内存回收机制lru
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本文基于linux-5.0内核源码分析
include/linux/mmzone.h
include/linux/pagevec.h
include/linux/mm_inline.h
include/linux/pagemap.h
include/linux/vmstat.h
mm/swap.c
mm/vmscan.c
mm/util.c
mm/rmap.c
1. lru_list
#define LRU_BASE 0
#define LRU_ACTIVE 1
#define LRU_FILE 2
// lru是双向链表: 内核根据页面类型(匿名页和文件页)与活跃性(活跃和不活跃), 分成5种类型lru链表
enum lru_list
// 0: inactive anonymous page lru list
LRU_INACTIVE_ANON = LRU_BASE,
// 1: active anonymous page lru list
LRU_ACTIVE_ANON = LRU_BASE + LRU_ACTIVE,
// 2: inactive page cache lru list
LRU_INACTIVE_FILE = LRU_BASE + LRU_FILE,
// 3: active page cache lru list
LRU_ACTIVE_FILE = LRU_BASE + LRU_FILE + LRU_ACTIVE,
// 4: unevictable page lru list
LRU_UNEVICTABLE,
NR_LRU_LISTS
;
2. lruvec
struct lruvec
// 每个lruvec都包含5个lru链表
struct list_head lists[NR_LRU_LISTS];
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
// 每个node都包含1个lruvec: pgdat标识lruvec所属的node
struct pglist_data *pgdat;
#endif
;
3. pagevec
/* 15 pointers + header align the pagevec structure to a power of two */
// 对比4.14.186的内核: PAGEVEC_SIZE为14
#define PAGEVEC_SIZE 15
// pagevec用于批量处理
struct pagevec
unsigned long nr;
bool percpu_pvec_drained;
// 每个pagevec都有1个15个page大小的数组
struct page *pages[PAGEVEC_SIZE];
;
4. lru_cache_add
// 将page添加到指定的lru链表
void lru_cache_add(struct page *page)
// 活跃且不可回收的页面不能加入lru链表
VM_BUG_ON_PAGE(PageActive(page) && PageUnevictable(page), page);
// 已经添加到lru链表的不能再重复添加
VM_BUG_ON_PAGE(PageLRU(page), page);
__lru_cache_add(page);
/*
*每个cpu定义1个pagevec
*/
// lru_add_pvec用于存放添加到lru链表的页面
static DEFINE_PER_CPU(struct pagevec, lru_add_pvec);
static DEFINE_PER_CPU(struct pagevec, lru_rotate_pvecs);
static DEFINE_PER_CPU(struct pagevec, lru_deactivate_file_pvecs);
static DEFINE_PER_CPU(struct pagevec, lru_lazyfree_pvecs);
#ifdef CONFIG_SMP
static DEFINE_PER_CPU(struct pagevec, activate_page_pvecs);
#endif
static void __lru_cache_add(struct page *page)
// 获取当前cpu的pagevec
struct pagevec *pvec = &get_cpu_var(lru_add_pvec);
get_page(page);
// 1.首先尝试通过pagevec_add将page添加到pagevec的pages数组
// 2.如果添加失败代表当前pagevec已满, 需要将pagevec的15个page批量提交到lru链表
// 3.如果是复合页也直接批量提交
if (!pagevec_add(pvec, page) || PageCompound(page))
__pagevec_lru_add(pvec);
// 更新lru_add_pvec
put_cpu_var(lru_add_pvec);
4.1 pagevec_add
// 将page添加到pagevec, 并返回剩余可用的空间
static inline unsigned pagevec_add(struct pagevec *pvec, struct page *page)
// 将page保存到pagevec的pages数组, 并将page数量加1
pvec->pages[pvec->nr++] = page;
// 返回剩余空间: 为0代表空间已满添加失败
return pagevec_space(pvec);
4.2 pagevec_space
// pagevec最多保存15个page, nr保存pagevec当前存储的page数: 两者之差等于pagevec剩余可用空间
static inline unsigned pagevec_space(struct pagevec *pvec)
return PAGEVEC_SIZE - pvec->nr;
4.3 __pagevec_lru_add
void __pagevec_lru_add(struct pagevec *pvec)
