Linux系统内存管理中存在着一个称之为OOM killer(Out-Of-Memory killer)的机制，该机制主要用于内存监控，监控进程的内存使用量，当系统的内存耗尽时，其将根据算法选择性地kill了部分进程。本文分析的内存溢出保护机制，也就是OOM killer机制了。
回到伙伴管理算法中涉及的一函数__alloc_pages_nodemask()，其里面调用的__alloc_pages_slowpath()并未展开深入，而内存溢出保护机制则在此函数中。
先行查看一下__alloc_pages_slowpath()的实现：
【file:/ mm/page_alloc.h】
static inline struct page *
__alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
struct zonelist *zonelist, enum zone_type high_zoneidx,
nodemask_t *nodemask, struct zone *preferred_zone,
int migratetype)
{
const gfp_t wait = gfp_mask & __GFP_WAIT;
struct page *page = NULL;
int alloc_flags;
unsigned long pages_reclaimed = 0;
unsigned long did_some_progress;
bool sync_migration = false;
bool deferred_compaction = false;
bool contended_compaction = false;
if (order >= MAX_ORDER) {
WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN));
return NULL;
}
if (IS_ENABLED(CONFIG_NUMA) &&
(gfp_mask & GFP_THISNODE) == GFP_THISNODE)
goto nopage;
restart:
if (!(gfp_mask & __GFP_NO_KSWAPD))
wake_all_kswapds(order, zonelist, high_zoneidx, preferred_zone);
alloc_flags = gfp_to_alloc_flags(gfp_mask);
if (!(alloc_flags & ALLOC_CPUSET) && !nodemask)
first_zones_zonelist(zonelist, high_zoneidx, NULL,
&preferred_zone);
rebalance:
page = get_page_from_freelist(gfp_mask, nodemask, order, zonelist,
high_zoneidx, alloc_flags & ~ALLOC_NO_WATERMARKS,
preferred_zone, migratetype);
if (page)
goto got_pg;
if (alloc_flags & ALLOC_NO_WATERMARKS) {
zonelist = node_zonelist(numa_node_id(), gfp_mask);
page = __alloc_pages_high_priority(gfp_mask, order,
zonelist, high_zoneidx, nodemask,
preferred_zone, migratetype);
if (page) {
goto got_pg;
}
}
if (!wait) {
WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL);
goto nopage;
}
if (current->flags & PF_MEMALLOC)
goto nopage;
if (test_thread_flag(TIF_MEMDIE) && !(gfp_mask & __GFP_NOFAIL))
goto nopage;
page = __alloc_pages_direct_compact(gfp_mask, order,
zonelist, high_zoneidx,
nodemask,
alloc_flags, preferred_zone,
migratetype, sync_migration,
&contended_compaction,
&deferred_compaction,
&did_some_progress);
if (page)
goto got_pg;
sync_migration = true;
if ((deferred_compaction || contended_compaction) &&
(gfp_mask & __GFP_NO_KSWAPD))
goto nopage;
page = __alloc_pages_direct_reclaim(gfp_mask, order,
zonelist, high_zoneidx,
nodemask,
alloc_flags, preferred_zone,
migratetype, &did_some_progress);
if (page)
goto got_pg;
if (!did_some_progress) {
if (oom_gfp_allowed(gfp_mask)) {
if (oom_killer_disabled)
goto nopage;
if ((current->flags & PF_DUMPCORE) &&
!(gfp_mask & __GFP_NOFAIL))
goto nopage;
page = __alloc_pages_may_oom(gfp_mask, order,
zonelist, high_zoneidx,
nodemask, preferred_zone,
migratetype);
if (page)
goto got_pg;
if (!(gfp_mask & __GFP_NOFAIL)) {
if (order > PAGE_ALLOC_COSTLY_ORDER)
goto nopage;
if (high_zoneidx < ZONE_NORMAL)
goto nopage;
}
goto restart;
}
}
pages_reclaimed += did_some_progress;
if (should_alloc_retry(gfp_mask, order, did_some_progress,
pages_reclaimed)) {
wait_iff_congested(preferred_zone, BLK_RW_ASYNC, HZ/50);
goto rebalance;
} else {
