lowmemorykiller驱动

一、内存页面回收

参考：http://www.wowotech.net/memory_management/233.html

内核会回收很少使用的内存页面来保证系统持续有内存使用。页面的回收方式有三种

• 页回写：如果一个很少使用的页的后备存储器是一个块设备（例如文件映射），则可以将内存直接同步到块设备，腾出的页面可以被重用。
• 页交换：如果页面没有后备存储器，则可以交换到特定swap分区，再次被访问时再交换回内存。
• 页丢弃：如果页面的后备存储器是一个文件，但文件内容在内存不能被修改（例如可执行文件），那么在当前不需要的情况下可直接丢弃。

触发页面回收的时机如下图所示：

 

当触发页面回收时，内核对下列分配的内存页进行回收处理：

• LRU列表中的页框
• slab缓存：所有注册到shrinker_list的shrinker都会被触发回收内存。

参考mm/vmscan.c

内核中定义了一个列表shrinker_list,外部通过register_shrinker、unregister_shrinker向该列表添加和移除shrinker。

shrinker的定义如下：

/*
 * A callback you can register to apply pressure to ageable caches.
 *
 * @count_objects should return the number of freeable items in the cache. If
 * there are no objects to free or the number of freeable items cannot be
 * determined, it should return 0. No deadlock checks should be done during the
 * count callback - the shrinker relies on aggregating scan counts that couldn't
 * be executed due to potential deadlocks to be run at a later call when the
 * deadlock condition is no longer pending.
 *
 * @scan_objects will only be called if @count_objects returned a non-zero
 * value for the number of freeable objects. The callout should scan the cache
 * and attempt to free items from the cache. It should then return the number
 * of objects freed during the scan, or SHRINK_STOP if progress cannot be made
 * due to potential deadlocks. If SHRINK_STOP is returned, then no further
 * attempts to call the @scan_objects will be made from the current reclaim
 * context.
 *
 * @flags determine the shrinker abilities, like numa awareness
 */
struct shrinker {
        unsigned long (*count_objects)(struct shrinker *,
                                       struct shrink_control *sc);
        unsigned long (*scan_objects)(struct shrinker *,
                                      struct shrink_control *sc);

        int seeks;      /* seeks to recreate an obj */
        long batch;     /* reclaim batch size, 0 = default */
        unsigned long flags;

        /* These are for internal use */
        struct list_head list;
        /* objs pending delete, per node */
        atomic_long_t *nr_deferred;
};

lowmemorykiller就是通过向shrinker_list中注册shrinker。

当内存不足时kswapd线程会遍历shrinker_list链表，并回调已注册的shrinker函数来回收内存page，kswapd还会周期性唤醒来执行内存操作。每个zone维护active_list和inactive_list链表，内核根据页面活动状态将page在这两个链表之间移动，最终通过shrink_slab和shrink_zone来回收内存页