/proc/meminfo MemAvaileble

在 /proc/meminfo.c:

    available = si_mem_available();
    ...

    show_val_kb(m, "MemTotal:       ", i.totalram);
    show_val_kb(m, "MemFree:        ", i.freeram);
    show_val_kb(m, "MemAvailable:   ", available);
    ...

在 mm/page_alloc.c

long si_mem_available(void)
{
    long available;
    unsigned long pagecache;
    unsigned long wmark_low = 0;
    unsigned long pages[NR_LRU_LISTS];
    unsigned long reclaimable;
    struct zone *zone;
    int lru;

    for (lru = LRU_BASE; lru < NR_LRU_LISTS; lru++)
        pages[lru] = global_node_page_state(NR_LRU_BASE + lru);

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

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

    /*
     * 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 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(NR_SLAB_RECLAIMABLE) +
            global_node_page_state(NR_KERNEL_MISC_RECLAIMABLE);
    available += reclaimable - min(reclaimable / 2, wmark_low);

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

所以 MemAvailable 是 free + cache + reclaimable slab (buffer?)