luaL_newstate() fails when limited amount of memory is available

[simonemainardi]

Lua fails to create luaL_newstate() when a limited amount of memory is available in the system. As other memory allocations (malloc/calloc/new) don't fail, the failure could be dependent from the lua implementation of luaL_newstate().

To reproduce systematically:

• change ntopng code to increase the num of buckets in the hosts and flows hash tables by a factor 8
• create a dummy network interface dummy0
• start ./ntopng -i dummy0 --capture-direction=1 -n 3 --dont-change-user --disable-login 1 -X 4000000 -x 4000000 -g 1 -m "0.0.0.0/0"
• send traffic on the dummy interface with pfsend -i stack:dummy0 -b 800000 -r 1

[simonemainardi]

This is the mmap that seems to fail

static void *mmap_map32(size_t size)
  {
  #if LJ_ALLOC_MMAP_PROBE
    static int fallback = 0;
    if (fallback)
  =>  return mmap_probe(size);
  #endif
    {
      int olderr = errno;
      void *ptr = mmap((void *)LJ_ALLOC_MMAP32_START, size, MMAP_PROT, MAP_32BIT|MMAP_FLAGS, -1, 0);
      errno = olderr;
      /* This only allows 1GB on Linux. So fallback to probing to get 2GB. */
  #if LJ_ALLOC_MMAP_PROBE
      if (ptr == MFAIL) {
        fallback = 1;
        return mmap_probe(size);
      }
  #endif
      return ptr;

MAP_32BIT (since Linux 2.4.20, 2.6)
              Put the mapping into the first 2 Gigabytes of the process
              address space.  This flag is supported only on x86-64, for
              64-bit programs.  It was added to allow thread stacks to be
              allocated somewhere in the first 2 GB of memory, so as to
              improve context-switch performance on some early 64-bit
              processors.  Modern x86-64 processors no longer have this
              performance problem, so use of this flag is not required on
              those systems.  The MAP_32BIT flag is ignored when MAP_FIXED
              is set.

[simonemainardi]

Issue is due to LuaJIT that only mmaps in the first 2GB of address space. That is, it looks for contiguous chunks of memory in the first 2GB.

ntopng/third-party/LuaJIT-2.1.0-git/src/lj_alloc.c

Line 258 in 91b3d69

if ((addr >> LJ_ALLOC_MBITS) == 0 && addr >= LJ_ALLOC_MMAP_PROBE_LOWER) {

This is not enough as, on large installations, the first 2GB could be easily be already used by ntopng (e.g., to keep the hash tables).

To allow LuaJIT to use more memory, it should be compiled with LUAJIT_ENABLE_GC64 but this causes ntopng to crash immediately. As of today, it seems that LUAJIT_ENABLE_GC64 isn't fully supported: LuaJIT/LuaJIT#25

Hence, we are considering to drop LuaJIT in favour of normal Lua. Indeed, the benefits of JIT in ntopng are limited as new interpreters are spawned continuously.

[simonemainardi]

fixed as part of 64d969f