// 批量处理pagevec的所有page: 针对每个page调用__pagevec_lru_add_fn方法
pagevec_lru_move_fn(pvec, __pagevec_lru_add_fn, NULL);
4.4 pagevec_lru_move_fn
static void pagevec_lru_move_fn(struct pagevec *pvec,
void (*move_fn)(struct page *page, struct lruvec *lruvec, void *arg),
void *arg)
int i;
struct pglist_data *pgdat = NULL;
struct lruvec *lruvec;
unsigned long flags = 0;
// 遍历pagevec中的每个page
for (i = 0; i < pagevec_count(pvec); i++)
struct page *page = pvec->pages[i];
// page所属的节点
struct pglist_data *pagepgdat = page_pgdat(page);
if (pagepgdat != pgdat)
if (pgdat)
spin_unlock_irqrestore(&pgdat->lru_lock, flags);
pgdat = pagepgdat;
spin_lock_irqsave(&pgdat->lru_lock, flags);
// 1.如果mem_cgroup_disabled: 则返回pglist_data的lruvec
// 2.否则返回mem_cgroup_per_node的lruvec
lruvec = mem_cgroup_page_lruvec(page, pgdat);
// 回调__pagevec_lru_add种定义的move_fn函数: __pagevec_lru_add_fn
(*move_fn)(page, lruvec, arg);
if (pgdat)
spin_unlock_irqrestore(&pgdat->lru_lock, flags);
// 释放并重新初始化pagevec
release_pages(pvec->pages, pvec->nr, pvec->cold);
pagevec_reinit(pvec);
4.5 __pagevec_lru_add_fn
static inline int page_is_file_cache(struct page *page)
// anonymous page通过磁盘上的swap分区或者在RAM开辟swap分区(zram)实现回收
// page cache通过drop或者writeback回收
// PG_swapbacked为0, 即page cache
return !PageSwapBacked(page);
// inactive list：包括inactive page cache和inactive anonymous page
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 void __pagevec_lru_add_fn(struct page *page, struct lruvec *lruvec,
void *arg)
// 4.14.186内核实现
// 判断是否文件缓存: 不需要swap分区支持的就是文件缓存
// int file = page_is_file_cache(page);
// 判断是否活跃
// int active = PageActive(page);
// 计算page的lru类型
// enum lru_list lru = page_lru(page);
// 将page添加到lruvec类型为lru的链表上, 然后更新node和zone的统计信息
// add_page_to_lru_list(page, lruvec, lru);
// 更新lruvec的zone_reclaim_stat成员信息
// update_page_reclaim_stat(lruvec, file, active);
// trace_mm_lru_insertion(page, lru);
enum lru_list lru;
// 判断page曾经是否不可回收, 同时清除其PG_unevictable标志位
int was_unevictable = TestClearPageUnevictable(page);
// 不能重复添加到lru链表
VM_BUG_ON_PAGE(PageLRU(page), page);
// 设置PG_lru标志位
SetPageLRU(page);
smp_mb();
// 判断page是否可回收
if (page_evictable(page))
// 获取page的lru链表类型
lru = page_lru(page);
update_page_reclaim_stat(lruvec, page_is_file_cache(page),
PageActive(page));
if (was_unevictable)
count_vm_event(UNEVICTABLE_PGRESCUED);
else
// page属于不可回收的lru链表
lru = LRU_UNEVICTABLE;
// 清除PG_active标志位
ClearPageActive(page);
// 设置PG_unevictable标志位
SetPageUnevictable(page);
if (!was_unevictable)
count_vm_event(UNEVICTABLE_PGCULLED);
// 将page添加到lruvec类型为lru的链表上, 然后更新node和zone的统计信息
add_page_to_lru_list(page, lruvec, lru);
trace_mm_lru_insertion(page, lru);
4.5.1 page_evictable
// 两种不可回收的情况
// 1.page->mapping被标记为不可回收
// 2.page属于1个被锁住的vma
int page_evictable(struct page *page)
