Linux mem 2.5 Buddy 内存回收机制

文章目录

• 1. 简介
• 2. LRU 组织
• • 2.1 LRU 链表
  • 2.2 LRU Cache
  • 2.3 LRU 移动操作
  • • 2.3.1 page 加入 LRU
    • 2.3.2 其他 LRU 移动操作
• 3. LRU 回收
• • 3.1 LRU 更新
  • 3.2 Swappiness
  • 3.3 反向映射
  • 3.4 代码实现
  • • 3.4.1 struct scan_control
    • 3.4.2 shrink_node()
    • 3.4.3 shrink_list()
    • 3.4.4 shrink_active_list()
    • 3.4.5 shrink_inactive_list()
    • 3.4.6 try_to_unmap()
    • 3.4.7 rmap_walk()
• 4. 其他回收方式
• • 4.1 Shrinker
  • • 4.1.1 register_shrinker()
    • 4.1.2 do_shrink_slab()
  • 4.2 内存整理 (Compact)
  • 4.3 内存合并 (KSM)
  • 4.4 OOM Killer
• 参考文档：

1. 简介

Buddy 的内存分配和释放算法还是比较简单明晰的。分配的时候优先找 order 相等的空闲内存链表，找不到的话就去找 order 更大的空闲内存链表；释放的话先释放到对应 order 的空闲内存链表，然后尝试和 buddy 内存进行合并，尽量合并成更大的空闲内存块。

但是内存管理加入 缺页(PageFault) 和 回收(Reclaim) 以后，情况就变得异常复杂了。Linux 为了用同样多的内存养活更多的进程和服务操碎了心：

• PageFault：对新创建的进程，除了内核态的内存不得不马上分配，对用户态的内存严格遵循 lazy 的延后分配策略，对私有数据设置 COW(Copy On Write) 策略只有在更改的时候才会触发 PageFault 重新分配物理页面，对新的文件映射也只是简单的分配VMA只有在实际访问的时候才会触发 PageFault 分配物理页面。
• Reclaim：在内存不够用的情况下，Linux 尝试回收一些内存。对于从文件映射的内存 (FileMap)，因为文件中有备份所以先丢弃掉内存，需要访问时再重新触发 PageFault 从文件中加载；对于没有文件映射的内存 (AnonMap)，可以先把它交换到 Swap 分区上去，需要访问时再重新触发 PageFault 从 Swap 中加载。

本篇文章就来详细的分析上述的 缺页(PageFault) 和 回收(Reclaim) 过程。

2. LRU 组织

用户态的内存(FileMap + AnonMap)的内存回收的主要来源。为了尽量减少内存回收锁引起的震荡，刚刚回收的内存马上又被访问需要重新分配。Linux 设计了 LRU (Last Recent Use) 链表，来优先回收最长时间没有访问的内存。

2.1 LRU 链表

设计了 5 组 LRU 链表：

enum lru_list {
	LRU_INACTIVE_ANON = LRU_BASE,							// inactive 匿名
	LRU_ACTIVE_ANON = LRU_BASE + LRU_ACTIVE,				// active 匿名
	LRU_INACTIVE_FILE = LRU_BASE + LRU_FILE,				// inactive 文件
	LRU_ACTIVE_FILE = LRU_BASE + LRU_FILE + LRU_ACTIVE,		// active 文件
	LRU_UNEVICTABLE,										// 不可回收内存
	NR_LRU_LISTS
};

在 4.8 版本以前是每个 zone 拥有独立的 lru 链表，在 4.8 版本以后改成了每个 node 一个 lru 链表：

typedef struct pglist_data {

	struct lruvec		lruvec;

	...
} pg_data_t;	

struct lruvec {
	struct list_head		lists[NR_LRU_LISTS];			// lru 链表
	struct zone_reclaim_stat	reclaim_stat;
	/* Evictions & activations on the inactive file list */
	atomic_long_t			inactive_age;
	/* Refaults at the time of last reclaim cycle */
	unsigned long			refaults;
#ifdef CONFIG_MEMCG
	struct pglist_data *pgdat;
#endif
};

2.2 LRU Cache

为了减少多个CPU在操作LRU链表时的拿锁冲突，系统设计了 PerCPU 的 lru cache。每个 cache 能容纳 14 个 page，一共定义了以下几类 cache：

static DEFINE_PER_CPU(struct pagevec, lru_add_pvec);				// 将不处于lru链表的新页放入到lru链表中
static DEFINE_PER_CPU(struct pagevec, lru_rotate_pvecs);			// 将非活动lru链表中的页移动到非活动lru链表尾部
static DEFINE_PER_CPU(struct pagevec, lru_deactivate_file_pvecs);	// 将处于活动lru链表的页移动到非活动lru链表
static DEFINE_PER_CPU(struct pagevec, lru_lazyfree_pvecs);
#ifdef CONFIG_SMP
static DEFINE_PER_CPU(struct pagevec, activate_page_pvecs);			// 将处于非活动lru链表的页移动到活动lru链表
#endif

/* 14 pointers + two long's align the pagevec structure to a power of two */
#define PAGEVEC_SIZE	14

struct pagevec {
	unsigned long nr;
	bool percpu_pvec_drained;
	struct page *pages[PAGEVEC_SIZE];
};

2.3 LRU 移动操作

page 可以加入到 lru 链表，并且根据条件在 active/inactive 链表间移动。

2.3.1 page 加入 LRU

关于 lru 的操作，其中最重要的是把 新分配的 page 加入到 lru 中。这部分工作一般由 do_page_fault() 来处理。

do_page_fault() 分配内存 page，以及把新 page 加入到 lru 的典型场景如下：

场景	入口函数	LRU函数调用关系	条件	LRU链表	LRU list	page flags
匿名内存第一次发生缺页	do_anonymous_page()	lru_cache_add_active_or_unevictable() → lru_cache_add() → __lru_cache_add() → __pagevec_lru_add() → pagevec_lru_move_fn() → __pagevec_lru_add_fn()	!(vma->vm_flags & VM_LOCKED)	匿名 active lru	pglist_data->lruvec.lists[LRU_ACTIVE_ANON]	PG_swapbacked + PG_active + PG_lru
↑	↑	lru_cache_add_active_or_unevictable() → add_page_to_unevictable_list()	(vma->vm_flags & VM_LOCKED)	不可回收 lru	pglist_data->lruvec.lists[LRU_UNEVICTABLE]	PG_swapbacked + PG_unevictable + PG_lru
匿名内存被swap出去后发生缺页	do_swap_page()	lru_cache_add_active_or_unevictable()	↑	↑	↑	↑
私有文件内存写操作缺页	do_cow_fault()	finish_fault() → alloc_set_pte() → lru_cache_add_active_or_unevictable()	!(vma->vm_flags & VM_LOCKED)	匿名 active lru	pglist_data->lruvec.lists[LRU_ACTIVE_ANON]	PG_swapbacked + PG_active + PG_lru
↑	↑	finish_fault() → alloc_set_pte() → lru_cache_add_active_or_unevictable()	(vma->vm_flags & VM_LOCKED)	不可回收 lru	pglist_data->lruvec.lists[LRU_UNEVICTABLE]	PG_swapbacked + PG_unevictable + PG_lru
私有内存写操作缺页	do_wp_page()	wp_page_copy() → lru_cache_add_active_or_unevictable()	↑	↑	↑	↑
文件内存读操作缺页	do_read_fault()	pagecache_get_page() → add_to_page_cache_lru() → lru_cache_add()	-	文件 lru	pglist_data->lruvec.lists[LRU_INACTIVE_FILE / LRU_ACTIVE_FILE]	PG_active + PG_lru
共享文件内存写操作缺页	do_shared_fault()	-	↑	↑	↑	↑

其中核心部分的代码分析如下：

static void __lru_cache_add(struct page *page)
{
	struct pagevec *pvec = &get_cpu_var(lru_add_pvec);

	get_page(page);
	/* (1) 首先把 page 加入到 lru cache 中 */
	if (!pagevec_add(pvec, page) || PageCompound(page))
		/* (2) 如果 lru cache 空间已满，把page加入到各自对应的 lru 链表中 */
		__pagevec_lru_add(pvec);
	put_cpu_var(lru_add_pvec);
}

↓

void __pagevec_lru_add(struct pagevec *pvec)
{
	/* (2.1) 遍历 lru cache 中的 page，根据 page 的标志把 page 加入到不同类型的 lru 链表 */
	pagevec_lru_move_fn(pvec, __pagevec_lru_add_fn, NULL);
}

