Mostly remove the VM_STACK OPTION.

This changes the definitions of a few items so that structures are the
same whether or not the option itself is enabled. This allows
people to enable and disable the option without recompilng the world.

As the author says:

|I ran into a problem pulling out the VM_STACK option.  I was aware of this
|when I first did the work, but then forgot about it.  The VM_STACK stuff
|has some code changes in the i386 branch.  There need to be corresponding
|changes in the alpha branch before it can come out completely.

what is done:
|
|1) Pull the VM_STACK option out of the header files it appears in.  This
|really shouldn't affect anything that executes with or without the rest
|of the VM_STACK patches.  The vm_map_entry will then always have one
|extra element (avail_ssize).  It just won't be used if the VM_STACK
|option is not turned on.
|
|I've also pulled the option out of vm_map.c.  This shouldn't harm anything,
|since the routines that are enabled as a result are not called unless
|the VM_STACK option is enabled elsewhere.
|
|2) Add what appears to be appropriate code the the alpha branch, still
|protected behind the VM_STACK switch.  I don't have an alpha machine,
|so we would need to get some testers with alpha machines to try it out.
|
|Once there is some testing, we can consider making the change permanent
|for both i386 and alpha.
|
[..]
|
|Once the alpha code is adequately tested, we can pull VM_STACK out
|everywhere.
|

Submitted by:	"Richard Seaman, Jr." <dick@tar.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/lib/libc/sys/mmap.2 b/lib/libc/sys/mmap.2
index 174a083..a2be590 100644
--- a/lib/libc/sys/mmap.2
+++ b/lib/libc/sys/mmap.2
@@ -130,6 +130,25 @@ system calls.
 Modifications are private.
 .It Dv MAP_SHARED
 Modifications are shared.
+.It Dv MAP_STACK
+This option is only available if your system has been compiled with
+VM_STACK defined when compiling the kernel. This is the default for
+i386 only. Consider adding -DVM_STACK to COPTFLAGS in your /etc/make.conf
+to enable this option for other architechures. MAP_STACK implies
+MAP_ANON, and
+.Fa offset 
+of 0.   
+.Fa fd 
+must be -1 and 
+.Fa prot 
+must include at least PROT_READ and PROT_WRITE.  This option creates
+a memory region that grows to at most 
+.Fa len 
+bytes in size, starting from the stack top and growing down.  The
+stack top is the starting address returned by the call, plus 
+.Fa len 
+bytes.  The bottom of the stack at maximum growth is the starting
+address returned by the call.
 .El
 .Pp
 The