/*
   *  linux/mm/oom_kill.c
   * 
   *  Copyright (C)  1998,2000  Rik van Riel
   *	Thanks go out to Claus Fischer for some serious inspiration and
   *	for goading me into coding this file...
   *  Copyright (C)  2010  Google, Inc.
   *	Rewritten by David Rientjes
   *
   *  The routines in this file are used to kill a process when
   *  we're seriously out of memory. This gets called from __alloc_pages()
   *  in mm/page_alloc.c when we really run out of memory.
   *
   *  Since we won't call these routines often (on a well-configured
   *  machine) this file will double as a 'coding guide' and a signpost
   *  for newbie kernel hackers. It features several pointers to major
   *  kernel subsystems and hints as to where to find out what things do.
   */
  
  #include <linux/oom.h>
  #include <linux/mm.h>
  #include <linux/err.h>
  #include <linux/gfp.h>
  #include <linux/sched.h>
  #include <linux/swap.h>
  #include <linux/timex.h>
  #include <linux/jiffies.h>
  #include <linux/cpuset.h>
  #include <linux/export.h>
  #include <linux/notifier.h>
  #include <linux/memcontrol.h>
  #include <linux/mempolicy.h>
  #include <linux/security.h>
  #include <linux/ptrace.h>
  #include <linux/freezer.h>
  #include <linux/ftrace.h>
  #include <linux/ratelimit.h>
  
  #define CREATE_TRACE_POINTS
  #include <trace/events/oom.h>
  
  int sysctl_panic_on_oom;
  int sysctl_oom_kill_allocating_task;
  int sysctl_oom_dump_tasks = 1;
  static DEFINE_SPINLOCK(zone_scan_lock);
  
  #ifdef CONFIG_NUMA
  /**
   * has_intersects_mems_allowed() - check task eligiblity for kill
   * @start: task struct of which task to consider
   * @mask: nodemask passed to page allocator for mempolicy ooms
   *
   * Task eligibility is determined by whether or not a candidate task, @tsk,
   * shares the same mempolicy nodes as current if it is bound by such a policy
   * and whether or not it has the same set of allowed cpuset nodes.
   */
  static bool has_intersects_mems_allowed(struct task_struct *start,
  					const nodemask_t *mask)
  {
  	struct task_struct *tsk;
  	bool ret = false;
  
  	rcu_read_lock();
  	for_each_thread(start, tsk) {
  		if (mask) {
  			/*
  			 * If this is a mempolicy constrained oom, tsk's
  			 * cpuset is irrelevant.  Only return true if its
  			 * mempolicy intersects current, otherwise it may be
  			 * needlessly killed.
  			 */
  			ret = mempolicy_nodemask_intersects(tsk, mask);
  		} else {
  			/*
  			 * This is not a mempolicy constrained oom, so only
  			 * check the mems of tsk's cpuset.
  			 */
  			ret = cpuset_mems_allowed_intersects(current, tsk);
  		}
  		if (ret)
  			break;
  	}
  	rcu_read_unlock();
  
  	return ret;
  }
  #else
  static bool has_intersects_mems_allowed(struct task_struct *tsk,
  					const nodemask_t *mask)
  {
  	return true;
  }
  #endif /* CONFIG_NUMA */
  
  /*
   * The process p may have detached its own ->mm while exiting or through
   * use_mm(), but one or more of its subthreads may still have a valid
   * pointer.  Return p, or any of its subthreads with a valid ->mm, with
   * task_lock() held.
   */
  struct task_struct *find_lock_task_mm(struct task_struct *p)
  {
  	struct task_struct *t;
  
  	rcu_read_lock();
  
  	for_each_thread(p, t) {
  		task_lock(t);
  		if (likely(t->mm))
  			goto found;
  		task_unlock(t);
  	}
  	t = NULL;
  found:
  	rcu_read_unlock();
  
  	return t;
  }
  
  /* return true if the task is not adequate as candidate victim task. */
  static bool oom_unkillable_task(struct task_struct *p,
  		const struct mem_cgroup *memcg, const nodemask_t *nodemask)
  {
  	if (is_global_init(p))
  		return true;
  	if (p->flags & PF_KTHREAD)
  		return true;
  
  	/* When mem_cgroup_out_of_memory() and p is not member of the group */
  	if (memcg && !task_in_mem_cgroup(p, memcg))
  		return true;
  