↓

static void __pagevec_lru_add_fn(struct page *page, struct lruvec *lruvec,
				 void *arg)
{
	int file = page_is_file_cache(page);
	int active = PageActive(page);
	/* (2.1.1) 根据 page 中的标志，获取到 page 想要加入的 lru 类型 */
	enum lru_list lru = page_lru(page);

	VM_BUG_ON_PAGE(PageLRU(page), page);

	/* (2.1.2) 加入 lru 链表的 page 设置 PG_lru 标志 */
	SetPageLRU(page);
	/* (2.1.3) 加入 lru 链表 */
	add_page_to_lru_list(page, lruvec, lru);
	update_page_reclaim_stat(lruvec, file, active);
	trace_mm_lru_insertion(page, lru);
}

↓

static __always_inline enum lru_list page_lru(struct page *page)
{
	enum lru_list lru;

	/* (2.1.1.1) page 标志设置了 PG_unevictable，lru = LRU_UNEVICTABLE */
	if (PageUnevictable(page))
		lru = LRU_UNEVICTABLE;
	/* (2.1.1.2) page 标志设置：
		PG_swapbacked，lru = LRU_INACTIVE_ANON
		PG_swapbacked +  PG_active，lru = LRU_ACTIVE_ANON
		，lru = LRU_INACTIVE_FILE
		PG_active，lru = LRU_ACTIVE_FILE
	 */
	else {
		lru = page_lru_base_type(page);
		if (PageActive(page))
			lru += LRU_ACTIVE;
	}
	return lru;
}

static inline enum lru_list page_lru_base_type(struct page *page)
{
	if (page_is_file_cache(page))
		return LRU_INACTIVE_FILE;
	return LRU_INACTIVE_ANON;
}

static inline int page_is_file_cache(struct page *page)
{
	return !PageSwapBacked(page);
}

2.3.2 其他 LRU 移动操作

action	function
将处于非活动链表中的页移动到非活动链表尾部	rotate_reclaimable_page() → pagevec_move_tail() → pagevec_move_tail_fn()
将活动lru链表中的页加入到非活动lru链表中	deactivate_page() → lru_deactivate_file_fn()
将非活动lru链表的页加入到活动lru链表	activate_page() → __activate_page()

3. LRU 回收

使用 LRU 回收内存的大概流程如下所示：

3.1 LRU 更新

为了减少对性能的影响，系统把加入到 LRU 的内存 page 分为 active 和 inactive，从 inactive 链表中回收内存。page 近期被访问过即 active，近期没有被访问即 inactive。

系统并不会设计一个定时器，而是通过判断两次扫描之间 PTE 中的 Accessed bit 没有被置位，从而来判断对应 page 有没有被访问过。每次扫描完会清理 Accessed bit：

一个 page 可能会被多个 vma 锁映射，系统通过 反向映射 找到所有 vma ，并统计 有多少个 vma 的 pte 被访问过 accessed。

page_referenced()

• 1、在 active 链表回收扫描函数 shrink_active_list() 中的处理：

shrink_node() → shrink_node_memcg() → shrink_list() → shrink_active_list():

shrink_active_list()
{
	...
			/* (1) page 对于的任一 vma 的 pte Accessed bit 有被置位，且是代码段
					这种 page 先重新放回 active 链表的表头
			  */
			if (page_referenced(page, 0, sc->target_mem_cgroup,
				    &vm_flags)) {
			nr_rotated += hpage_nr_pages(page);
			/*
			 * Identify referenced, file-backed active pages and
			 * give them one more trip around the active list. So
			 * that executable code get better chances to stay in
			 * memory under moderate memory pressure.  Anon pages
			 * are not likely to be evicted by use-once streaming
			 * IO, plus JVM can create lots of anon VM_EXEC pages,
			 * so we ignore them here.
			 */
			if ((vm_flags & VM_EXEC) && page_is_file_cache(page)) {
				list_add(&page->lru, &l_active);
				continue;
			}
		}

		/* (2) 对于其他 page，将其从 active 链表移动到 inactive 链表 */
		ClearPageActive(page);	/* we are de-activating */
		list_add(&page->lru, &l_inactive);
	...
}

• 2、inactive 链表回收扫描函数 shrink_inactive_list() 中的处理：

shrink_node() → shrink_node_memcg() → shrink_list() → shrink_inactive_list() → shrink_page_list() → page_check_references():

static enum page_references page_check_references(struct page *page,
						  struct scan_control *sc)
{
	int referenced_ptes, referenced_page;
	unsigned long vm_flags;

	/* (1) page 对于的任一 vma 的 pte Accessed bit 有被置位 */
	referenced_ptes = page_referenced(page, 1, sc->target_mem_cgroup,
					  &vm_flags);
	/* (2) 判断 page 结构中的 PG_referenced 标志并清除 */
	referenced_page = TestClearPageReferenced(page);

	/*
	 * Mlock lost the isolation race with us.  Let try_to_unmap()
	 * move the page to the unevictable list.
	 */
	if (vm_flags & VM_LOCKED)
		return PAGEREF_RECLAIM;

	/* (3) Accessed > 0 */
	if (referenced_ptes) {
		/* (3.1) 情况1：匿名内存 + Accessed，page 从 inactive 链表移回 active 链表 */
		if (PageSwapBacked(page))
			return PAGEREF_ACTIVATE;
		/*
		 * All mapped pages start out with page table
		 * references from the instantiating fault, so we need
		 * to look twice if a mapped file page is used more
		 * than once.
		 *
		 * Mark it and spare it for another trip around the
		 * inactive list.  Another page table reference will
		 * lead to its activation.
		 *
		 * Note: the mark is set for activated pages as well
		 * so that recently deactivated but used pages are
		 * quickly recovered.
		 */
		/* (3.2) 文件内存 + Accessed，设置 PG_referenced 标志  */
		SetPageReferenced(page);

		/* (3.3) 情况2：文件内存 + Accessed>1 + 之前PG_referenced，page 从 inactive 链表移回 active 链表
				相当于两次扫描都是 Accessed 被置位，第一次扫描是情况4 设置了 PG_referenced 并且保留在 inactive 链表，第二次扫描 到了当前的情况2
		 */
		if (referenced_page || referenced_ptes > 1)
			return PAGEREF_ACTIVATE;

		/*
		 * Activate file-backed executable pages after first usage.
		 */
		/* (3.3) 情况3：文件内存 + Accessed + 代码段，page 从 inactive 链表移回 active 链表 */
		if (vm_flags & VM_EXEC)
			return PAGEREF_ACTIVATE;

		/* (3.3) 情况4：文件内存 + Accessed + 其他情况，page 保留在 inactive 链表不回收 */
		return PAGEREF_KEEP;
	}

	/* (4) Accessed == 0，page 可以回收 */
	/* Reclaim if clean, defer dirty pages to writeback */
	if (referenced_page && !PageSwapBacked(page))
		return PAGEREF_RECLAIM_CLEAN;

	return PAGEREF_RECLAIM;
}

3.2 Swappiness

对于内存回收时是优先回收匿名内存还是文件内存，由/proc/sys/vm/swappiness这个参数来指定。

swappiness的值从0到100不等，默认一般是60（只是一个经验值），这个值越高，则回收的时候越优先选择anonymous pages。当swappiness等于100的时候，anonymous pages和page cache就具有相同的优先级。

shrink_node() → shrink_node_memcg() → get_scan_count():

3.3 反向映射

page 通过 反向映射能查找到所有映射的 vma，这个也是重中之重。写了一篇独立的文章来说明：Rmap 内存反向映射机制。

3.4 代码实现

关于 lru 回收的核心代码由 shrink_node() 实现，不论是 get_page_from_freelist()、__alloc_pages_slowpath() 中启动的哪种回收，最后都会调用到 shrink_node()。

3.4.1 struct scan_control

虽然都是调用 shrink_node()，但是传入的参数不一样内存回收的行为也不一样。struct scan_control 定义了回收参数，简称 sc。

struct scan_control {
	/* How many pages shrink_list() should reclaim */
	/* 需要回收的page数量 */
	unsigned long nr_to_reclaim;

	/* This context's GFP mask */
	/* 申请内存时使用的分配标志 */
	gfp_t gfp_mask;

	/* Allocation order */
	/* 申请内存时使用的order值 */
	int order;

	/*
	 * Nodemask of nodes allowed by the caller. If NULL, all nodes
	 * are scanned.
	 */
	nodemask_t	*nodemask;

	/*
	 * The memory cgroup that hit its limit and as a result is the
	 * primary target of this reclaim invocation.
	 */
	struct mem_cgroup *target_mem_cgroup;

