注：本文分析基于3.10.0-693.el7内核版本，即CentOS 7.4
1、关于drop_caches
通常在内存不足时，我们习惯通过echo 3 > /proc/sys/vm/drop_caches 的方式手动清理系统缓存，
[root@localhost ~]# free -m
total used free shared buff/cache available
Mem: 7822 3436 2068 40 2317 3997
Swap: 0 0 0
[root@localhost ~]# echo 3 > /proc/sys/vm/drop_caches
[root@localhost ~]# free -m
total used free shared buff/cache available
Mem: 7822 3433 4036 40 352 4037
Swap: 0 0 0对于数字3的含义，我们可以通过内核文档了解其具体含义，
To free pagecache:
echo 1 > /proc/sys/vm/drop_caches
To free reclaimable slab objects (includes dentries and inodes):
echo 2 > /proc/sys/vm/drop_caches
To free slab objects and pagecache:
echo 3 > /proc/sys/vm/drop_caches2、释放pagecache
在之前我们知道当内存低于某个阈值时，会触发脏页回写，提交回写work到对应BDI设备上，由BDI writebacke进程回写脏页释放内存。这和drop_caches中的echo 1类似，都是释放脏页，因此其最后路径是一致的。
int drop_caches_sysctl_handler(ctl_table *table, int write,
void __user *buffer, size_t *length, loff_t *ppos)
{
int ret;
ret <span >=</span> <span >proc_dointvec_minmax</span><span >(</span>table<span >,</span> write<span >,</span> buffer<span >,</span> length<span >,</span> ppos<span >)</span><span >;</span>
<span >if</span> <span >(</span>ret<span >)</span>
<span >return</span> ret<span >;</span>
<span >if</span> <span >(</span>write<span >)</span> <span >{<!-- --></span>
<span >static</span> <span >int</span> stfu<span >;</span>
<span >// echo 1 > drop_caches</span>
<span >if</span> <span >(</span>sysctl_drop_caches <span >&</span> <span >1</span><span >)</span> <span >{<!-- --></span>
<span >iterate_supers</span><span >(</span>drop_pagecache_sb<span >,</span> <span class="token constant">NULL</span><span >)</span><span >;</span>
<span >count_vm_event</span><span >(</span>DROP_PAGECACHE<span >)</span><span >;</span>
<span >}</span>
<span >// echo 2 > drop_caches</span>
<span >if</span> <span >(</span>sysctl_drop_caches <span >&</span> <span >2</span><span >)</span> <span >{<!-- --></span>
<span >drop_slab</span><span >(</span><span >)</span><span >;</span>
<span >count_vm_event</span><span >(</span>DROP_SLAB<span >)</span><span >;</span>
<span >}</span>
<span >if</span> <span >(</span><span >!</span>stfu<span >)</span> <span >{<!-- --></span>
<span >pr_info</span><span >(</span><span class="token string">"%s (%d): drop_caches: %d\n"</span><span >,</span>
current<span >-></span>comm<span >,</span> <span >task_pid_nr</span><span >(</span>current<span >)</span><span >,</span>
sysctl_drop_caches<span >)</span><span >;</span>
<span >}</span>
<span >//置位，否则就一直在回收了</span>
stfu <span >|</span><span >=</span> sysctl_drop_caches <span >&</span> <span >4</span><span >;</span>
<span >}</span>
<span >return</span> <span >0</span><span >;</span>
}
ret <span >=</span> <span >proc_dointvec_minmax</span><span >(</span>table<span >,</span> write<span >,</span> buffer<span >,</span> length<span >,</span> ppos<span >)</span><span >;</span>
<span >if</span> <span >(</span>ret<span >)</span>
<span >return</span> ret<span >;</span>
<span >if</span> <span >(</span>write<span >)</span> <span >{<!-- --></span>
<span >static</span> <span >int</span> stfu<span >;</span>
<span >// echo 1 > drop_caches</span>
<span >if</span> <span >(</span>sysctl_drop_caches <span >&</span> <span >1</span><span >)</span> <span >{<!-- --></span>
<span >iterate_supers</span><span >(</span>drop_pagecache_sb<span >,</span> <span class="token constant">NULL</span><span >)</span><span >;</span>
<span >count_vm_event</span><span >(</span>DROP_PAGECACHE<span >)</span><span >;</span>
<span >}</span>
<span >// echo 2 > drop_caches</span>
<span >if</span> <span >(</span>sysctl_drop_caches <span >&</span> <span >2</span><span >)</span> <span >{<!-- --></span>
