malloc/mallopt question

[Steve Ellcey]

I have a basic question about the glibc malloc and mallopt implementations.
I am looking at do_set_mmap_threshold in malloc/malloc.c and I see:

static __always_inline int
do_set_mmap_threshold (size_t value)
{
  /* Forbid setting the threshold too high.  */
  if (value <= HEAP_MAX_SIZE / 2)
    {
      LIBC_PROBE (memory_mallopt_mmap_threshold, 3, value, mp_.mmap_threshold,
                  mp_.no_dyn_threshold);
      mp_.mmap_threshold = value;
      mp_.no_dyn_threshold = 1;
      return 1;
    }
  return 0;
}

How does checking that 'value' is less than or equal to HEAP_MAX_SIZE/2
make sure it is not 'too high'?  Also the mallopt man page says the
limits are 0 and 'DEFAULT_MMAP_THRESHOLD_MAX'.  I don't see the upper
limit being enforced here (or anywhere else).  What am I missing?

The hidden agenda here is to undestand if the mmap threshold can really
be set to something larger than DEFAULT_MMAP_THRESHOLD_MAX somehow to
get some large mallocs allocated with sbrk instead of mmap.

Steve Ellcey
sellcey@marvell.com