	/* Scan (total_size >> priority) pages at once */
	/* 扫描优先级，代表一次扫描(total_size >> priority)个页框
     * 优先级越低，一次扫描的页框数量就越多
     * 优先级越高，一次扫描的数量就越少
     * 默认优先级为12
     */
	int priority;

	/* The highest zone to isolate pages for reclaim from */
	enum zone_type reclaim_idx;

	/* Writepage batching in laptop mode; RECLAIM_WRITE */
	/* 是否能够进行回写操作(与分配标志的__GFP_IO和__GFP_FS有关) */
	unsigned int may_writepage:1;

	/* Can mapped pages be reclaimed? */
	/* 能否进行unmap操作，就是将所有映射了此页的页表项清空 */
	unsigned int may_unmap:1;

	/* Can pages be swapped as part of reclaim? */
	/* 是否能够进行swap交换，如果不能，在内存回收时则不扫描匿名页lru链表 */
	unsigned int may_swap:1;

	/*
	 * Cgroups are not reclaimed below their configured memory.low,
	 * unless we threaten to OOM. If any cgroups are skipped due to
	 * memory.low and nothing was reclaimed, go back for memory.low.
	 */
	unsigned int memcg_low_reclaim:1;
	unsigned int memcg_low_skipped:1;

	unsigned int hibernation_mode:1;

	/* One of the zones is ready for compaction */
	/* 扫描结束后会标记，用于内存回收判断是否需要进行内存压缩 */
	unsigned int compaction_ready:1;

	/* Incremented by the number of inactive pages that were scanned */
	/* 已经扫描的页框数量 */
	unsigned long nr_scanned;

	/* Number of pages freed so far during a call to shrink_zones() */
	/* 已经回收的页框数量 */
	unsigned long nr_reclaimed;
};

3.4.2 shrink_node()

shrink_node() → shrink_node_memcg():

static void shrink_node_memcg(struct pglist_data *pgdat, struct mem_cgroup *memcg,
			      struct scan_control *sc, unsigned long *lru_pages)
{
	/* (1) 获取到当前 node 的 lruvec */
	struct lruvec *lruvec = mem_cgroup_lruvec(pgdat, memcg);
	unsigned long nr[NR_LRU_LISTS];
	unsigned long targets[NR_LRU_LISTS];
	unsigned long nr_to_scan;
	enum lru_list lru;
	unsigned long nr_reclaimed = 0;
	unsigned long nr_to_reclaim = sc->nr_to_reclaim;
	struct blk_plug plug;
	bool scan_adjusted;

	/* (2) 根据sc参数 和 swappiness 参数，计算出每种类型 lru 链表需要扫描的个数，放在 nr[] 数组中 */
	get_scan_count(lruvec, memcg, sc, nr, lru_pages);

	/* Record the original scan target for proportional adjustments later */
	memcpy(targets, nr, sizeof(nr));

	/*
	 * Global reclaiming within direct reclaim at DEF_PRIORITY is a normal
	 * event that can occur when there is little memory pressure e.g.
	 * multiple streaming readers/writers. Hence, we do not abort scanning
	 * when the requested number of pages are reclaimed when scanning at
	 * DEF_PRIORITY on the assumption that the fact we are direct
	 * reclaiming implies that kswapd is not keeping up and it is best to
	 * do a batch of work at once. For memcg reclaim one check is made to
	 * abort proportional reclaim if either the file or anon lru has already
	 * dropped to zero at the first pass.
	 */
	scan_adjusted = (global_reclaim(sc) && !current_is_kswapd() &&
			 sc->priority == DEF_PRIORITY);

	blk_start_plug(&plug);
	/* (3) 有扫描额度，循环进行扫描 */
	while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
					nr[LRU_INACTIVE_FILE]) {
		unsigned long nr_anon, nr_file, percentage;
		unsigned long nr_scanned;

		/* (4) 逐个对可回收的 lru 链表进行回收扫描，包括：
				LRU_INACTIVE_ANON
				LRU_ACTIVE_ANON
				LRU_INACTIVE_FILE
				LRU_ACTIVE_FILE
		 */
		for_each_evictable_lru(lru) {
			if (nr[lru]) {
				nr_to_scan = min(nr[lru], SWAP_CLUSTER_MAX);
				nr[lru] -= nr_to_scan;

				/* (4.1) 核心：对 active 和 inactive 链表进行回收扫描 */
				nr_reclaimed += shrink_list(lru, nr_to_scan,
							    lruvec, sc);
			}
		}

		cond_resched();

		if (nr_reclaimed < nr_to_reclaim || scan_adjusted)
			continue;

		/*
		 * For kswapd and memcg, reclaim at least the number of pages
		 * requested. Ensure that the anon and file LRUs are scanned
		 * proportionally what was requested by get_scan_count(). We
		 * stop reclaiming one LRU and reduce the amount scanning
		 * proportional to the original scan target.
		 */
		nr_file = nr[LRU_INACTIVE_FILE] + nr[LRU_ACTIVE_FILE];
		nr_anon = nr[LRU_INACTIVE_ANON] + nr[LRU_ACTIVE_ANON];

		/*
		 * It's just vindictive to attack the larger once the smaller
		 * has gone to zero.  And given the way we stop scanning the
		 * smaller below, this makes sure that we only make one nudge
		 * towards proportionality once we've got nr_to_reclaim.
		 */
		if (!nr_file || !nr_anon)
			break;

		if (nr_file > nr_anon) {
			unsigned long scan_target = targets[LRU_INACTIVE_ANON] +
						targets[LRU_ACTIVE_ANON] + 1;
			lru = LRU_BASE;
			percentage = nr_anon * 100 / scan_target;
		} else {
			unsigned long scan_target = targets[LRU_INACTIVE_FILE] +
						targets[LRU_ACTIVE_FILE] + 1;
			lru = LRU_FILE;
			percentage = nr_file * 100 / scan_target;
		}

		/* Stop scanning the smaller of the LRU */
		nr[lru] = 0;
		nr[lru + LRU_ACTIVE] = 0;

		/*
		 * Recalculate the other LRU scan count based on its original
		 * scan target and the percentage scanning already complete
		 */
		lru = (lru == LRU_FILE) ? LRU_BASE : LRU_FILE;
		nr_scanned = targets[lru] - nr[lru];
		nr[lru] = targets[lru] * (100 - percentage) / 100;
		nr[lru] -= min(nr[lru], nr_scanned);

		lru += LRU_ACTIVE;
		nr_scanned = targets[lru] - nr[lru];
		nr[lru] = targets[lru] * (100 - percentage) / 100;
		nr[lru] -= min(nr[lru], nr_scanned);

		scan_adjusted = true;
	}
	blk_finish_plug(&plug);
	sc->nr_reclaimed += nr_reclaimed;

	/*
	 * Even if we did not try to evict anon pages at all, we want to
	 * rebalance the anon lru active/inactive ratio.
	 */
	if (inactive_list_is_low(lruvec, false, sc, true))
		shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
				   sc, LRU_ACTIVE_ANON);
}

3.4.3 shrink_list()

static unsigned long shrink_list(enum lru_list lru, unsigned long nr_to_scan,
				 struct lruvec *lruvec, struct scan_control *sc)
{
	/* (4.1.1) 扫描 active 链表
				只有在 inactive 链表数量过小的时候，才会启动对 active 链表的扫描
				扫描 active 链表不会产生 page 回收，只会把某些没有访问的 page 移动到 inactive 链表
	 */
	if (is_active_lru(lru)) {
		if (inactive_list_is_low(lruvec, is_file_lru(lru), sc, true))
			shrink_active_list(nr_to_scan, lruvec, sc, lru);
		return 0;
	}

	/* (4.1.2) 扫描 inactive 链表。真正启动对内存 page 的回收 */
	return shrink_inactive_list(nr_to_scan, lruvec, sc, lru);
}

