Provide introductory description of the default pager.

Reviewed by:	alc
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

sys/vm/default_pager.c

@@ -57,6 +57,16 @@ static boolean_t	default_pager_haspage(vm_object_t, vm_pindex_t, int *,
 
 /*
  * pagerops for OBJT_DEFAULT - "default pager".
+ *
+ * This pager handles anonymous (no handle) swap-backed memory, just
+ * like the swap pager.  It allows several optimizations based on the
+ * fact that no pages of a default object can be swapped out.  The
+ * most important optimization is in vm_fault(), where the pager is
+ * never asked for a non-resident page.  Instead, a freshly allocated
+ * zeroed page is used.
+ *
+ * On the first request to page out a page from a default object, the
+ * object is converted to swap pager type.
  */
 struct pagerops defaultpagerops = {
 	.pgo_alloc =	default_pager_alloc,