Better document --enable-ivsalloc.

INSTALL

@@ -124,9 +124,10 @@ any of the following arguments (not a definitive list) to 'configure':
     option documentation for usage details.
 
 --enable-ivsalloc
-    Enable validation code, which verifies that pointers reside within
-    jemalloc-owned chunks before dereferencing them.  This incurs a minor
-    performance hit.
+    Enable validation code for malloc_usable_size() and sallocx(), which
+    verifies that pointers reside within jemalloc-owned extents before
+    dereferencing metadata.  This incurs a minor performance hit, and causes
+    the functions to return 0 for failed lookups.
 
 --enable-prof
     Enable heap profiling and leak detection functionality.  See the "opt.prof"

include/jemalloc/internal/jemalloc_internal.h.in

@@ -1061,12 +1061,19 @@ ivsalloc(tsdn_t *tsdn, const void *ptr)
 {
 	extent_t *extent;
 
-	/* Return 0 if ptr is not within a chunk managed by jemalloc. */
+	/*
+	 * Return 0 if ptr is not within an extent managed by jemalloc.  This
+	 * function has two extra costs relative to isalloc():
+	 * - The extent_lookup() call cannot claim to be a dependent lookup,
+	 *   which induces rtree lookup load dependencies.
+	 * - The lookup may fail, so there is an extra branch to check for
+	 *   failure.
+	 * */
 	extent = extent_lookup(tsdn, ptr, false);
 	if (extent == NULL)
 		return (0);
 	assert(extent_active_get(extent));
-	/* Only arena chunks should be looked up via interior pointers. */
+	/* Only slab members should be looked up via interior pointers. */
 	assert(extent_addr_get(extent) == ptr || extent_slab_get(extent));
 
 	return (isalloc(tsdn, extent, ptr));

include/jemalloc/internal/jemalloc_internal_defs.h.in

@@ -195,7 +195,7 @@
 
 /*
  * JEMALLOC_IVSALLOC enables ivsalloc(), which verifies that pointers reside
- * within jemalloc-owned chunks before dereferencing them.
+ * within jemalloc-owned extents before dereferencing them.
  */
 #undef JEMALLOC_IVSALLOC