↓

/*
 * The inactive anon list should be small enough that the VM never has
 * to do too much work.
 * inactive anon 链表应该足够小，这样VM就不会做太多的工作。
 *
 * The inactive file list should be small enough to leave most memory
 * to the established workingset on the scan-resistant active list,
 * but large enough to avoid thrashing the aggregate readahead window.
 * inactive 文件链表应该足够小，以便将大部分内存留给抗扫描的 active 链表上的已建立的工作集，但又需要足够大以避免冲击聚合预读窗口。
 *
 * Both inactive lists should also be large enough that each inactive
 * page has a chance to be referenced again before it is reclaimed.
 * 两个 inactive 链表也应该足够大，以便每个非活动页面在被回收之前有机会再次被引用。
 *
 * If that fails and refaulting is observed, the inactive list grows.
 * 如果失败，并观察到 refaulting，非活动列表增长。
 *
 * The inactive_ratio is the target ratio of ACTIVE to INACTIVE pages
 * on this LRU, maintained by the pageout code. An inactive_ratio
 * of 3 means 3:1 or 25% of the pages are kept on the inactive list.
 *  inactive_ratio 是这个LRU上ACTIVE和INACTIVE页面的目标比率，由分页输出代码维护。inactive_ratio为3意味着3:1或25%的页面保持在非活动列表中。
 *
 * total     target    max
 * memory    ratio     inactive
 * -------------------------------------
 *   10MB       1         5MB
 *  100MB       1        50MB
 *    1GB       3       250MB
 *   10GB      10       0.9GB
 *  100GB      31         3GB
 *    1TB     101        10GB
 *   10TB     320        32GB
 */
static bool inactive_list_is_low(struct lruvec *lruvec, bool file,
				 struct scan_control *sc, bool trace)
{
	enum lru_list active_lru = file * LRU_FILE + LRU_ACTIVE;
	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
	enum lru_list inactive_lru = file * LRU_FILE;
	unsigned long inactive, active;
	unsigned long inactive_ratio;
	unsigned long refaults;
	unsigned long gb;

	/*
	 * If we don't have swap space, anonymous page deactivation
	 * is pointless.
	 */
	if (!file && !total_swap_pages)
		return false;

	inactive = lruvec_lru_size(lruvec, inactive_lru, sc->reclaim_idx);
	active = lruvec_lru_size(lruvec, active_lru, sc->reclaim_idx);

	/*
	 * When refaults are being observed, it means a new workingset
	 * is being established. Disable active list protection to get
	 * rid of the stale workingset quickly.
	 */
	/* inactive page 比率的计算 */
	refaults = lruvec_page_state(lruvec, WORKINGSET_ACTIVATE);
	if (file && lruvec->refaults != refaults) {
		inactive_ratio = 0;
	} else {
		gb = (inactive + active) >> (30 - PAGE_SHIFT);
		if (gb)
			inactive_ratio = int_sqrt(10 * gb);
		else
			inactive_ratio = 1;
	}

	if (trace)
		trace_mm_vmscan_inactive_list_is_low(pgdat->node_id, sc->reclaim_idx,
			lruvec_lru_size(lruvec, inactive_lru, MAX_NR_ZONES), inactive,
			lruvec_lru_size(lruvec, active_lru, MAX_NR_ZONES), active,
			inactive_ratio, file);

	return inactive * inactive_ratio < active;
}

3.4.4 shrink_active_list()

该函数负责对 active lru list 进行扫描，以得到更多的 inactive page。

static void shrink_active_list(unsigned long nr_to_scan,
			       struct lruvec *lruvec,
			       struct scan_control *sc,
			       enum lru_list lru)
{
	unsigned long nr_taken;
	unsigned long nr_scanned;
	unsigned long vm_flags;
	LIST_HEAD(l_hold);	/* The pages which were snipped off */
	LIST_HEAD(l_active);
	LIST_HEAD(l_inactive);
	struct page *page;
	struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
	unsigned nr_deactivate, nr_activate;
	unsigned nr_rotated = 0;
	isolate_mode_t isolate_mode = 0;
	int file = is_file_lru(lru);
	struct pglist_data *pgdat = lruvec_pgdat(lruvec);

	lru_add_drain();

	if (!sc->may_unmap)
		isolate_mode |= ISOLATE_UNMAPPED;

	spin_lock_irq(&pgdat->lru_lock);

	/* (4.1.1.1) 从目标链表 结尾 中取出 nr_to_scan 个 page，暂存在 l_hold 链表中 */
	nr_taken = isolate_lru_pages(nr_to_scan, lruvec, &l_hold,
				     &nr_scanned, sc, isolate_mode, lru);

	__mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, nr_taken);
	reclaim_stat->recent_scanned[file] += nr_taken;

	__count_vm_events(PGREFILL, nr_scanned);
	count_memcg_events(lruvec_memcg(lruvec), PGREFILL, nr_scanned);

	spin_unlock_irq(&pgdat->lru_lock);

	/* (4.1.1.2) 逐个扫描 l_hold 链表中的 page，进行处理 */
	while (!list_empty(&l_hold)) {
		cond_resched();
		page = lru_to_page(&l_hold);
		list_del(&page->lru);

		/* (4.1.1.2.1) 如果 page 不是可回收的，将其放回 lru 链表 */
		if (unlikely(!page_evictable(page))) {
			putback_lru_page(page);
			continue;
		}

		/* (4.1.1.2.2) 如果 buffer_heads 超过限制，尝试释放是 buffer_heads 的 page */
		if (unlikely(buffer_heads_over_limit)) {
			if (page_has_private(page) && trylock_page(page)) {
				if (page_has_private(page))
					try_to_release_page(page, 0);
				unlock_page(page);
			}
		}

		/* (4.1.1.2.3) 查询 page 所有的反向映射 vma，其中 pte 中的 accessed bit 是否被置位
						如果有任一 vma 的 accessed 被置位，说明该 page 被访问过，是 referenced
		 */
		if (page_referenced(page, 0, sc->target_mem_cgroup,
				    &vm_flags)) {
			nr_rotated += hpage_nr_pages(page);
			/*
			 * Identify referenced, file-backed active pages and
			 * give them one more trip around the active list. So
			 * that executable code get better chances to stay in
			 * memory under moderate memory pressure.  Anon pages
			 * are not likely to be evicted by use-once streaming
			 * IO, plus JVM can create lots of anon VM_EXEC pages,
			 * so we ignore them here.
			 * 识别引用的、有文件支持的活动页面，并让它们在活动列表中多走一圈。
			 * 因此，在适度的内存压力下，可执行代码有更好的机会留在内存中。
			 * Anon页不太可能被一次性使用的流IO逐出，而且JVM可以创建大量的Anon VM_EXEC页，所以我们在这里忽略它们。
			 */
			/* 对于 file 代码段，如果被访问过，再给一次机会，先不要 移入到 inactive 链表，准备放回 active 链表  */
			if ((vm_flags & VM_EXEC) && page_is_file_cache(page)) {
				list_add(&page->lru, &l_active);
				continue;
			}
		}

		/* (4.1.1.2.4) 对于其他的 page，清除 PG_active 标志，准备放进 inactive 链表 */
		ClearPageActive(page);	/* we are de-activating */
		list_add(&page->lru, &l_inactive);
	}

	/*
	 * Move pages back to the lru list.
	 */
	spin_lock_irq(&pgdat->lru_lock);
	/*
	 * Count referenced pages from currently used mappings as rotated,
	 * even though only some of them are actually re-activated.  This
	 * helps balance scan pressure between file and anonymous pages in
	 * get_scan_count.
	 */
	reclaim_stat->recent_rotated[file] += nr_rotated;

	/* (4.1.1.3) 将临时链表 l_active 中的 page 放回原来的 active 链表 */
	nr_activate = move_active_pages_to_lru(lruvec, &l_active, &l_hold, lru);
	/* (4.1.1.4) 将临时链表 l_inactive 中的 page 移进正式的 inactive 链表 */
	nr_deactivate = move_active_pages_to_lru(lruvec, &l_inactive, &l_hold, lru - LRU_ACTIVE);
	__mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);
	spin_unlock_irq(&pgdat->lru_lock);

	mem_cgroup_uncharge_list(&l_hold);
	/* (4.1.1.5) 将临时链表 l_hold 中剩余无人要的的 page 释放 */
	free_unref_page_list(&l_hold);
	trace_mm_vmscan_lru_shrink_active(pgdat->node_id, nr_taken, nr_activate,
			nr_deactivate, nr_rotated, sc->priority, file);
}

3.4.5 shrink_inactive_list()

该函数负责对 inactive lru list 进行扫描，尝试回收内存 page。

static noinline_for_stack unsigned long
shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
		     struct scan_control *sc, enum lru_list lru)
{
	LIST_HEAD(page_list);
	unsigned long nr_scanned;
	unsigned long nr_reclaimed = 0;
	unsigned long nr_taken;
	struct reclaim_stat stat = {};
	isolate_mode_t isolate_mode = 0;
	int file = is_file_lru(lru);
	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
	struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
	bool stalled = false;

	while (unlikely(too_many_isolated(pgdat, file, sc))) {
		if (stalled)
			return 0;

		/* wait a bit for the reclaimer. */
		msleep(100);
		stalled = true;