  	/* p may not have freeable memory in nodemask */
  	if (!has_intersects_mems_allowed(p, nodemask))
  		return true;
  
  	return false;
  }
  
  /**
   * oom_badness - heuristic function to determine which candidate task to kill
   * @p: task struct of which task we should calculate
   * @totalpages: total present RAM allowed for page allocation
   *
   * The heuristic for determining which task to kill is made to be as simple and
   * predictable as possible.  The goal is to return the highest value for the
   * task consuming the most memory to avoid subsequent oom failures.
   */
  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;
  	}
  
  	/*
  	 * The baseline for the badness score is the proportion of RAM that each
  	 * task's rss, pagetable and swap space use.
  	 */
  	points = get_mm_rss(p->mm) + atomic_long_read(&p->mm->nr_ptes) +
  		 get_mm_counter(p->mm, MM_SWAPENTS);
  	task_unlock(p);
  
  	/*
  	 * Root processes get 3% bonus, just like the __vm_enough_memory()
  	 * implementation used by LSMs.
  	 */
  	if (has_capability_noaudit(p, CAP_SYS_ADMIN))
  		points -= (points * 3) / 100;
  
  	/* Normalize to oom_score_adj units */
  	adj *= totalpages / 1000;
  	points += adj;
  
  	/*
  	 * Never return 0 for an eligible task regardless of the root bonus and
  	 * oom_score_adj (oom_score_adj can't be OOM_SCORE_ADJ_MIN here).
  	 */
  	return points > 0 ? points : 1;
  }
  
  /*
   * Determine the type of allocation constraint.
   */
  #ifdef CONFIG_NUMA
  static enum oom_constraint constrained_alloc(struct zonelist *zonelist,
  				gfp_t gfp_mask, nodemask_t *nodemask,
  				unsigned long *totalpages)
  {
  	struct zone *zone;
  	struct zoneref *z;
  	enum zone_type high_zoneidx = gfp_zone(gfp_mask);
  	bool cpuset_limited = false;
  	int nid;
  
  	/* Default to all available memory */
  	*totalpages = totalram_pages + total_swap_pages;
  
  	if (!zonelist)
  		return CONSTRAINT_NONE;
  	/*
  	 * Reach here only when __GFP_NOFAIL is used. So, we should avoid
  	 * to kill current.We have to random task kill in this case.
  	 * Hopefully, CONSTRAINT_THISNODE...but no way to handle it, now.
  	 */
  	if (gfp_mask & __GFP_THISNODE)
  		return CONSTRAINT_NONE;
  
  	/*
  	 * This is not a __GFP_THISNODE allocation, so a truncated nodemask in
  	 * the page allocator means a mempolicy is in effect.  Cpuset policy
  	 * is enforced in get_page_from_freelist().
  	 */
  	if (nodemask && !nodes_subset(node_states[N_MEMORY], *nodemask)) {
  		*totalpages = total_swap_pages;
  		for_each_node_mask(nid, *nodemask)
  			*totalpages += node_spanned_pages(nid);
  		return CONSTRAINT_MEMORY_POLICY;
  	}
  
  	/* Check this allocation failure is caused by cpuset's wall function */
  	for_each_zone_zonelist_nodemask(zone, z, zonelist,
  			high_zoneidx, nodemask)
  		if (!cpuset_zone_allowed_softwall(zone, gfp_mask))
  			cpuset_limited = true;
  
  	if (cpuset_limited) {
  		*totalpages = total_swap_pages;
  		for_each_node_mask(nid, cpuset_current_mems_allowed)
  			*totalpages += node_spanned_pages(nid);
  		return CONSTRAINT_CPUSET;
  	}
  	return CONSTRAINT_NONE;
  }
  #else
  static enum oom_constraint constrained_alloc(struct zonelist *zonelist,
  				gfp_t gfp_mask, nodemask_t *nodemask,
  				unsigned long *totalpages)
  {
  	*totalpages = totalram_pages + total_swap_pages;
  	return CONSTRAINT_NONE;
  }
  #endif
  
  enum oom_scan_t oom_scan_process_thread(struct task_struct *task,
  		unsigned long totalpages, const nodemask_t *nodemask,
  		bool force_kill)
  {
  	if (task->exit_state)
  		return OOM_SCAN_CONTINUE;
  	if (oom_unkillable_task(task, NULL, nodemask))
  		return OOM_SCAN_CONTINUE;
  
