内存Available 是怎么计算的

前段时间在看/proc/meminfo，计算内核态内存，用户态内存，还有X内存。

在看这些的时候发现个问题就是 MemAvailable 竟然大于 Buffers + Cached + MemFree，因为在我印象里一直认为Available会少于buff + cache + free的。

文件：fs/proc/meminfo.c  

于是扒代码分析：

	for_each_zone(zone)
		wmark_low += zone->watermark[WMARK_LOW];

	/*
	 * Estimate the amount of memory available for userspace allocations,
	 * without causing swapping.
	 *
	 * Free memory cannot be taken below the low watermark, before the
	 * system starts swapping.
	 */
	available = i.freeram - wmark_low;

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

	/*
	 * Part of the reclaimable swap consists of items that are in use,
	 * and cannot be freed. Cap this