		/* We are about to die and free our memory. Return now. */
		if (fatal_signal_pending(current))
			return SWAP_CLUSTER_MAX;
	}

	lru_add_drain();

	if (!sc->may_unmap)
		isolate_mode |= ISOLATE_UNMAPPED;

	spin_lock_irq(&pgdat->lru_lock);

	/* (4.1.2.1) 尝试从 inactive 链表中摘取 nr_to_scan 个 page，放到临时链表  page_list */
	nr_taken = isolate_lru_pages(nr_to_scan, lruvec, &page_list,
				     &nr_scanned, sc, isolate_mode, lru);

	__mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, nr_taken);
	reclaim_stat->recent_scanned[file] += nr_taken;

	if (current_is_kswapd()) {
		if (global_reclaim(sc))
			__count_vm_events(PGSCAN_KSWAPD, nr_scanned);
		count_memcg_events(lruvec_memcg(lruvec), PGSCAN_KSWAPD,
				   nr_scanned);
	} else {
		if (global_reclaim(sc))
			__count_vm_events(PGSCAN_DIRECT, nr_scanned);
		count_memcg_events(lruvec_memcg(lruvec), PGSCAN_DIRECT,
				   nr_scanned);
	}
	spin_unlock_irq(&pgdat->lru_lock);

	if (nr_taken == 0)
		return 0;

	/* (4.1.2.2) 扫描临时链表  page_list，执行真正的回收动作 */
	nr_reclaimed = shrink_page_list(&page_list, pgdat, sc, 0,
				&stat, false);

	spin_lock_irq(&pgdat->lru_lock);

	if (current_is_kswapd()) {
		if (global_reclaim(sc))
			__count_vm_events(PGSTEAL_KSWAPD, nr_reclaimed);
		count_memcg_events(lruvec_memcg(lruvec), PGSTEAL_KSWAPD,
				   nr_reclaimed);
	} else {
		if (global_reclaim(sc))
			__count_vm_events(PGSTEAL_DIRECT, nr_reclaimed);
		count_memcg_events(lruvec_memcg(lruvec), PGSTEAL_DIRECT,
				   nr_reclaimed);
	}

	/* (4.1.2.3) 扫描返回时 page_list 中存放的是不能回收需要放回 inactive/active 链表的 page */
	putback_inactive_pages(lruvec, &page_list);

	__mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);

	spin_unlock_irq(&pgdat->lru_lock);

	mem_cgroup_uncharge_list(&page_list);
	/* (4.1.2.3) 释放掉 page_list 中剩余的 page */
	free_unref_page_list(&page_list);

	/*
	 * If reclaim is isolating dirty pages under writeback, it implies
	 * that the long-lived page allocation rate is exceeding the page
	 * laundering rate. Either the global limits are not being effective
	 * at throttling processes due to the page distribution throughout
	 * zones or there is heavy usage of a slow backing device. The
	 * only option is to throttle from reclaim context which is not ideal
	 * as there is no guarantee the dirtying process is throttled in the
	 * same way balance_dirty_pages() manages.
	 *
	 * Once a zone is flagged ZONE_WRITEBACK, kswapd will count the number
	 * of pages under pages flagged for immediate reclaim and stall if any
	 * are encountered in the nr_immediate check below.
	 */
	if (stat.nr_writeback && stat.nr_writeback == nr_taken)
		set_bit(PGDAT_WRITEBACK, &pgdat->flags);

	/*
	 * If dirty pages are scanned that are not queued for IO, it
	 * implies that flushers are not doing their job. This can
	 * happen when memory pressure pushes dirty pages to the end of
	 * the LRU before the dirty limits are breached and the dirty
	 * data has expired. It can also happen when the proportion of
	 * dirty pages grows not through writes but through memory
	 * pressure reclaiming all the clean cache. And in some cases,
	 * the flushers simply cannot keep up with the allocation
	 * rate. Nudge the flusher threads in case they are asleep.
	 */
	if (stat.nr_unqueued_dirty == nr_taken)
		wakeup_flusher_threads(WB_REASON_VMSCAN);

	/*
	 * Legacy memcg will stall in page writeback so avoid forcibly
	 * stalling here.
	 */
	if (sane_reclaim(sc)) {
		/*
		 * Tag a zone as congested if all the dirty pages scanned were
		 * backed by a congested BDI and wait_iff_congested will stall.
		 */
		if (stat.nr_dirty && stat.nr_dirty == stat.nr_congested)
			set_bit(PGDAT_CONGESTED, &pgdat->flags);

		/* Allow kswapd to start writing pages during reclaim. */
		if (stat.nr_unqueued_dirty == nr_taken)
			set_bit(PGDAT_DIRTY, &pgdat->flags);

		/*
		 * If kswapd scans pages marked marked for immediate
		 * reclaim and under writeback (nr_immediate), it implies
		 * that pages are cycling through the LRU faster than
		 * they are written so also forcibly stall.
		 */
		if (stat.nr_immediate && current_may_throttle())
			congestion_wait(BLK_RW_ASYNC, HZ/10);
	}

	/*
	 * Stall direct reclaim for IO completions if underlying BDIs or zone
	 * is congested. Allow kswapd to continue until it starts encountering
	 * unqueued dirty pages or cycling through the LRU too quickly.
	 */
	if (!sc->hibernation_mode && !current_is_kswapd() &&
	    current_may_throttle())
		wait_iff_congested(pgdat, BLK_RW_ASYNC, HZ/10);

	trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
			nr_scanned, nr_reclaimed,
			stat.nr_dirty,  stat.nr_writeback,
			stat.nr_congested, stat.nr_immediate,
			stat.nr_activate, stat.nr_ref_keep,
			stat.nr_unmap_fail,
			sc->priority, file);
	return nr_reclaimed;
}

↓

static unsigned long shrink_page_list(struct list_head *page_list,
				      struct pglist_data *pgdat,
				      struct scan_control *sc,
				      enum ttu_flags ttu_flags,
				      struct reclaim_stat *stat,
				      bool force_reclaim)
{
	LIST_HEAD(ret_pages);
	LIST_HEAD(free_pages);
	int pgactivate = 0;
	unsigned nr_unqueued_dirty = 0;
	unsigned nr_dirty = 0;
	unsigned nr_congested = 0;
	unsigned nr_reclaimed = 0;
	unsigned nr_writeback = 0;
	unsigned nr_immediate = 0;
	unsigned nr_ref_keep = 0;
	unsigned nr_unmap_fail = 0;

	cond_resched();

	/* (4.1.2.2.1) 扫描临时链表  page_list，尝试逐个回收其中的 page */
	while (!list_empty(page_list)) {
		struct address_space *mapping;
		struct page *page;
		int may_enter_fs;
		enum page_references references = PAGEREF_RECLAIM_CLEAN;
		bool dirty, writeback;

		cond_resched();

		page = lru_to_page(page_list);
		list_del(&page->lru);

		if (!trylock_page(page))
			goto keep;

		VM_BUG_ON_PAGE(PageActive(page), page);

		sc->nr_scanned++;

		if (unlikely(!page_evictable(page)))
			goto activate_locked;

		if (!sc->may_unmap && page_mapped(page))
			goto keep_locked;

		/* Double the slab pressure for mapped and swapcache pages */
		if ((page_mapped(page) || PageSwapCache(page)) &&
		    !(PageAnon(page) && !PageSwapBacked(page)))
			sc->nr_scanned++;

		may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
			(PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));

		/*
		 * The number of dirty pages determines if a zone is marked
		 * reclaim_congested which affects wait_iff_congested. kswapd
		 * will stall and start writing pages if the tail of the LRU
		 * is all dirty unqueued pages.
		 */
		page_check_dirty_writeback(page, &dirty, &writeback);
		if (dirty || writeback)
			nr_dirty++;

		if (dirty && !writeback)
			nr_unqueued_dirty++;

		/*
		 * Treat this page as congested if the underlying BDI is or if
		 * pages are cycling through the LRU so quickly that the
		 * pages marked for immediate reclaim are making it to the
		 * end of the LRU a second time.
		 */
		mapping = page_mapping(page);
		if (((dirty || writeback) && mapping &&
		     inode_write_congested(mapping->host)) ||
		    (writeback && PageReclaim(page)))
			nr_congested++;

