diff --git a/lib/libc/sys/mmap.2 b/lib/libc/sys/mmap.2
index b633cb11942..0b1f35fedca 100644
--- a/lib/libc/sys/mmap.2
+++ b/lib/libc/sys/mmap.2
@@ -28,7 +28,7 @@
 .\"	@(#)mmap.2	8.4 (Berkeley) 5/11/95
 .\" $FreeBSD$
 .\"
-.Dd November 6, 2009
+.Dd August 28, 2010
 .Dt MMAP 2
 .Os
 .Sh NAME
@@ -211,6 +211,19 @@ implements a coherent file system buffer cache.
 However, it may be
 used to associate dirty VM pages with file system buffers and thus cause
 them to be flushed to physical media sooner rather than later.
+.It Dv MAP_PREFAULT_READ
+Immediately update the calling process's lowest-level virtual address
+translation structures, such as its page table, so that every memory
+resident page within the region is mapped for read access.
+Ordinarily these structures are updated lazily.
+The effect of this option is to eliminate any soft faults that would
+otherwise occur on the initial read accesses to the region.
+Although this option does not preclude
+.Fa prot
+from including
+.Dv PROT_WRITE ,
+it does not eliminate soft faults on the initial write accesses to the
+region.
 .It Dv MAP_PRIVATE
 Modifications are private.
 .It Dv MAP_SHARED
diff --git a/sys/sys/mman.h b/sys/sys/mman.h
index 12c013383de..379ed14b72b 100644
--- a/sys/sys/mman.h
+++ b/sys/sys/mman.h
@@ -90,6 +90,7 @@
  * Extended flags
  */
 #define	MAP_NOCORE	 0x00020000 /* dont include these pages in a coredump */
+#define	MAP_PREFAULT_READ 0x00040000 /* prefault mapping for reading */
 #endif /* __BSD_VISIBLE */
 
 #if __POSIX_VISIBLE >= 199309
diff --git a/sys/sys/param.h b/sys/sys/param.h
index 1c1ecec61e8..fec4b46336d 100644
--- a/sys/sys/param.h
+++ b/sys/sys/param.h
@@ -58,7 +58,7 @@
  *		in the range 5 to 9.
  */
 #undef __FreeBSD_version
-#define __FreeBSD_version 900018	/* Master, propagated to newvers */
+#define __FreeBSD_version 900019	/* Master, propagated to newvers */
 
 #ifndef LOCORE
 #include <sys/types.h>
diff --git a/sys/vm/vm_mmap.c b/sys/vm/vm_mmap.c
index bd9f98fd588..2071cc7772d 100644
--- a/sys/vm/vm_mmap.c
+++ b/sys/vm/vm_mmap.c
@@ -1467,9 +1467,10 @@ vm_mmap(vm_map_t map, vm_offset_t *addr, vm_size_t size, vm_prot_t prot,
 		 */
 		if (handle == 0)
 			foff = 0;
-	} else {
+	} else if (flags & MAP_PREFAULT_READ)
+		docow = MAP_PREFAULT;
+	else
 		docow = MAP_PREFAULT_PARTIAL;
-	}
 
 	if ((flags & (MAP_ANON|MAP_SHARED)) == 0)
 		docow |= MAP_COPY_ON_WRITE;