLOC_PURGE_MADVISE_FREE
 #undef JEMALLOC_PURGE_MADVISE_DONTNEED
 
-/*
- * Defined if transparent huge pages are supported via the MADV_[NO]HUGEPAGE
- * arguments to madvise(2).
- */
+/* Defined if transparent huge page support is enabled. */
 #undef JEMALLOC_THP
 
 /* Define if operating system has alloca.h header. */

include/jemalloc/internal/private_symbols.txt

@@ -392,6 +392,7 @@ opt_quarantine
 opt_redzone
 opt_stats_print
 opt_tcache
+opt_thp
 opt_utrace
 opt_xmalloc
 opt_zero