		/*
		 * If a page at the tail of the LRU is under writeback, there
		 * are three cases to consider.
		 *
		 * 1) If reclaim is encountering an excessive number of pages
		 *    under writeback and this page is both under writeback and
		 *    PageReclaim then it indicates that pages are being queued
		 *    for IO but are being recycled through the LRU before the
		 *    IO can complete. Waiting on the page itself risks an
		 *    indefinite stall if it is impossible to writeback the
		 *    page due to IO error or disconnected storage so instead
		 *    note that the LRU is being scanned too quickly and the
		 *    caller can stall after page list has been processed.
		 *
		 * 2) Global or new memcg reclaim encounters a page that is
		 *    not marked for immediate reclaim, or the caller does not
		 *    have __GFP_FS (or __GFP_IO if it's simply going to swap,
		 *    not to fs). In this case mark the page for immediate
		 *    reclaim and continue scanning.
		 *
		 *    Require may_enter_fs because we would wait on fs, which
		 *    may not have submitted IO yet. And the loop driver might
		 *    enter reclaim, and deadlock if it waits on a page for
		 *    which it is needed to do the write (loop masks off
		 *    __GFP_IO|__GFP_FS for this reason); but more thought
		 *    would probably show more reasons.
		 *
		 * 3) Legacy memcg encounters a page that is already marked
		 *    PageReclaim. memcg does not have any dirty pages
		 *    throttling so we could easily OOM just because too many
		 *    pages are in writeback and there is nothing else to
		 *    reclaim. Wait for the writeback to complete.
		 *
		 * In cases 1) and 2) we activate the pages to get them out of
		 * the way while we continue scanning for clean pages on the
		 * inactive list and refilling from the active list. The
		 * observation here is that waiting for disk writes is more
		 * expensive than potentially causing reloads down the line.
		 * Since they're marked for immediate reclaim, they won't put
		 * memory pressure on the cache working set any longer than it
		 * takes to write them to disk.
		 */
		/* (4.1.2.2.2) 需要回写的page的处理：放回 active 链表 */
		if (PageWriteback(page)) {
			/* Case 1 above */
			if (current_is_kswapd() &&
			    PageReclaim(page) &&
			    test_bit(PGDAT_WRITEBACK, &pgdat->flags)) {
				nr_immediate++;
				goto activate_locked;

			/* Case 2 above */
			} else if (sane_reclaim(sc) ||
			    !PageReclaim(page) || !may_enter_fs) {
				/*
				 * This is slightly racy - end_page_writeback()
				 * might have just cleared PageReclaim, then
				 * setting PageReclaim here end up interpreted
				 * as PageReadahead - but that does not matter
				 * enough to care.  What we do want is for this
				 * page to have PageReclaim set next time memcg
				 * reclaim reaches the tests above, so it will
				 * then wait_on_page_writeback() to avoid OOM;
				 * and it's also appropriate in global reclaim.
				 */
				SetPageReclaim(page);
				nr_writeback++;
				goto activate_locked;

			/* Case 3 above */
			} else {
				unlock_page(page);
				wait_on_page_writeback(page);
				/* then go back and try same page again */
				list_add_tail(&page->lru, page_list);
				continue;
			}
		}

		if (!force_reclaim)
			references = page_check_references(page, sc);

		/* (4.1.2.2.3) 读取 Accessed 和 PG_referenced 标志，来决定 page 是回收、还是放回 inactive/active 链表 */
		switch (references) {
		case PAGEREF_ACTIVATE:
			goto activate_locked;
		case PAGEREF_KEEP:
			nr_ref_keep++;
			goto keep_locked;
		case PAGEREF_RECLAIM:
		case PAGEREF_RECLAIM_CLEAN:
			; /* try to reclaim the page below */
		}

		/*
		 * Anonymous process memory has backing store?
		 * Try to allocate it some swap space here.
		 * Lazyfree page could be freed directly
		 */
		/* (4.1.2.2.4) 匿名可交换 page 的回收处理：把page交换到swap，并设置对应的 mapping */
		if (PageAnon(page) && PageSwapBacked(page)) {
			if (!PageSwapCache(page)) {
				if (!(sc->gfp_mask & __GFP_IO))
					goto keep_locked;
				if (PageTransHuge(page)) {
					/* cannot split THP, skip it */
					if (!can_split_huge_page(page, NULL))
						goto activate_locked;
					/*
					 * Split pages without a PMD map right
					 * away. Chances are some or all of the
					 * tail pages can be freed without IO.
					 */
					if (!compound_mapcount(page) &&
					    split_huge_page_to_list(page,
								    page_list))
						goto activate_locked;
				}
				if (!add_to_swap(page)) {
					if (!PageTransHuge(page))
						goto activate_locked;
					/* Fallback to swap normal pages */
					if (split_huge_page_to_list(page,
								    page_list))
						goto activate_locked;
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
					count_vm_event(THP_SWPOUT_FALLBACK);
#endif
					if (!add_to_swap(page))
						goto activate_locked;
				}

				may_enter_fs = 1;

				/* Adding to swap updated mapping */
				mapping = page_mapping(page);
			}
		} else if (unlikely(PageTransHuge(page))) {
			/* Split file THP */
			if (split_huge_page_to_list(page, page_list))
				goto keep_locked;
		}

		/*
		 * The page is mapped into the page tables of one or more
		 * processes. Try to unmap it here.
		 */
		/* (4.1.2.2.5) 匿名/文件 page 的回收处理：遍历反向映射的vma，解除所有 vma mmu的映射，并释放 page */
		if (page_mapped(page)) {
			enum ttu_flags flags = ttu_flags | TTU_BATCH_FLUSH;

			if (unlikely(PageTransHuge(page)))
				flags |= TTU_SPLIT_HUGE_PMD;
			if (!try_to_unmap(page, flags)) {
				nr_unmap_fail++;
				goto activate_locked;
			}
		}

		/* (4.1.2.2.6) dirty page 的处理：放回 active 链表 */
		if (PageDirty(page)) {
			/*
			 * Only kswapd can writeback filesystem pages
			 * to avoid risk of stack overflow. But avoid
			 * injecting inefficient single-page IO into
			 * flusher writeback as much as possible: only
			 * write pages when we've encountered many
			 * dirty pages, and when we've already scanned
			 * the rest of the LRU for clean pages and see
			 * the same dirty pages again (PageReclaim).
			 */
			if (page_is_file_cache(page) &&
			    (!current_is_kswapd() || !PageReclaim(page) ||
			     !test_bit(PGDAT_DIRTY, &pgdat->flags))) {
				/*
				 * Immediately reclaim when written back.
				 * Similar in principal to deactivate_page()
				 * except we already have the page isolated
				 * and know it's dirty
				 */
				inc_node_page_state(page, NR_VMSCAN_IMMEDIATE);
				SetPageReclaim(page);

				goto activate_locked;
			}

			if (references == PAGEREF_RECLAIM_CLEAN)
				goto keep_locked;
			if (!may_enter_fs)
				goto keep_locked;
			if (!sc->may_writepage)
				goto keep_locked;

			/*
			 * Page is dirty. Flush the TLB if a writable entry
			 * potentially exists to avoid CPU writes after IO
			 * starts and then write it out here.
			 */
			try_to_unmap_flush_dirty();
			switch (pageout(page, mapping, sc)) {
			case PAGE_KEEP:
				goto keep_locked;
			case PAGE_ACTIVATE:
				goto activate_locked;
			case PAGE_SUCCESS:
				if (PageWriteback(page))
					goto keep;
				if (PageDirty(page))
					goto keep;

				/*
				 * A synchronous write - probably a ramdisk.  Go
				 * ahead and try to reclaim the page.
				 */
				if (!trylock_page(page))
					goto keep;
				if (PageDirty(page) || PageWriteback(page))
					goto keep_locked;
				mapping = page_mapping(page);
			case PAGE_CLEAN:
				; /* try to free the page below */
			}
		}

		/*
		 * If the page has buffers, try to free the buffer mappings
		 * associated with this page. If we succeed we try to free
		 * the page as well.
		 *
		 * We do this even if the page is PageDirty().
		 * try_to_release_page() does not perform I/O, but it is
		 * possible for a page to have PageDirty set, but it is actually
		 * clean (all its buffers are clean).  This happens if the
		 * buffers were written out directly, with submit_bh(). ext3
		 * will do this, as well as the blockdev mapping.
		 * try_to_release_page() will discover that cleanness and will
		 * drop the buffers and mark the page clean - it can be freed.
		 *
		 * Rarely, pages can have buffers and no ->mapping.  These are
		 * the pages which were not successfully invalidated in
		 * truncate_complete_page().  We try to drop those buffers here
		 * and if that worked, and the page is no longer mapped into
		 * process address space (page_count == 1) it can be freed.
		 * Otherwise, leave the page on the LRU so it is swappable.
		 */
		/* (4.1.2.2.6) head buffer 的处理：直接释放 page */
		if (page_has_private(page)) {
			if (!try_to_release_page(page, sc->gfp_mask))
				goto activate_locked;
			if (!mapping && page_count(page) == 1) {
				unlock_page(page);
				if (put_page_testzero(page))
					goto free_it;
				else {
					/*
					 * rare race with speculative reference.
					 * the speculative reference will free
					 * this page shortly, so we may
					 * increment nr_reclaimed here (and
					 * leave it off the LRU).
					 */
					nr_reclaimed++;
					continue;
				}
			}
		}