page = __alloc_pages_direct_compact(gfp_mask, order,
zonelist, high_zoneidx,
nodemask,
alloc_flags, preferred_zone,
migratetype, sync_migration,
&contended_compaction,
&deferred_compaction,
&did_some_progress);
if (page)
goto got_pg;
}
nopage:
warn_alloc_failed(gfp_mask, order, NULL);
return page;
got_pg:
if (kmemcheck_enabled)
kmemcheck_pagealloc_alloc(page, order, gfp_mask);
return page;
}
该函数首先判断调用者是否禁止唤醒kswapd线程，若不做禁止则唤醒线程进行内存回收工作，然后通过gfp_to_alloc_flags()对内存分配标识进行调整，而后再次调用get_page_from_freelist()尝试分配，如果分配到则退出。否则继续尝试内存分配，继续尝试分配则先行判断是否设置了ALLOC_NO_WATERMARKS标识，如果设置了，则将忽略watermark，调用__alloc_pages_high_priority()进行分配。
__alloc_pages_high_priority()函数实现：
【file:/ mm/page_alloc.h】
static inline struct page *
__alloc_pages_high_priority(gfp_t gfp_mask, unsigned int order,
struct zonelist *zonelist, enum zone_type high_zoneidx,
nodemask_t *nodemask, struct zone *preferred_zone,
int migratetype)
{
struct page *page;
do {
page = get_page_from_freelist(gfp_mask, nodemask, order,
zonelist, high_zoneidx, ALLOC_NO_WATERMARKS,
preferred_zone, migratetype);
if (!page && gfp_mask & __GFP_NOFAIL)
wait_iff_congested(preferred_zone, BLK_RW_ASYNC, HZ/50);
} while (!page && (gfp_mask & __GFP_NOFAIL));
return page;
}
可以看到该函数根据分配标识__GFP_NOFAIL不断地调用get_page_from_freelist()循环尝试去获得内存。
接着回到__alloc_pages_slowpath()中，其从__alloc_pages_high_priority()退出后继而判断是否设置了__GFP_WAIT标识，如果设置则表示内存分配运行休眠，否则直接以分配内存失败而退出。接着将会调用__alloc_pages_direct_compact()和__alloc_pages_direct_reclaim()尝试回收内存并尝试分配。基于上面的多种尝试内存分配仍然失败的情况，将会调用__alloc_pages_may_oom()触发OOM killer机制。OOM killer将进程kill后会重新再次尝试内存分配，最后则是分配失败或分配成功的收尾处理。
__alloc_pages_slowpath()暂且分析至此，回到本文重点函数__alloc_pages_may_oom()中进一步进行分析。
【file:/ mm/page_alloc.h】
static inline struct page *
__alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
struct zonelist *zonelist, enum zone_type high_zoneidx,
nodemask_t *nodemask, struct zone *preferred_zone,
int migratetype)
{
struct page *page;
if (!try_set_zonelist_oom(zonelist, gfp_mask)) {
schedule_timeout_uninterruptible(1);
return NULL;
}
page = get_page_from_freelist(gfp_mask|__GFP_HARDWALL, nodemask,
order, zonelist, high_zoneidx,
ALLOC_WMARK_HIGH|ALLOC_CPUSET,
preferred_zone, migratetype);
if (page)
goto out;
if (!(gfp_mask & __GFP_NOFAIL)) {
if (order > PAGE_ALLOC_COSTLY_ORDER)
goto out;
if (high_zoneidx < ZONE_NORMAL)
goto out;
if (gfp_mask & __GFP_THISNODE)
goto out;
}
out_of_memory(zonelist, gfp_mask, order, nodemask, false);
out:
clear_zonelist_oom(zonelist, gfp_mask);
return page;
}
该函数首先通过try_set_zonelist_oom()判断OOM killer是否已经在其他核进行killing操作，如果没有的情况下将会在try_set_zonelist_oom()内部进行锁操作，确保只有一个核执行killing的操作。继而调用get_page_from_freelist()在高watermark的情况下尝试再次获取内存，不过这里注定会失败。接着就是调用到了关键函数out_of_memory()。最后函数退出时将会调用clear_zonelist_oom()清除掉try_set_zonelist_oom()里面的锁操作。
着重分析一下out_of_memory()：
【file:/ mm/oom_kill.c】
void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
int order, nodemask_t *nodemask, bool force_kill)
{
const nodemask_t *mpol_mask;
struct task_struct *p;
unsigned long totalpages;
unsigned long freed = 0;
unsigned int uninitialized_var(points);