int ret;
/* Prevent address_space of inode and swap cache from being freed */
rcu_read_lock();
// 首先判断page是否可以回收, 其次判断page是否设置PG_mlocked标志位
ret = !mapping_unevictable(page_mapping(page)) && !PageMlocked(page);
rcu_read_unlock();
return ret;
4.5.2 page_mapping
struct address_space *page_mapping(struct page *page)
struct address_space *mapping;
page = compound_head(page);
/* This happens if someone calls flush_dcache_page on slab page */
if (unlikely(PageSlab(page)))
return NULL;
// swap缓存
if (unlikely(PageSwapCache(page)))
swp_entry_t entry;
entry.val = page_private(page);
// 返回swapper_spaces数组的address_space元素
return swap_address_space(entry);
mapping = page->mapping;
// 如果是匿名映射则返回NULL
if ((unsigned long)mapping & PAGE_MAPPING_ANON)
return NULL;
// 返回page映射的address_space
return (void *)((unsigned long)mapping & ~PAGE_MAPPING_FLAGS);
4.5.3 mapping_unevictable
/*
* Bits in mapping->flags.
*/
enum mapping_flags
AS_EIO = 0, /* IO error on async write */
AS_ENOSPC = 1, /* ENOSPC on async write */
AS_MM_ALL_LOCKS = 2, /* under mm_take_all_locks() */
AS_UNEVICTABLE = 3, /* e.g., ramdisk, SHM_LOCK */
AS_EXITING = 4, /* final truncate in progress */
/* writeback related tags are not used */
AS_NO_WRITEBACK_TAGS = 5,
;
static inline int mapping_unevictable(struct address_space *mapping)
// 判断address_space->flags是否含有AS_UNEVICTABLE标志位
if (mapping)
return test_bit(AS_UNEVICTABLE, &mapping->flags);
return !!mapping;
4.6 add_page_to_lru_list
static __always_inline void add_page_to_lru_list(struct page *page,
struct lruvec *lruvec, enum lru_list lru)
// 更新node和zone中的lru链表大小: page_zonenum返回page对应的zone索引
update_lru_size(lruvec, lru, page_zonenum(page), hpage_nr_pages(page));
// 将page插入到lruvec对应的链表末尾
list_add(&page->lru, &lruvec->lists[lru]);
4.6.1 update_lru_size
static __always_inline void update_lru_size(struct lruvec *lruvec,
enum lru_list lru, enum zone_type zid,
int nr_pages)
// 继续调用__update_lru_size
__update_lru_size(lruvec, lru, zid, nr_pages);
#ifdef CONFIG_MEMCG
// memory cgroup使能时更新mem_cgroup_per_node
mem_cgroup_update_lru_size(lruvec, lru, zid, nr_pages);
#endif
4.6.2 __update_lru_size
static __always_inline void __update_lru_size(struct lruvec *lruvec,
enum lru_list lru, enum zone_type zid,
int nr_pages)
// lruvec对应的节点
struct pglist_data *pgdat = lruvec_pgdat(lruvec);
// 更新node统计信息
__mod_node_page_state(pgdat, NR_LRU_BASE + lru, nr_pages);
// 更新zone统计信息
__mod_zone_page_state(&pgdat->node_zones[zid],
NR_ZONE_LRU_BASE + lru, nr_pages);
4.6.3 __mod_node_page_state
enum node_stat_item
NR_LRU_BASE,
NR_INACTIVE_ANON = NR_LRU_BASE, /* must match order of LRU_[IN]ACTIVE */
NR_ACTIVE_ANON, /* " " " " " */
NR_INACTIVE_FILE, /* " " " " " */
NR_ACTIVE_FILE, /* " " " " " */
NR_UNEVICTABLE, /* " " " " " */
...
