static bool
arena_run_split_small(arena_t *arena, arena_run_t *run, size_t size,
    szind_t binind)
{
	arena_chunk_t *chunk;
	arena_chunk_map_misc_t *miscelm;
	size_t flag_dirty, flag_decommitted, run_ind, need_pages, i;

	assert(binind != BININD_INVALID);

	chunk = (arena_chunk_t *)CHUNK_ADDR2BASE(run);
	miscelm = arena_run_to_miscelm(run);
	run_ind = arena_miscelm_to_pageind(miscelm);
	flag_dirty = arena_mapbits_dirty_get(chunk, run_ind);
	flag_decommitted = arena_mapbits_decommitted_get(chunk, run_ind);
        // size 是 bin 对应的 run_size 
	need_pages = (size >> LG_PAGE);
	assert(need_pages > 0);

	if (flag_decommitted != 0 && arena->chunk_hooks.commit(chunk, chunksize,
	    run_ind << LG_PAGE, size, arena->ind))
		return (true);

	arena_run_split_remove(arena, chunk, run_ind, flag_dirty,
	    flag_decommitted, need_pages);

        // 设置分配出去的 page 对应的 map_bits
	for (i = 0; i < need_pages; i++) {
		size_t flag_unzeroed = arena_mapbits_unzeroed_get(chunk,
		    run_ind+i);
		arena_mapbits_small_set(chunk, run_ind+i, i, binind,
		    flag_unzeroed);
		if (config_debug && flag_dirty == 0 && flag_unzeroed == 0)
			arena_run_page_validate_zeroed(chunk, run_ind+i);
	}
	JEMALLOC_VALGRIND_MAKE_MEM_UNDEFINED((void *)((uintptr_t)chunk +
	    (run_ind << LG_PAGE)), (need_pages << LG_PAGE));
	return (false);
}

// 从 run_ind 对应 run 中分配出 need_pages，剩余的再次插入到 avail_runs 中
static void
arena_run_split_remove(arena_t *arena, arena_chunk_t *chunk, size_t run_ind,
    size_t flag_dirty, size_t flag_decommitted, size_t need_pages)
{
	size_t total_pages, rem_pages;

	assert(flag_dirty == 0 || flag_decommitted == 0);

	total_pages = arena_mapbits_unallocated_size_get(chunk, run_ind) >>
	    LG_PAGE;
	assert(arena_mapbits_dirty_get(chunk, run_ind+total_pages-1) ==
	    flag_dirty);
	assert(need_pages <= total_pages);
	rem_pages = total_pages - need_pages;

	arena_avail_remove(arena, chunk, run_ind, total_pages);
	if (flag_dirty != 0)
		arena_run_dirty_remove(arena, chunk, run_ind, total_pages);
	arena_cactive_update(arena, need_pages, 0);
	arena->nactive += need_pages;

	/* Keep track of trailing unused pages for later use. */
	if (rem_pages > 0) {
		size_t flags = flag_dirty | flag_decommitted;
		size_t flag_unzeroed_mask = (flags == 0) ?  CHUNK_MAP_UNZEROED :
		    0;

            // 设置 run 对应的 page 的信息，设置开头和结尾的 page 对应的 map_bits 的
            // 未分配内存大小
		arena_mapbits_unallocated_set(chunk, run_ind+need_pages,
		    (rem_pages << LG_PAGE), flags |
		    (arena_mapbits_unzeroed_get(chunk, run_ind+need_pages) &
		    flag_unzeroed_mask));
		arena_mapbits_unallocated_set(chunk, run_ind+total_pages-1,
		    (rem_pages << LG_PAGE), flags |
		    (arena_mapbits_unzeroed_get(chunk, run_ind+total_pages-1) &
		    flag_unzeroed_mask));
		if (flag_dirty != 0) {
			arena_run_dirty_insert(arena, chunk, run_ind+need_pages,
			    rem_pages);
		}
		arena_avail_insert(arena, chunk, run_ind+need_pages, rem_pages);
	}
}