long si_mem_available(void)
{
	long available;
	unsigned long pagecache;
	unsigned long wmark_low = 0;
	unsigned long reclaimable;
	struct zone *zone;

	for_each_zone(zone)
		wmark_low += low_wmark_pages(zone);

	/*
	 * Estimate the amount of memory available for userspace allocations,
	 * without causing swapping or OOM.
	 */
	available = global_zone_page_state(NR_FREE_PAGES) - totalreserve_pages;

	/*
	 * Not all the page cache can be freed, otherwise the system will
	 * start swapping or thrashing. Assume at least half of the page
	 * cache, or the low watermark worth of cache, needs to stay.
	 */
	pagecache = global_node_page_state(NR_ACTIVE_FILE) +
		global_node_page_state(NR_INACTIVE_FILE);
	pagecache -= min(pagecache / 2, wmark_low);
	available += pagecache;

	/*
	 * Part of the reclaimable slab and other kernel memory consists of
	 * items that are in use, and cannot be freed. Cap this estimate at the
	 * low watermark.
	 */
	reclaimable = global_node_page_state_pages(NR_SLAB_RECLAIMABLE_B) +
		global_node_page_state(NR_KERNEL_MISC_RECLAIMABLE);
	reclaimable -= min(reclaimable / 2, wmark_low);
	available += reclaimable;

	if (available < 0)
		available = 0;
	return available;
}

Warm-up