lowmemorykiller

简称lmk，是安卓平台特有的特性，代码路径在drivers/staging/android/lowmemorykiller.c (kernel 3.18)，sysfs node在/sys/module/lowmemorykiller/下，它的前身是linux OOM，主要两个参数是minfree和adj，minfree的设置主要根据framework层代码算出，但也有厂商会自己修改，要做充分的UX测试后给出。

主体扫描

static unsigned long lowmem_scan(struct shrinker *s, struct shrink_control *sc)
{
	struct task_struct *tsk;
	struct task_struct *selected = NULL;
	unsigned long rem = 0;
	int tasksize;
	int i;
	short min_score_adj = OOM_SCORE_ADJ_MAX + 1;
	int selected_tasksize = 0;
	short selected_oom_score_adj;
	int array_size = ARRAY_SIZE(lowmem_adj);
	int other_free = global_page_state(NR_FREE_PAGES) - totalreserve_pages;
	int other_file = global_page_state(NR_FILE_PAGES) -
						global_page_state(NR_SHMEM) -
						total_swapcache_pages();

	if (lowmem_adj_size < array_size)
		array_size = lowmem_adj_size;
	if (lowmem_minfree_size < array_size)
		array_size = lowmem_minfree_size;
	for (i = 0; i < array_size; i++) {
		if (other_free < lowmem_minfree[i] &&
		    other_file < lowmem_minfree[i]) {
			min_score_adj = lowmem_adj[i];
			break;
		}
	}

	lowmem_print(3, "lowmem_scan %lu, %x, ofree %d %d, ma %hd\n",
			sc->nr_to_scan, sc->gfp_mask, other_free,
			other_file, min_score_adj);

	if (min_score_adj == OOM_SCORE_ADJ_MAX + 1) {
		lowmem_print(5, "lowmem_scan %lu, %x, return 0\n",
			     sc->nr_to_scan, sc->gfp_mask);
		return 0;
	}

	selected_oom_score_adj = min_score_adj;
        ...
}

首先看看能不能找到oom_score_adj，如果找不到就认为内存充足不杀进程，如果找到就按一定的策略选出某个进程杀掉。

判断内存是否充足的条件就是other_free和other_file两个都必须同时小于lowmem_minfree中的用户设定值,other_free基本上是free pages，other_file基本上是file pages，两者可以分别看成proc/meminfo的MemFree和Cached大小，一般来说Cached远大于MemFree。

static unsigned long lowmem_scan(struct shrinker *s, struct shrink_control *sc)
{
        ...
	rcu_read_lock();
	for_each_process(tsk) {
		struct task_struct *p;
		short oom_score_adj;

		if (tsk->flags & PF_KTHREAD)
			continue;

		p = find_lock_task_mm(tsk);
		if (!p)
			continue;

		if (test_tsk_thread_flag(p, TIF_MEMDIE) &&
		    time_before_eq(jiffies, lowmem_deathpending_timeout)) {
			task_unlock(p);
			rcu_read_unlock();
			return 0;
		}
		oom_score_adj = p->signal->oom_score_adj;
		if (oom_score_adj < min_score_adj) {
			task_unlock(p);
			continue;
		}
		tasksize = get_mm_rss(p->mm);
		task_unlock(p);
		if (tasksize <= 0)
			continue;
		if (selected) {
			if (oom_score_adj < selected_oom_score_adj)
				continue;
			if (oom_score_adj == selected_oom_score_adj &&
			    tasksize <= selected_tasksize)
				continue;
		}
		selected = p;
		selected_tasksize = tasksize;
		selected_oom_score_adj = oom_score_adj;
		lowmem_print(2, "select %d (%s), adj %hd, size %d, to kill\n",
			     p->pid, p->comm, oom_score_adj, tasksize);
	}
	if (selected) {
		lowmem_print(1, "send sigkill to %d (%s), adj %hd, size %d\n",
			     selected->pid, selected->comm,
			     selected_oom_score_adj, selected_tasksize);
		lowmem_deathpending_timeout = jiffies + HZ;
		set_tsk_thread_flag(selected, TIF_MEMDIE);
		send_sig(SIGKILL, selected, 0);
		rem += selected_tasksize;
	}

	lowmem_print(4, "lowmem_scan %lu, %x, return %lu\n",
		     sc->nr_to_scan, sc->gfp_mask, rem);
	rcu_read_unlock();
	return rem;
}

遍历所有的进程for_each_process，当前任务如果是PF_KTHREAD就不能杀，如果不是用find_lock_task_mm遍历当前进程子线程，找到持有->mm的子线程。

如果该线程有TIF_MEMDIE标记就不杀了，如果没有看下该线程的oom_score_adj要比min_score_adj小就直接continue到下一个进程如果比较大而且rss有值(>0)就作为备选后再查看下一个进程。

下一个进程必须比上一次选中的得分要大 or 得分相同但是rss比上一次的占用多的就被认为是更搓的task，要杀。

lmk是如何被调用的

[<c082a824>] (lowmem_scan) from [<c01e0ba4>] (shrink_slab_node+0x204/0x3d0)
[<c01e0ba4>] (shrink_slab_node) from [<c01e12b8>] (shrink_slab+0x70/0xe4)
[<c01e12b8>] (shrink_slab) from [<c01e3ba8>] (try_to_free_pages+0x3c0/0x74c)
[<c01e3ba8>] (try_to_free_pages) from [<c01d9188>] (__alloc_pages_nodemask+0x578/0x92c)
static struct shrinker lowmem_shrinker = {
	.scan_objects = lowmem_scan,
	.count_objects = lowmem_count,
	.seeks = DEFAULT_SEEKS * 16
};

主要是回收内存流程时会调用，lowmem_scan挂到了register_shrinker里, shrink_slab_node里会scan_objects。