Reduce 32GB RLIMIT_DATA on FreeBSD/x64 to allocate low-2GB memory.

Thanks to Tony Finch.

src/lj_alloc.c

@@ -171,12 +171,20 @@ static LJ_AINLINE int CALL_MUNMAP(void *ptr, size_t size)
 /* Actually this only gives us max. 1GB in current Linux kernels. */
 #define CALL_MMAP(s)	mmap(NULL, (s), MMAP_PROT, MAP_32BIT|MMAP_FLAGS, -1, 0)
 
-#elif defined(__MACH__) && defined(__APPLE__)
+#elif (defined(__MACH__) && defined(__APPLE__)) || \
+      defined(__FreeBSD__) || defined(__FreeBSD_kernel__)
 
-/* OSX mmap() uses a naive first-fit linear search. That's perfect for us.
-** But -pagezero_size must be set, otherwise the lower 4GB are blocked.
+/* OSX and FreeBSD mmap() use a naive first-fit linear search.
+** That's perfect for us. Except that -pagezero_size must be set for OSX,
+** otherwise the lower 4GB are blocked. And the 32GB RLIMIT_DATA needs
+** to be reduced to 250MB on FreeBSD.
 */
+#if defined(__FreeBSD__) || defined(__FreeBSD_kernel__)
+#include <sys/resource.h>
+#define MMAP_REGION_START	((uintptr_t)0x10000000)
+#else
 #define MMAP_REGION_START	((uintptr_t)0x10000)
+#endif
 #define MMAP_REGION_END		((uintptr_t)0x80000000)
 
 static LJ_AINLINE void *CALL_MMAP(size_t size)
@@ -184,6 +192,15 @@ static LJ_AINLINE void *CALL_MMAP(size_t size)
   /* Hint for next allocation. Doesn't need to be thread-safe. */
   static uintptr_t alloc_hint = MMAP_REGION_START;
   int retry = 0;
+#if defined(__FreeBSD__) || defined(__FreeBSD_kernel__)
+  static int rlimit_modified = 0;
+  if (LJ_UNLIKELY(rlimit_modified == 0)) {
+    struct rlimit rlim;
+    rlim.rlim_cur = rlim.rlim_max = MMAP_REGION_START;
+    setrlimit(RLIMIT_DATA, &rlim);  /* Ignore result. May fail below. */
+    rlimit_modified = 1;
+  }
+#endif
   for (;;) {
     void *p = mmap((void *)alloc_hint, size, MMAP_PROT, MMAP_FLAGS, -1, 0);
     if ((uintptr_t)p >= MMAP_REGION_START &&
@@ -199,12 +216,6 @@ static LJ_AINLINE void *CALL_MMAP(size_t size)
   return CMFAIL;
 }
 
-#elif defined(__FreeBSD__) || defined(__FreeBSD_kernel__)
-
-/* FreeBSD 64 bit kernel ignores mmap() hints for lower 32GB of memory. */
-/* See: grep -C15 RLIMIT_DATA /usr/src/sys/vm/vm_mmap.c */
-#error "No support for 64 bit FreeBSD"
-
 #else
 
 #error "NYI: need an equivalent of MAP_32BIT for this 64 bit OS"