NR_VM_NODE_STAT_ITEMS
;
static inline void __mod_node_page_state(struct pglist_data *pgdat,
enum node_stat_item item, int delta)
// delta代表新增的page数量
node_page_state_add(delta, pgdat, item);
static inline void node_page_state_add(long x, struct pglist_data *pgdat,
enum node_stat_item item)
// 更新node的vm_stat统计
atomic_long_add(x, &pgdat->vm_stat[item]);
// 更新全局的vm_node_stat统计
atomic_long_add(x, &vm_node_stat[item]);
4.6.4 __mod_zone_page_state
enum zone_stat_item
/* First 128 byte cacheline (assuming 64 bit words) */
NR_FREE_PAGES,
NR_ZONE_LRU_BASE, /* Used only for compaction and reclaim retry */
NR_ZONE_INACTIVE_ANON = NR_ZONE_LRU_BASE,
NR_ZONE_ACTIVE_ANON,
NR_ZONE_INACTIVE_FILE,
NR_ZONE_ACTIVE_FILE,
NR_ZONE_UNEVICTABLE,
...
NR_VM_ZONE_STAT_ITEMS ;
static inline void __mod_zone_page_state(struct zone *zone,
enum zone_stat_item item, long delta)
// delta代表新增的page数量
zone_page_state_add(delta, zone, item);
static inline void zone_page_state_add(long x, struct zone *zone,
enum zone_stat_item item)
// 更新zone的vm_stat统计
atomic_long_add(x, &zone->vm_stat[item]);
// 更新全局的vm_zone_stat统计
atomic_long_add(x, &vm_zone_stat[item]);
5. mark_page_accessed(二次机会法)
// 当page被访问时会有以下三种PG_active和PG_referenced的组合
// 一.不活跃且未被引用 -> 转换为不活跃且被引用
// 二.不活跃且被引用 -> 转换为活跃且未被引用
// 三.活跃且未被引用 -> 转换为活跃且被引用
void mark_page_accessed(struct page *page)
page = compound_head(page);
// 1. PG_active为0, 即inactive page
// 2. PG_unevictable为0, 即可回收的page
// 3. PG_referenced为1, 即已经被使用的page
// 对应第二种组合: inactive,referenced -> active,unreferenced
if (!PageActive(page) && !PageUnevictable(page) &&
PageReferenced(page))
// PG_lru为1, 即在lru链表中
if (PageLRU(page))
// 激活page: 将page从inactive list迁移到active list
activate_page(page);
else
// 激活page: 将PG_active标志位设置为1
__lru_cache_activate_page(page);
// 清除PG_referenced标志位
ClearPageReferenced(page);
if (page_is_file_cache(page))
workingset_activation(page);
else if (!PageReferenced(page))
// 对应第一种和第三种组合
// inactive,unreferenced -> inactive,referenced
// active,unreferenced -> active,referenced
// 只需设置PG_referenced标志位
SetPageReferenced(page);
if (page_is_idle(page))
clear_page_idle(page);
5.1 activate_page
// 支持对称多处理器
#ifdef CONFIG_SMP
// 每个cpu都有1个pagevec用于保存active page
static DEFINE_PER_CPU(struct pagevec, activate_page_pvecs);
void activate_page(struct page *page)
page = compound_head(page);
// page需要满足在lru链表, inactive和evictable三个条件
if (PageLRU(page) && !PageActive(page) && !PageUnevictable(page))
// 获取当前cpu的activate_page_pvecs
struct pagevec *pvec = &get_cpu_var(activate_page_pvecs);
get_page(page);
// 同前面介绍过的__lru_cache_add类似
// 1.首先尝试调用pagevec_add将page添加到pagevec
// 2.如果添加失败代表pagevec已满, 则将pagevec批量激活
if (!pagevec_add(pvec, page) || PageCompound(page))
pagevec_lru_move_fn(pvec, __activate_page, NULL);
// 更新activate_page_pvecs
put_cpu_var(activate_page_pvecs);
#剩余内存无法满足申请时,系统会怎么做?