vss: Add documentation of MAP_POPULATE.

It should be clear why the vss should ask the kernel to prefault the
page tables for the mapping, but it's less clear why we might need
to define the preprocessor symbol.

diff --git a/vss.c b/vss.c
index f6ec839..73c7231 100644 (file)
--- a/vss.c
+++ b/vss.c
@@ -941,6 +941,7 @@ static int recv_afs_msg(int afs_socket, int *fd, uint32_t *code, uint32_t *data)
 }
 
 #ifndef MAP_POPULATE
+/** As of 2018, neither FreeBSD-11.2 nor NetBSD-8.0 have MAP_POPULATE. */
 #define MAP_POPULATE 0
 #endif