		/* (4.1.2.2.4) 匿名不可交换 page 的回收处理：继续保持 page 在 inactive 链表 */
		if (PageAnon(page) && !PageSwapBacked(page)) {
			/* follow __remove_mapping for reference */
			if (!page_ref_freeze(page, 1))
				goto keep_locked;
			if (PageDirty(page)) {
				page_ref_unfreeze(page, 1);
				goto keep_locked;
			}

			count_vm_event(PGLAZYFREED);
			count_memcg_page_event(page, PGLAZYFREED);
		} else if (!mapping || !__remove_mapping(mapping, page, true))
			goto keep_locked;
		/*
		 * At this point, we have no other references and there is
		 * no way to pick any more up (removed from LRU, removed
		 * from pagecache). Can use non-atomic bitops now (and
		 * we obviously don't have to worry about waking up a process
		 * waiting on the page lock, because there are no references.
		 */
		__ClearPageLocked(page);
free_it:
		nr_reclaimed++;

		/*
		 * Is there need to periodically free_page_list? It would
		 * appear not as the counts should be low
		 */
		if (unlikely(PageTransHuge(page))) {
			mem_cgroup_uncharge(page);
			(*get_compound_page_dtor(page))(page);
		} else
			list_add(&page->lru, &free_pages);
		continue;

		/* (4.1.2.2.5)  把 page 从 inactive 链表移动到 active 链表的处理：设置 PG_active 标志 */
activate_locked:
		/* Not a candidate for swapping, so reclaim swap space. */
		if (PageSwapCache(page) && (mem_cgroup_swap_full(page) ||
						PageMlocked(page)))
			try_to_free_swap(page);
		VM_BUG_ON_PAGE(PageActive(page), page);
		if (!PageMlocked(page)) {
			SetPageActive(page);
			pgactivate++;
			count_memcg_page_event(page, PGACTIVATE);
		}
		/* (4.1.2.2.6) 不回收page，继续保留在 inactive 链表中 */
keep_locked:
		unlock_page(page);
keep:
		list_add(&page->lru, &ret_pages);
		VM_BUG_ON_PAGE(PageLRU(page) || PageUnevictable(page), page);
	}

	mem_cgroup_uncharge_list(&free_pages);
	try_to_unmap_flush();
	free_unref_page_list(&free_pages);

	list_splice(&ret_pages, page_list);
	count_vm_events(PGACTIVATE, pgactivate);

	if (stat) {
		stat->nr_dirty = nr_dirty;
		stat->nr_congested = nr_congested;
		stat->nr_unqueued_dirty = nr_unqueued_dirty;
		stat->nr_writeback = nr_writeback;
		stat->nr_immediate = nr_immediate;
		stat->nr_activate = pgactivate;
		stat->nr_ref_keep = nr_ref_keep;
		stat->nr_unmap_fail = nr_unmap_fail;
	}
	return nr_reclaimed;
}

3.4.6 try_to_unmap()

遍历反向映射的vma，解除所有 vma mmu的映射，并释放 page：

bool try_to_unmap(struct page *page, enum ttu_flags flags)
{
	struct rmap_walk_control rwc = {
		.rmap_one = try_to_unmap_one,
		.arg = (void *)flags,
		.done = page_mapcount_is_zero,
		.anon_lock = page_lock_anon_vma_read,
	};

	/*
	 * During exec, a temporary VMA is setup and later moved.
	 * The VMA is moved under the anon_vma lock but not the
	 * page tables leading to a race where migration cannot
	 * find the migration ptes. Rather than increasing the
	 * locking requirements of exec(), migration skips
	 * temporary VMAs until after exec() completes.
	 */
	if ((flags & (TTU_MIGRATION|TTU_SPLIT_FREEZE))
	    && !PageKsm(page) && PageAnon(page))
		rwc.invalid_vma = invalid_migration_vma;

	/* (1) 根据 page 的反向映射，遍历所有关联的 vma */
	if (flags & TTU_RMAP_LOCKED)
		rmap_walk_locked(page, &rwc);
	else
		rmap_walk(page, &rwc);

	return !page_mapcount(page) ? true : false;
}

↓

try_to_unmap_one()
{

	/* 解除 mmu 映射，释放 page 内存 */

}

3.4.7 rmap_walk()

反向映射的遍历方法：

void rmap_walk(struct page *page, struct rmap_walk_control *rwc)
{
	if (unlikely(PageKsm(page)))
		/* (1) KSM 内存的反向映射遍历 */
		rmap_walk_ksm(page, rwc);
	else if (PageAnon(page))
		/* (2) 匿名内存的反向映射遍历 */
		rmap_walk_anon(page, rwc, false);
	else
		/* (3) 文件内存的反向映射遍历 */
		rmap_walk_file(page, rwc, false);
}

|→

static void rmap_walk_anon(struct page *page, struct rmap_walk_control *rwc,
		bool locked)
{
	struct anon_vma *anon_vma;
	pgoff_t pgoff_start, pgoff_end;
	struct anon_vma_chain *avc;

	/* (2.1) 找到 page 对应的 anon_vma 结构 */
	if (locked) {
		anon_vma = page_anon_vma(page);
		/* anon_vma disappear under us? */
		VM_BUG_ON_PAGE(!anon_vma, page);
	} else {
		anon_vma = rmap_walk_anon_lock(page, rwc);
	}
	if (!anon_vma)
		return;

	/* (2.2) 计算 page 在vma中的偏移 pgoff */
	pgoff_start = page_to_pgoff(page);
	pgoff_end = pgoff_start + hpage_nr_pages(page) - 1;
	/* (2.3) 逐个遍历 anon_vma 树中符合条件的 vma */
	anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root,
			pgoff_start, pgoff_end) {
		struct vm_area_struct *vma = avc->vma;
		unsigned long address = vma_address(page, vma);

		cond_resched();

		if (rwc->invalid_vma && rwc->invalid_vma(vma, rwc->arg))
			continue;

		if (!rwc->rmap_one(page, vma, address, rwc->arg))
			break;
		if (rwc->done && rwc->done(page))
			break;
	}

	if (!locked)
		anon_vma_unlock_read(anon_vma);
}

|→

static void rmap_walk_file(struct page *page, struct rmap_walk_control *rwc,
		bool locked)
{
	/* (3.1) 找到 page 对应的 address_space mapping 结构 */
	struct address_space *mapping = page_mapping(page);
	pgoff_t pgoff_start, pgoff_end;
	struct vm_area_struct *vma;

	/*
	 * The page lock not only makes sure that page->mapping cannot
	 * suddenly be NULLified by truncation, it makes sure that the
	 * structure at mapping cannot be freed and reused yet,
	 * so we can safely take mapping->i_mmap_rwsem.
	 */
	VM_BUG_ON_PAGE(!PageLocked(page), page);

	if (!mapping)
		return;

	/* (3.2) 计算 page 在文件中的偏移 pgoff */
	pgoff_start = page_to_pgoff(page);
	pgoff_end = pgoff_start + hpage_nr_pages(page) - 1;
	if (!locked)
		i_mmap_lock_read(mapping);
	/* (3.3) 逐个遍历 mapping 树中符合条件的 vma */
	vma_interval_tree_foreach(vma, &mapping->i_mmap,
			pgoff_start, pgoff_end) {
		unsigned long address = vma_address(page, vma);

		cond_resched();

		if (rwc->invalid_vma && rwc->invalid_vma(vma, rwc->arg))
			continue;

		if (!rwc->rmap_one(page, vma, address, rwc->arg))
			goto done;
		if (rwc->done && rwc->done(page))
			goto done;
	}

done:
	if (!locked)
		i_mmap_unlock_read(mapping);
}

4. 其他回收方式

除了从 LRU 链表中回收内存，系统还有一些手段来回收内存。

4.1 Shrinker

除了用户态的匿名内存和文件内存。内核模块可以把一些可回收的资源注册成 shrinker，在内存紧张的情况下尝试进行回收。类如 android 下大名鼎鼎的 lmk 驱动。

