Refactor purging and splitting/merging.

Split purging into lazy and forced variants.  Use the forced variant for
zeroing dss.

Add support for NULL function pointers as an opt-out mechanism for the
dalloc, commit, decommit, purge_lazy, purge_forced, split, and merge
fields of extent_hooks_t.

Add short-circuiting checks in large_ralloc_no_move_{shrink,expand}() so
that no attempt is made if splitting/merging is not supported.

This resolves #268.

include/jemalloc/internal/arena.h

@@ -198,8 +198,8 @@ struct arena_s {
 
 	/*
 	 * Current count of pages within unused extents that are potentially
-	 * dirty, and for which madvise(... MADV_DONTNEED) has not been called.
-	 * By tracking this, we can institute a limit on how much dirty unused
+	 * dirty, and for which pages_purge_*() has not been called.  By
+	 * tracking this, we can institute a limit on how much dirty unused
 	 * memory is mapped for each arena.
 	 */
 	size_t			ndirty;

include/jemalloc/internal/extent.h

@@ -133,7 +133,10 @@ bool	extent_commit_wrapper(tsdn_t *tsdn, arena_t *arena,
 bool	extent_decommit_wrapper(tsdn_t *tsdn, arena_t *arena,
     extent_hooks_t **r_extent_hooks, extent_t *extent, size_t offset,
     size_t length);
-bool	extent_purge_wrapper(tsdn_t *tsdn, arena_t *arena,
+bool	extent_purge_lazy_wrapper(tsdn_t *tsdn, arena_t *arena,
+    extent_hooks_t **r_extent_hooks, extent_t *extent, size_t offset,
+    size_t length);
+bool	extent_purge_forced_wrapper(tsdn_t *tsdn, arena_t *arena,
     extent_hooks_t **r_extent_hooks, extent_t *extent, size_t offset,
     size_t length);
 extent_t	*extent_split_wrapper(tsdn_t *tsdn, arena_t *arena,

include/jemalloc/internal/pages.h

@@ -24,6 +24,23 @@
 #define	HUGEPAGE_CEILING(s)						\
 	(((s) + HUGEPAGE_MASK) & ~HUGEPAGE_MASK)
 
+/* PAGES_CAN_PURGE_LAZY is defined if lazy purging is supported. */
+#if defined(_WIN32) || defined(JEMALLOC_PURGE_MADVISE_FREE)
+#  define PAGES_CAN_PURGE_LAZY
+#endif
+/*
+ * PAGES_CAN_PURGE_FORCED is defined if forced purging is supported.
+ *
+ * The only supported way to hard-purge on Windows is to decommit and then
+ * re-commit, but doing so is racy, and if re-commit fails it's a pain to
+ * propagate the "poisoned" memory state.  Since we typically decommit as the
+ * next step after purging on Windows anyway, there's no point in adding such
+ * complexity.
+ */
+#if !defined(_WIN32) && defined(JEMALLOC_PURGE_MADVISE_DONTNEED)
+#  define PAGES_CAN_PURGE_FORCED
+#endif
+
 #endif /* JEMALLOC_H_TYPES */
 /******************************************************************************/
 #ifdef JEMALLOC_H_STRUCTS
@@ -32,13 +49,29 @@
 /******************************************************************************/
 #ifdef JEMALLOC_H_EXTERNS
 
+static const bool pages_can_purge_lazy =
+#ifdef PAGES_CAN_PURGE_LAZY
+    true
+#else
+    false
+#endif
+    ;
+static const bool pages_can_purge_forced =
+#ifdef PAGES_CAN_PURGE_FORCED
+    true
+#else
+    false
+#endif
+    ;
+
 void	*pages_map(void *addr, size_t size, bool *commit);
 void	pages_unmap(void *addr, size_t size);
 void	*pages_trim(void *addr, size_t alloc_size, size_t leadsize,
     size_t size, bool *commit);
 bool	pages_commit(void *addr, size_t size);
 bool	pages_decommit(void *addr, size_t size);
-bool	pages_purge(void *addr, size_t size);
+bool	pages_purge_lazy(void *addr, size_t size);
+bool	pages_purge_forced(void *addr, size_t size);
 bool	pages_huge(void *addr, size_t size);
 bool	pages_nohuge(void *addr, size_t size);
 void	pages_boot(void);

include/jemalloc/internal/private_symbols.txt

@@ -179,7 +179,8 @@ extent_merge_wrapper
 extent_past_get
 extent_prof_tctx_get
 extent_prof_tctx_set
-extent_purge_wrapper
+extent_purge_forced_wrapper
+extent_purge_lazy_wrapper
 extent_retained_get
 extent_ring_insert
 extent_ring_remove
@@ -327,7 +328,8 @@ pages_decommit
 pages_huge
 pages_map
 pages_nohuge
-pages_purge
+pages_purge_forced
+pages_purge_lazy
 pages_trim
 pages_unmap
 pind2sz

include/jemalloc/jemalloc_typedefs.h.in

@@ -61,7 +61,8 @@ struct extent_hooks_s {
 	extent_dalloc_t		*dalloc;
 	extent_commit_t		*commit;
 	extent_decommit_t	*decommit;
-	extent_purge_t		*purge;
+	extent_purge_t		*purge_lazy;
+	extent_purge_t		*purge_forced;
 	extent_split_t		*split;
 	extent_merge_t		*merge;
 };