<span >drop_slab</span><span >(</span><span >)</span><span >;</span>
<span >count_vm_event</span><span >(</span>DROP_SLAB<span >)</span><span >;</span>
<span >}</span>
<span >if</span> <span >(</span><span >!</span>stfu<span >)</span> <span >{<!-- --></span>
<span >pr_info</span><span >(</span><span class="token string">"%s (%d): drop_caches: %d\n"</span><span >,</span>
current<span >-></span>comm<span >,</span> <span >task_pid_nr</span><span >(</span>current<span >)</span><span >,</span>
sysctl_drop_caches<span >)</span><span >;</span>
<span >}</span>
<span >//置位，否则就一直在回收了</span>
stfu <span >|</span><span >=</span> sysctl_drop_caches <span >&</span> <span >4</span><span >;</span>
<span >}</span>
<span >return</span> <span >0</span><span >;</span>可见，echo 1时，会调用drop_pagecache_sb去释放pagecache，我们继续往下查，
drop_pagecache_sb ->
iput ->
iput_final->
write_inode_now -> #提交writeback_control，立即回写
writeback_single_inode ->
__writeback_single_inode ->
do_writepages #调用对应文件系统的writepage写回磁盘在BDI回写里，一开始提交的是wb_writeback_work，等到实际要执行回写操作时，都会转换为writeback_control，再去执行回写。
因此，echo 1的操作就是，遍历每个超级块，调用drop_pagecache_sb，drop_pagecache_sb中会遍历该超级块所有的inode，对其关联的pagecache进行回写。与BDI不同的是，该操作是立马执行，不需要等待周期执行或者inode过期。
3、释放slab cache
而对于echo 2的情况，就比较复杂一点，
static void drop_slab(void)
{
int nr_objects;
struct shrink_control shrink = {
.gfp_mask = GFP_KERNEL,
};
//上次回收缓存数量高于10，就再进行一次回收
//这个条件其实挺苛刻的，回收后整个系统空闲slab不会超过10
do {
nr_objects = shrink_slab(&shrink, 1000, 1000);
} while (nr_objects > 10);
}
unsigned long shrink_slab(struct shrink_control shrink,
unsigned long nr_pages_scanned,
unsigned long lru_pages)
{
struct shrinker shrinker;
unsigned long ret = 0;
...
//遍历系统中所有的shrinker，回收各个slab管理区的空闲缓存
list_for_each_entry(shrinker, &shrinker_list, list) {
unsigned long long delta;
long total_scan;
long max_pass;
int shrink_ret = 0;
long nr;
long new_nr;
//获取批处理数量，默认每次回收128，对于超级块而言是1024
long batch_size = shrinker->batch ? shrinker->batch
: SHRINK_BATCH;
//获取该slab管理区可回收的缓存数量
max_pass = do_shrinker_shrink(shrinker, shrink, 0);
if (max_pass <= 0)
continue;
nr <span >=</span> <span >atomic_long_xchg</span><span >(</span><span >&</span>shrinker<span >-></span>nr_in_batch<span >,</span> <span >0</span><span >)</span><span >;</span>
total_scan <span >=</span> nr<span >;</span>
<span >//计算该slab管理区此次缓存回收额度，一堆操作</span>
<span >//针对手动释放缓存的场景，基本上是两倍的max_pass，也就是尽可能去释放</span>
<span >//对于kswap或其他路径上，不会超过一倍的max_pass</span>
delta <span >=</span> <span >(</span><span >4</span> <span >*</span> nr_pages_scanned<span >)</span> <span >/</span> shrinker<span >-></span>seeks<span >;</span>
delta <span >*</span><span >=</span> max_pass<span >;</span>
<span >do_div</span><span >(</span>delta<span >,</span> lru_pages <span >+</span> <span >1</span><span >)</span><span >;</span>
total_scan <span >+</span><span >=</span> delta<span >;</span>
<span >if</span> <span >(</span>total_scan <span ><</span> <span >0</span><span >)</span> <span >{<!-- --></span>
<span >printk</span><span >(</span>KERN_ERR <span class="token string">"shrink_slab: %pF negative objects to "</span>
<span class="token string">"delete nr=%ld\n"</span><span >,</span>
shrinker<span >-></span>shrink<span >,</span> total_scan<span >)</span><span >;</span>
total_scan <span >=</span> max_pass<span >;</span>
<span >}</span>
<span >//如果delta偏小，意味着系统中inactive的缓存偏少，我们回收的额度也不能设置太大</span>
<span >if</span> <span >(</span>delta <span ><</span> max_pass <span >/</span> <span >4</span><span >)</span>
total_scan <span >=</span> <span >min</span><span >(</span>total_scan<span >,</span> max_pass <span >/</span> <span >2</span><span >)</span><span >;</span>
<span >//控制回收总额上限，避免死循环</span>