4.1.1 register_shrinker()

shrinker 的注册函数：

int register_shrinker(struct shrinker *shrinker)
{
	int err = prealloc_shrinker(shrinker);

	if (err)
		return err;
	register_shrinker_prepared(shrinker);
	return 0;
}

↓

void register_shrinker_prepared(struct shrinker *shrinker)
{
	down_write(&shrinker_rwsem);
	/* (1) 把新的 shrinker 加入到全局链表 shrinker_list */
	list_add_tail(&shrinker->list, &shrinker_list);
	up_write(&shrinker_rwsem);
}

4.1.2 do_shrink_slab()

内存回收时，调用 shrinker 的回收函数：

shrink_node() → shrink_slab():

static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
				 struct mem_cgroup *memcg,
				 unsigned long nr_scanned,
				 unsigned long nr_eligible)
{
	struct shrinker *shrinker;
	unsigned long freed = 0;

	if (memcg && (!memcg_kmem_enabled() || !mem_cgroup_online(memcg)))
		return 0;

	if (nr_scanned == 0)
		nr_scanned = SWAP_CLUSTER_MAX;

	if (!down_read_trylock(&shrinker_rwsem)) {
		/*
		 * If we would return 0, our callers would understand that we
		 * have nothing else to shrink and give up trying. By returning
		 * 1 we keep it going and assume we'll be able to shrink next
		 * time.
		 */
		freed = 1;
		goto out;
	}

	/* (1) 遍历全局链表 shrinker_list 中的 shrinker，逐个进行回收尝试 */
	list_for_each_entry(shrinker, &shrinker_list, list) {
		struct shrink_control sc = {
			.gfp_mask = gfp_mask,
			.nid = nid,
			.memcg = memcg,
		};

		/*
		 * If kernel memory accounting is disabled, we ignore
		 * SHRINKER_MEMCG_AWARE flag and call all shrinkers
		 * passing NULL for memcg.
		 */
		if (memcg_kmem_enabled() &&
		    !!memcg != !!(shrinker->flags & SHRINKER_MEMCG_AWARE))
			continue;

		if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
			sc.nid = 0;

		/* (2) 调用 shrinker 的回收 */
		freed += do_shrink_slab(&sc, shrinker, nr_scanned, nr_eligible);
		/*
		 * Bail out if someone want to register a new shrinker to
		 * prevent the regsitration from being stalled for long periods
		 * by parallel ongoing shrinking.
		 */
		if (rwsem_is_contended(&shrinker_rwsem)) {
			freed = freed ? : 1;
			break;
		}
	}

	up_read(&shrinker_rwsem);
out:
	cond_resched();
	return freed;
}

↓

static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
				    struct shrinker *shrinker,
				    unsigned long nr_scanned,
				    unsigned long nr_eligible)
{
	unsigned long freed = 0;
	unsigned long long delta;
	long total_scan;
	long freeable;
	long nr;
	long new_nr;
	int nid = shrinkctl->nid;
	long batch_size = shrinker->batch ? shrinker->batch
					  : SHRINK_BATCH;
	long scanned = 0, next_deferred;

	freeable = shrinker->count_objects(shrinker, shrinkctl);
	if (freeable == 0)
		return 0;

	/*
	 * copy the current shrinker scan count into a local variable
	 * and zero it so that other concurrent shrinker invocations
	 * don't also do this scanning work.
	 */
	nr = atomic_long_xchg(&shrinker->nr_deferred[nid], 0);

	total_scan = nr;
	delta = (4 * nr_scanned) / shrinker->seeks;
	delta *= freeable;
	do_div(delta, nr_eligible + 1);
	total_scan += delta;
	if (total_scan < 0) {
		pr_err("shrink_slab: %pF negative objects to delete nr=%ld\n",
		       shrinker->scan_objects, total_scan);
		total_scan = freeable;
		next_deferred = nr;
	} else
		next_deferred = total_scan;

	/*
	 * We need to avoid excessive windup on filesystem shrinkers
	 * due to large numbers of GFP_NOFS allocations causing the
	 * shrinkers to return -1 all the time. This results in a large
	 * nr being built up so when a shrink that can do some work
	 * comes along it empties the entire cache due to nr >>>
	 * freeable. This is bad for sustaining a working set in
	 * memory.
	 *
	 * Hence only allow the shrinker to scan the entire cache when
	 * a large delta change is calculated directly.
	 */
	if (delta < freeable / 4)
		total_scan = min(total_scan, freeable / 2);

	/*
	 * Avoid risking looping forever due to too large nr value:
	 * never try to free more than twice the estimate number of
	 * freeable entries.
	 */
	if (total_scan > freeable * 2)
		total_scan = freeable * 2;

	trace_mm_shrink_slab_start(shrinker, shrinkctl, nr,
				   nr_scanned, nr_eligible,
				   freeable, delta, total_scan);

	/*
	 * Normally, we should not scan less than batch_size objects in one
	 * pass to avoid too frequent shrinker calls, but if the slab has less
	 * than batch_size objects in total and we are really tight on memory,
	 * we will try to reclaim all available objects, otherwise we can end
	 * up failing allocations although there are plenty of reclaimable
	 * objects spread over several slabs with usage less than the
	 * batch_size.
	 *
	 * We detect the "tight on memory" situations by looking at the total
	 * number of objects we want to scan (total_scan). If it is greater
	 * than the total number of objects on slab (freeable), we must be
	 * scanning at high prio and therefore should try to reclaim as much as
	 * possible.
	 */
	while (total_scan >= batch_size ||
	       total_scan >= freeable) {
		unsigned long ret;
		unsigned long nr_to_scan = min(batch_size, total_scan);

		shrinkctl->nr_to_scan = nr_to_scan;
		shrinkctl->nr_scanned = nr_to_scan;
		ret = shrinker->scan_objects(shrinker, shrinkctl);
		if (ret == SHRINK_STOP)
			break;
		freed += ret;

		count_vm_events(SLABS_SCANNED, shrinkctl->nr_scanned);
		total_scan -= shrinkctl->nr_scanned;
		scanned += shrinkctl->nr_scanned;

		cond_resched();
	}

	if (next_deferred >= scanned)
		next_deferred -= scanned;
	else
		next_deferred = 0;
	/*
	 * move the unused scan count back into the shrinker in a
	 * manner that handles concurrent updates. If we exhausted the
	 * scan, there is no need to do an update.
	 */
	if (next_deferred > 0)
		new_nr = atomic_long_add_return(next_deferred,
						&shrinker->nr_deferred[nid]);
	else
		new_nr = atomic_long_read(&shrinker->nr_deferred[nid]);

	trace_mm_shrink_slab_end(shrinker, nid, freed, nr, new_nr, total_scan);
	return freed;
}

4.2 内存整理 (Compact)

Compact 功能是尝试把多个内存碎片，规整成一大块连续的内存。具体功能这里就不详细展开。

参考：Linux中的Memory Compaction

4.3 内存合并 (KSM)

共享内存的概念在现代操作系统中很常用了，比如，一个程序启动时会与父进程共用它的全部内存。但子或父进程需要修改共享内存的时候，linux便再分配新内存，然后copy原区域内容到新内存。这个过程就叫copy on write。

而KSM(Kernel Samepage Merging）是linux的新属性，它做的东西刚好与共享内存相反。 当linux启用了KSM之后，KSM会检查多个运行中的进程，并比对它们的内存。如果任何区域或者分页是一样的，KSM就会毫不犹豫地合并他们成一个分页。 那么新分页也是被标记成copy on write。如果VM要修改内存的话，那么linux就会分配新的内存给这个VM。KSM可以在KVM大有作为。

参考：玩转KVM: 聊聊KSM内存合并

4.4 OOM Killer

Linux OOM(Out Of Memory) killer，就是系统在内存极度紧张的情况下，开始通过杀进程来释放内存了。

原生 Linux 启动杀进程的时机非常晚，因为系统很难区分出哪些进程可以被杀哪些进程不能被杀。而 Android 下启动 LMK 来杀进程释放内存的时机非常早，因为 Android 可以区分出进程的前台和后台，后台的进程大部分是可以被杀掉的。

参考：Linux内核OOM killer机制

参考文档：

1.linux内存源码分析 - 内存回收(lru链表)
2.linux内存源码分析 - 内存回收(匿名页反向映射)
3.linux内存源码分析 - 内存碎片整理(实现流程)
4.linux内存源码分析 - 内存碎片整理(同步关系)
5.page reclaim 参数
6.kernel-4.9内存回收核心流程
7.Linux内存管理 (21)OOM
8.linux内核page结构体的PG_referenced和PG_active标志
9.Linux中的内存回收[一]
10.Page 页帧管理详解
11.Linux中的Memory Compaction
12.玩转KVM: 聊聊KSM内存合并
13.Linux内核OOM killer机制