/*
* Compute the header size such that it is large enough to contain the
* page map.  The page map is biased to omit entries for the header
* itself, so some iteration is necessary to compute the map bias.
*
* 1) Compute safe header_size and map_bias values that include enough
*    space for an unbiased page map.
* 2) Refine map_bias based on (1) to omit the header pages in the page
*    map.  The resulting map_bias may be one too small.
* 3) Refine map_bias based on (2).  The result will be >= the result
*    from (2), and will always be correct.
*/
map_bias = 0;
for (i = 0; i < 3; i++) {
    size_t header_size = offsetof(arena_chunk_t, map_bits) +
        ((sizeof(arena_chunk_map_bits_t) +
        sizeof(arena_chunk_map_misc_t)) * (chunk_npages-map_bias));
    map_bias = (header_size + PAGE_MASK) >> LG_PAGE;
}