Android Low memory killer（下）

本文参照：http://blog.sina.com.cn/s/blog_4d66a3cb0100prfe.html
http://tech.it168.com/a2011/0805/1228/000001228471_2.shtml
在阅读本文前，请先阅读《Android
Low memory killer（上）》
在了解了Low Memory Killer的原理之后，我来看其具体实现，lowmemorykiller.c的lowmem_shrink函数。
static int lowmem_shrink(int nr_to_scan,
gfp_t gfp_mask)

{

struct task_struct *p;

struct task_struct *selected = NULL;

int rem = 0;

int tasksize;

int i;

int min_adj = OOM_ADJUST_MAX + 1;

int selected_tasksize = 0;

int array_size = ARRAY_SIZE(lowmem_adj);

int other_free = global_page_state(NR_FREE_PAGES);

int other_file = global_page_state(NR_FILE_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_adj = lowmem_adj[i];

break;

}

}

if(nr_to_scan > 0)

lowmem_print(3, "lowmem_shrink
%d, %x, ofree %d %d, ma %d\n", nr_to_scan,

gfp_mask, other_free, other_file, min_adj);

rem = global_page_state(NR_ACTIVE_ANON) +

global_page_state(NR_ACTIVE_FILE) +

global_page_state(NR_INACTIVE_ANON) +

global_page_state(NR_INACTIVE_FILE);

if (nr_to_scan <= 0 ||
min_adj == OOM_ADJUST_MAX + 1)
{

lowmem_print(5, "lowmem_shrink
%d, %x, return %d\n", nr_to_scan, gfp_mask,

rem);

return rem;

}

read_lock(&tasklist_lock);

for_each_process(p) {

if (p->oomkilladj < min_adj
|| !p->mm)

continue;

tasksize = get_mm_rss(p->mm);

if (tasksize <= 0)

continue;

if (selected) {

if (p->oomkilladj < selected->oomkilladj)

continue;

if (p->oomkilladj == selected->oomkilladj &&

tasksize <= selected_tasksize)

continue;

}

selected = p;

selected_tasksize = tasksize;

lowmem_print(2, "select
%d (%s), adj %d, size %d, to kill\n",

p->pid, p->comm,
p->oomkilladj, tasksize);

}

if(selected != NULL)
{

lowmem_print(1, "send
sigkill to %d (%s), adj %d, size %d\n",

selected->pid, selected->comm,

selected->oomkilladj, selected_tasksize);

force_sig(SIGKILL, selected);

rem -= selected_tasksize;

}

lowmem_print(4, "lowmem_shrink
%d, %x, return %d\n", nr_to_scan, gfp_mask, rem);

read_unlock(&tasklist_lock);

return rem;

}
首先通过global_page_state获取当前剩余内存大小；接着检测lowmem_adj和lowmem_minfree数组的大小(元素个数)是否一致，如果不一致则以最小数组的大小为基准；然后根据剩余内存和内存阈值数组lowmem_minfree查找当前的内存警戒数min_adj。接着遍历所有进程，找到oom_adj大于min_adj并且oom_adj最大的进程。
进程的oom_adj小于警戒阈值，则无视。进程的oom_adj大于等于于警戒阈值，则获取这个进程所占用的内存大小tasksize，如果小于比我们当前选出进程的内存，则无视。如果大于则选中这个进程。
经过for_each的遍历，selected 就是我们选出要释放掉的bad进程，它具有下面两个条件：
第一、Oom_adj大于当前警戒阈值并且最大。
第二、在同样大小的oom_adj中，占用内存最多。
最后，我们释放掉这个进程的内存，通过force_sig(SIGKILL, selected)来向进程发送一个不可以忽略或阻塞的SIGKILL信号。

在lowmem_shrink函数中多处用到了global_page_state函数。
它被定义在了common/include/linux/vmstat.h中，

static inline unsigned long global_page_state(enum
zone_stat_item item)
{
long x = atomic_long_read(&vm_stat[item]);
#ifdef CONFIG_SMP
if (x < 0)
x = 0;
#endif
return x;
}

global_page_state函数的参数NR_FREE_PAGES等使用zone_stat_item枚举，被定义在common/include/linux/mmzone.h中，具体代码如下：
enum zone_stat_item {

NR_FREE_PAGES,

NR_LRU_BASE,

NR_INACTIVE_ANON = NR_LRU_BASE,

NR_ACTIVE_ANON,

NR_INACTIVE_FILE,

NR_ACTIVE_FILE,

#ifdef CONFIG_UNEVICTABLE_LRU

NR_UNEVICTABLE,

NR_MLOCK,

#else

NR_UNEVICTABLE = NR_ACTIVE_FILE, /* 避免编译错误*/

NR_MLOCK = NR_ACTIVE_FILE,

#endif

NR_ANON_PAGES, /* 匿名映射页面*/

NR_FILE_MAPPED, /*映射页面*/

NR_FILE_PAGES,

NR_FILE_DIRTY,

NR_WRITEBACK,

NR_SLAB_RECLAIMABLE,

NR_SLAB_UNRECLAIMABLE,

NR_PAGETABLE,

NR_UNSTABLE_NFS,

NR_BOUNCE,

NR_VMSCAN_WRITE,

NR_WRITEBACK_TEMP, /* 使用临时缓冲区*/

#ifdef CONFIG_NUMA

NUMA_HIT, /* 在预定节点上分配*/

NUMA_MISS, /* 在非预定节点上分配*/

NUMA_FOREIGN,

NUMA_INTERLEAVE_HIT,

NUMA_LOCAL, /* 从本地页面分配*/

NUMA_OTHER, /* 从其他节点分配 */

#endif

NR_VM_ZONE_STAT_ITEMS };

到此结束！