enum oom_constraint constraint = CONSTRAINT_NONE;
int killed = 0;
blocking_notifier_call_chain(&oom_notify_list, 0, &freed);
if (freed > 0)
return;
if (fatal_signal_pending(current) || current->flags & PF_EXITING) {
set_thread_flag(TIF_MEMDIE);
return;
}
constraint = constrained_alloc(zonelist, gfp_mask, nodemask,
&totalpages);
mpol_mask = (constraint == CONSTRAINT_MEMORY_POLICY) ? nodemask : NULL;
check_panic_on_oom(constraint, gfp_mask, order, mpol_mask);
if (sysctl_oom_kill_allocating_task && current->mm &&
!oom_unkillable_task(current, NULL, nodemask) &&
current->signal->oom_score_adj != OOM_SCORE_ADJ_MIN) {
get_task_struct(current);
oom_kill_process(current, gfp_mask, order, 0, totalpages, NULL,
nodemask,
"Out of memory (oom_kill_allocating_task)");
goto out;
}
p = select_bad_process(&points, totalpages, mpol_mask, force_kill);
if (!p) {
dump_header(NULL, gfp_mask, order, NULL, mpol_mask);
panic("Out of memory and no killable processes...\n");
}
if (p != (void *)-1UL) {
oom_kill_process(p, gfp_mask, order, points, totalpages, NULL,
nodemask, "Out of memory");
killed = 1;
}
out:
if (killed)
schedule_timeout_killable(1);
}
该函数首先调用blocking_notifier_call_chain()进行OOM的内核通知链回调处理；接着的if (fatal_signal_pending(current) || current->flags &
PF_EXITING)判断则是用于检查是否有SIGKILL信号挂起或者正在信号处理中，如果有则退出；再接着通过constrained_alloc()检查内存分配限制以及check_panic_on_oom()检查是否报linux内核panic；继而判断sysctl_oom_kill_allocating_task变量及进程检查，如果符合条件判断，则将当前分配的内存kill掉；否则最后，将通过select_bad_process()选出最佳的进程，进而调用oom_kill_process()对其进行kill操作。
最后分析一下select_bad_process()和oom_kill_process()，其中select_bad_process()的实现：
【file:/ mm/oom_kill.c】
static struct task_struct *select_bad_process(unsigned int *ppoints,
unsigned long totalpages, const nodemask_t *nodemask,
bool force_kill)
{
struct task_struct *g, *p;
struct task_struct *chosen = NULL;
unsigned long chosen_points = 0;
rcu_read_lock();
for_each_process_thread(g, p) {
unsigned int points;
switch (oom_scan_process_thread(p, totalpages, nodemask,
force_kill)) {
case OOM_SCAN_SELECT:
chosen = p;
chosen_points = ULONG_MAX;
case OOM_SCAN_CONTINUE:
continue;
case OOM_SCAN_ABORT:
rcu_read_unlock();
return (struct task_struct *)(-1UL);
case OOM_SCAN_OK:
break;
};
points = oom_badness(p, NULL, nodemask, totalpages);
if (!points || points < chosen_points)
continue;
if (points == chosen_points && thread_group_leader(chosen))
continue;
chosen = p;
chosen_points = points;
}
if (chosen)
get_task_struct(chosen);
rcu_read_unlock();
*ppoints = chosen_points * 1000 / totalpages;
return chosen;
}
此函数通过for_each_process_thread()宏遍历所有进程，进而借用oom_scan_process_thread()获得进程扫描类型然后通过switch-case作特殊化处理，例如存在某进程退出中则中断扫描、某进程占用内存过多且被标识为优先kill掉则优选等特殊处理。而正常情况则会通过oom_badness()计算出进程的分值，然后根据最高分值将进程控制块返回回去。
顺便研究一下oom_badness()的实现:
【file:/ mm/oom_kill.c】
unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
const nodemask_t *nodemask, unsigned long totalpages)
{
long points;
long adj;
if (oom_unkillable_task(p, memcg, nodemask))
return 0;
p = find_lock_task_mm(p);
if (!p)
return 0;
adj = (long)p->signal->oom_score_adj;
if (adj == OOM_SCORE_ADJ_MIN) {
