rts: Enable two-step allocator on FreeBSD

Previously we would prevent any operating system not providing the
MEM_NORESERVE flag
from using the two-step allocator. Afterall, Linux will reserve
swap-space for
a mapping unless this flag is given, which is most certainly not what
we want.

However, it seems that FreeBSD provides the reservation-only mapping
behavior
that we expect despite not providing the MEM_NORESERVE macro. In fact,
it
provided the macro until 2014, when it was removed on account of not
being
implemented in the kernel. However, empirical evidence suggests that
just plain
mmap does what we want.

Reviewers: erikd, simonmar

Subscribers: rwbarton, thomie, erikd, carter

GHC Trac Issues: #15348

Differential Revision: https://phabricator.haskell.org/D4939

2 files changed, 17 insertions, 11 deletions

diff --git a/configure.ac b/configure.ac
index ac464b6d2f..1976530df9 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1182,22 +1182,23 @@ if test "$ac_cv_sizeof_void_p" -eq 8 ; then
     if test "x$EnableLargeAddressSpace" = "xyes" ; then
         if test "$ghc_host_os" = "darwin" ; then
             use_large_address_space=yes
-	elif test "$ghc_host_os" = "openbsd" ; then
-	    # as of OpenBSD 5.8 (2015), OpenBSD does not support mmap with MAP_NORESERVE.
-	    # The flag MAP_NORESERVE is supported for source compatibility reasons,
-	    # but is completely ignored by OS mmap
+        elif test "$ghc_host_os" = "openbsd" ; then
+            # as of OpenBSD 5.8 (2015), OpenBSD does not support mmap with MAP_NORESERVE.
+            # The flag MAP_NORESERVE is supported for source compatibility reasons,
+            # but is completely ignored by OS mmap
             use_large_address_space=no
         else
-            AC_CHECK_DECLS([MAP_NORESERVE, MADV_FREE, MADV_DONTNEED],[],[],
+            AC_CHECK_DECLS([MAP_NORESERVE, MAP_GUARD, MADV_FREE, MADV_DONTNEED],[],[],
                 [
                 #include <unistd.h>
                 #include <sys/types.h>
                 #include <sys/mman.h>
                 #include <fcntl.h>
             ])
-            if test "$ac_cv_have_decl_MAP_NORESERVE" = "yes" &&
-                test "$ac_cv_have_decl_MADV_FREE" = "yes" ||
-                test "$ac_cv_have_decl_MADV_DONTNEED" = "yes" ; then
+            if ( test "$ac_cv_have_decl_MAP_NORESERVE" = "yes" ||
+                 test "$ac_cv_have_decl_MAP_GUARD" = "yes" ) &&
+                ( test "$ac_cv_have_decl_MADV_FREE" = "yes" ||
+                test "$ac_cv_have_decl_MADV_DONTNEED" = "yes" ) ; then
                     use_large_address_space=yes
             fi
         fi
diff --git a/rts/posix/OSMem.c b/rts/posix/OSMem.c
index e63e798be9..08f96350c9 100644
--- a/rts/posix/OSMem.c
+++ b/rts/posix/OSMem.c
@@ -102,8 +102,10 @@ void osMemInit(void)
  The naming is chosen from the Win32 API (VirtualAlloc) which does the
  same thing and has done so forever, while support for this in Unix systems
  has only been added recently and is hidden in the posix portability mess.
- It is confusing because to get the reserve behavior we need MAP_NORESERVE
- (which tells the kernel not to allocate backing space), but heh...
+ The Linux manpage suggests that mmap must be passed MAP_NORESERVE in order
+ to get reservation-only behavior. It is confusing because to get the reserve
+ behavior we need MAP_NORESERVE (which tells the kernel not to allocate backing
+ space), but heh...
 */
 enum
 {
@@ -161,7 +163,10 @@ my_mmap (void *addr, W_ size, int operation)
     else
         prot = PROT_NONE;
     if (operation == MEM_RESERVE)
-# if defined(MAP_NORESERVE)
+# if defined(MAP_GUARD)
+        // Provided by FreeBSD
+        flags = MAP_GUARD;
+# elif defined(MAP_NORESERVE)
         flags = MAP_NORESERVE;
 # else
 #  if defined(USE_LARGE_ADDRESS_SPACE)