<span >if</span> <span >(</span>total_scan <span >></span> max_pass <span >*</span> <span >2</span><span >)</span>
total_scan <span >=</span> max_pass <span >*</span> <span >2</span><span >;</span>
<span >trace_mm_shrink_slab_start</span><span >(</span>shrinker<span >,</span> shrink<span >,</span> nr<span >,</span>
nr_pages_scanned<span >,</span> lru_pages<span >,</span>
max_pass<span >,</span> delta<span >,</span> total_scan<span >)</span><span >;</span>
<span >//循环回收缓存</span>
<span >while</span> <span >(</span>total_scan <span >>=</span> batch_size<span >)</span> <span >{<!-- --></span>
<span >int</span> nr_before<span >;</span>
<span >//记录处理前缓存数量</span>
nr_before <span >=</span> <span >do_shrinker_shrink</span><span >(</span>shrinker<span >,</span> shrink<span >,</span> <span >0</span><span >)</span><span >;</span>
<span >//回收后缓存数量</span>
shrink_ret <span >=</span> <span >do_shrinker_shrink</span><span >(</span>shrinker<span >,</span> shrink<span >,</span>
batch_size<span >)</span><span >;</span>
<span >if</span> <span >(</span>shrink_ret <span >==</span> <span >-</span><span >1</span><span >)</span>
<span >break</span><span >;</span>
<span >//统计此次回收的缓存数量</span>
<span >if</span> <span >(</span>shrink_ret <span ><</span> nr_before<span >)</span>
ret <span >+</span><span >=</span> nr_before <span >-</span> shrink_ret<span >;</span>
<span >count_vm_events</span><span >(</span>SLABS_SCANNED<span >,</span> batch_size<span >)</span><span >;</span>
<span >//减少扫描总额</span>
total_scan <span >-</span><span >=</span> batch_size<span >;</span>
<span >cond_resched</span><span >(</span><span >)</span><span >;</span>
<span >}</span>
<span >//如果剩下的额度不够一个batch_size，留着下次使用，记录在nr_in_batch</span>
<span >if</span> <span >(</span>total_scan <span >></span> <span >0</span><span >)</span>
new_nr <span >=</span> <span >atomic_long_add_return</span><span >(</span>total_scan<span >,</span>
<span >&</span>shrinker<span >-></span>nr_in_batch<span >)</span><span >;</span>
<span >else</span>
new_nr <span >=</span> <span >atomic_long_read</span><span >(</span><span >&</span>shrinker<span >-></span>nr_in_batch<span >)</span><span >;</span>
<span >trace_mm_shrink_slab_end</span><span >(</span>shrinker<span >,</span> shrink_ret<span >,</span> nr<span >,</span> new_nr<span >)</span><span >;</span>
<span >}</span>
<span >up_read</span><span >(</span><span >&</span>shrinker_rwsem<span >)</span><span >;</span>
out:
cond_resched();
return ret;
}
nr <span >=</span> <span >atomic_long_xchg</span><span >(</span><span >&</span>shrinker<span >-></span>nr_in_batch<span >,</span> <span >0</span><span >)</span><span >;</span>
total_scan <span >=</span> nr<span >;</span>
<span >//计算该slab管理区此次缓存回收额度，一堆操作</span>
<span >//针对手动释放缓存的场景，基本上是两倍的max_pass，也就是尽可能去释放</span>
<span >//对于kswap或其他路径上，不会超过一倍的max_pass</span>
delta <span >=</span> <span >(</span><span >4</span> <span >*</span> nr_pages_scanned<span >)</span> <span >/</span> shrinker<span >-></span>seeks<span >;</span>
delta <span >*</span><span >=</span> max_pass<span >;</span>
<span >do_div</span><span >(</span>delta<span >,</span> lru_pages <span >+</span> <span >1</span><span >)</span><span >;</span>
total_scan <span >+</span><span >=</span> delta<span >;</span>
<span >if</span> <span >(</span>total_scan <span ><</span> <span >0</span><span >)</span> <span >{<!-- --></span>
<span >printk</span><span >(</span>KERN_ERR <span class="token string">"shrink_slab: %pF negative objects to "</span>
<span class="token string">"delete nr=%ld\n"</span><span >,</span>
shrinker<span >-></span>shrink<span >,</span> total_scan<span >)</span><span >;</span>