task_unlock(p);
return 0;
}
points = get_mm_rss(p->mm) + atomic_long_read(&p->mm->nr_ptes) +
get_mm_counter(p->mm, MM_SWAPENTS);
task_unlock(p);
if (has_capability_noaudit(p, CAP_SYS_ADMIN))
points -= (points * 3) / 100;
adj *= totalpages / 1000;
points += adj;
return points > 0 ? points : 1;
}
计算进程分值的函数中，首先排除了不可OOM kill的进程以及oom_score_adj值为OOM_SCORE_ADJ_MIN(即-1000)的进程，其中oom_score_adj取值范围是-1000到1000；接着就是计算进程的RSS、页表以及SWAP空间的使用量占RAM的比重，如果该进程是超级进程，则去除3%的权重；最后将oom_score_adj和points归一后，但凡小于0值的都返回1，其他的则返回原值。由此可知，分值越低的则越不会被kill，而且该值可以通过修改oom_score_adj进行调整。
最后分析一下找到了最“bad”的进程后，其享受的“待遇”oom_kill_process()：
【file:/ mm/oom_kill.c】
void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
unsigned int points, unsigned long totalpages,
struct mem_cgroup *memcg, nodemask_t *nodemask,
const char *message)
{
struct task_struct *victim = p;
struct task_struct *child;
struct task_struct *t;
struct mm_struct *mm;
unsigned int victim_points = 0;
static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
DEFAULT_RATELIMIT_BURST);
if (p->flags & PF_EXITING) {
set_tsk_thread_flag(p, TIF_MEMDIE);
put_task_struct(p);
return;
}
if (__ratelimit(&oom_rs))
dump_header(p, gfp_mask, order, memcg, nodemask);
task_lock(p);
pr_err("%s: Kill process %d (%s) score %d or sacrifice child\n",
message, task_pid_nr(p), p->comm, points);
task_unlock(p);
read_lock(&tasklist_lock);
for_each_thread(p, t) {
list_for_each_entry(child, &t->children, sibling) {
unsigned int child_points;
if (child->mm == p->mm)
continue;
child_points = oom_badness(child, memcg, nodemask,
totalpages);
if (child_points > victim_points) {
put_task_struct(victim);
victim = child;
victim_points = child_points;
get_task_struct(victim);
}
}
}
read_unlock(&tasklist_lock);
p = find_lock_task_mm(victim);
if (!p) {
put_task_struct(victim);
return;
} else if (victim != p) {
get_task_struct(p);
put_task_struct(victim);
victim = p;
}
mm = victim->mm;
pr_err("Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB\n",
task_pid_nr(victim), victim->comm, K(victim->mm->total_vm),
K(get_mm_counter(victim->mm, MM_ANONPAGES)),
K(get_mm_counter(victim->mm, MM_FILEPAGES)));
task_unlock(victim);
rcu_read_lock();
for_each_process(p)
if (p->mm == mm && !same_thread_group(p, victim) &&
!(p->flags & PF_KTHREAD)) {
if (p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN)
continue;
task_lock(p);
pr_err("Kill process %d (%s) sharing same memory\n",
task_pid_nr(p), p->comm);
task_unlock(p);
do_send_sig_info(SIGKILL, SEND_SIG_FORCED, p, true);
}
rcu_read_unlock();
set_tsk_thread_flag(victim, TIF_MEMDIE);
do_send_sig_info(SIGKILL, SEND_SIG_FORCED, victim, true);
put_task_struct(victim);
}
该函数将会判断当前被kill的进程情况，如果该进程处于退出状态，则设置TIF_MEMDIE标志，不做kill操作；接着会通过list_for_each_entry()遍历该进程的子进程信息，如果某个子进程拥有不同的mm且合适被kill掉，将会优先考虑将该子进程替代父进程kill掉，这样可以避免kill掉父进程带来的接管子进程的工作开销；再往下通过find_lock_task_mm()找到持有mm锁的进程，如果进程处于退出状态，则return，否则继续处理，若此时的进程与传入的不是同一个时则更新victim；继而接着通过for_each_process()查找与当前被kill进程使用到了同样的共享内存的进程进行一起kill掉，kill之前将对应的进程添加标识TIF_MEMDIE，而kill的动作则是通过发送SICKILL信号给对应进程，由被kill进程从内核态返回用户态时进行处理。
至此，OOM kill处理分析完毕。