total_scan <span >=</span> max_pass<span >;</span>
<span >}</span>
<span >//如果delta偏小，意味着系统中inactive的缓存偏少，我们回收的额度也不能设置太大</span>
<span >if</span> <span >(</span>delta <span ><</span> max_pass <span >/</span> <span >4</span><span >)</span>
total_scan <span >=</span> <span >min</span><span >(</span>total_scan<span >,</span> max_pass <span >/</span> <span >2</span><span >)</span><span >;</span>
<span >//控制回收总额上限，避免死循环</span>
<span >if</span> <span >(</span>total_scan <span >></span> max_pass <span >*</span> <span >2</span><span >)</span>
total_scan <span >=</span> max_pass <span >*</span> <span >2</span><span >;</span>
<span >trace_mm_shrink_slab_start</span><span >(</span>shrinker<span >,</span> shrink<span >,</span> nr<span >,</span>
nr_pages_scanned<span >,</span> lru_pages<span >,</span>
max_pass<span >,</span> delta<span >,</span> total_scan<span >)</span><span >;</span>
<span >//循环回收缓存</span>
<span >while</span> <span >(</span>total_scan <span >>=</span> batch_size<span >)</span> <span >{<!-- --></span>
<span >int</span> nr_before<span >;</span>
<span >//记录处理前缓存数量</span>
nr_before <span >=</span> <span >do_shrinker_shrink</span><span >(</span>shrinker<span >,</span> shrink<span >,</span> <span >0</span><span >)</span><span >;</span>
<span >//回收后缓存数量</span>
shrink_ret <span >=</span> <span >do_shrinker_shrink</span><span >(</span>shrinker<span >,</span> shrink<span >,</span>
batch_size<span >)</span><span >;</span>
<span >if</span> <span >(</span>shrink_ret <span >==</span> <span >-</span><span >1</span><span >)</span>
<span >break</span><span >;</span>
<span >//统计此次回收的缓存数量</span>
<span >if</span> <span >(</span>shrink_ret <span ><</span> nr_before<span >)</span>
ret <span >+</span><span >=</span> nr_before <span >-</span> shrink_ret<span >;</span>
<span >count_vm_events</span><span >(</span>SLABS_SCANNED<span >,</span> batch_size<span >)</span><span >;</span>
<span >//减少扫描总额</span>
total_scan <span >-</span><span >=</span> batch_size<span >;</span>
<span >cond_resched</span><span >(</span><span >)</span><span >;</span>
<span >}</span>
<span >//如果剩下的额度不够一个batch_size，留着下次使用，记录在nr_in_batch</span>
<span >if</span> <span >(</span>total_scan <span >></span> <span >0</span><span >)</span>
new_nr <span >=</span> <span >atomic_long_add_return</span><span >(</span>total_scan<span >,</span>
<span >&</span>shrinker<span >-></span>nr_in_batch<span >)</span><span >;</span>
<span >else</span>
new_nr <span >=</span> <span >atomic_long_read</span><span >(</span><span >&</span>shrinker<span >-></span>nr_in_batch<span >)</span><span >;</span>
<span >trace_mm_shrink_slab_end</span><span >(</span>shrinker<span >,</span> shrink_ret<span >,</span> nr<span >,</span> new_nr<span >)</span><span >;</span>
<span >}</span>
<span >up_read</span><span >(</span><span >&</span>shrinker_rwsem<span >)</span><span >;</span>空闲slab缓存计算和回收都是在do_shrinker_shrink完成，它其实调用的是一个函数指针，不同slab管理区有自己定义的shrink函数，第三个入参nr_to_scan为0时，是计算空闲slab缓存；不为空时，表示扫描和回收缓存的数量。
static inline int do_shrinker_shrink(struct shrinker *shrinker,
struct shrink_control *sc,
unsigned long nr_to_scan)
{
int objects;
sc->nr_to_scan = nr_to_scan;
objects = (*shrinker->shrink)(shrinker, sc);
<span >if</span> <span >(</span>objects <span ><</span> <span >-</span><span >1</span><span >)</span>
<span >return</span> INT_MAX<span >;</span>
<span >return</span> objects<span >;</span>
}
<span >if</span> <span >(</span>objects <span ><</span> <span >-</span><span >1</span><span >)</span>
<span >return</span> INT_MAX<span >;</span>
<span >return</span> objects<span >;</span>总的来说，drop_slab就是调用每个slab管理区定义的shrink函数，先计算出可回收的slab缓存数量，然后确定扫描数量，最后调用shrink函数执行缓存扫描和回收。