  	/*
  	 * This task already has access to memory reserves and is being killed.
  	 * Don't allow any other task to have access to the reserves.
  	 */
  	if (test_tsk_thread_flag(task, TIF_MEMDIE)) {
  		if (unlikely(frozen(task)))
  			__thaw_task(task);
  		if (!force_kill)
  			return OOM_SCAN_ABORT;
  	}
  	if (!task->mm)
  		return OOM_SCAN_CONTINUE;
  
  	/*
  	 * If task is allocating a lot of memory and has been marked to be
  	 * killed first if it triggers an oom, then select it.
  	 */
  	if (oom_task_origin(task))
  		return OOM_SCAN_SELECT;
  
  	if (task->flags & PF_EXITING && !force_kill) {
  		/*
  		 * If this task is not being ptraced on exit, then wait for it
  		 * to finish before killing some other task unnecessarily.
  		 */
  		if (!(task->group_leader->ptrace & PT_TRACE_EXIT))
  			return OOM_SCAN_ABORT;
  	}
  	return OOM_SCAN_OK;
  }
  
  /*
   * Simple selection loop. We chose the process with the highest
   * number of 'points'.  Returns -1 on scan abort.
   *
   * (not docbooked, we don't want this one cluttering up the manual)
   */
  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;
  			/* fall through */
  		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;
  		/* Prefer thread group leaders for display purposes */
  		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;
  }
  
  /**
   * dump_tasks - dump current memory state of all system tasks
   * @memcg: current's memory controller, if constrained
   * @nodemask: nodemask passed to page allocator for mempolicy ooms
   *
   * Dumps the current memory state of all eligible tasks.  Tasks not in the same
   * memcg, not in the same cpuset, or bound to a disjoint set of mempolicy nodes
   * are not shown.
   * State information includes task's pid, uid, tgid, vm size, rss, nr_ptes,
   * swapents, oom_score_adj value, and name.
   */
  static void dump_tasks(const struct mem_cgroup *memcg, const nodemask_t *nodemask)
  {
  	struct task_struct *p;
  	struct task_struct *task;
  
  	pr_info("[ pid ]   uid  tgid total_vm      rss nr_ptes swapents oom_score_adj name
  ");
  	rcu_read_lock();
  	for_each_process(p) {
  		if (oom_unkillable_task(p, memcg, nodemask))
  			continue;
  
  		task = find_lock_task_mm(p);
  		if (!task) {
  			/*
  			 * This is a kthread or all of p's threads have already
  			 * detached their mm's.  There's no need to report
  			 * them; they can't be oom killed anyway.
  			 */
  			continue;
  		}
  
  		pr_info("[%5d] %5d %5d %8lu %8lu %7ld %8lu         %5hd %s
  ",
  			task->pid, from_kuid(&init_user_ns, task_uid(task)),
  			task->tgid, task->mm->total_vm, get_mm_rss(task->mm),
  			atomic_long_read(&task->mm->nr_ptes),
  			get_mm_counter(task->mm, MM_SWAPENTS),
  			task->signal->oom_score_adj, task->comm);
  		task_unlock(task);
  	}
  	rcu_read_unlock();
  }
  
  static void dump_header(struct task_struct *p, gfp_t gfp_mask, int order,
  			struct mem_cgroup *memcg, const nodemask_t *nodemask)
  {
  	task_lock(current);
  	pr_warning("%s invoked oom-killer: gfp_mask=0x%x, order=%d, "
  		"oom_score_adj=%hd
  ",
  		current->comm, gfp_mask, order,
  		current->signal->oom_score_adj);
  	cpuset_print_task_mems_allowed(current);
  	task_unlock(current);
  	dump_stack();
  	if (memcg)
  		mem_cgroup_print_oom_info(memcg, p);
  	else
  		show_mem(SHOW_MEM_FILTER_NODES);
  	if (sysctl_oom_dump_tasks)
  		dump_tasks(memcg, nodemask);
  }
  
  /*
   * Number of OOM killer invocations (including memcg OOM killer).
   * Primarily used by PM freezer to check for potential races with
   * OOM killed frozen task.
   */
  static atomic_t oom_kills = ATOMIC_INIT(0);
  
  int oom_kills_count(void)
  {
  	return atomic_read(&oom_kills);
  }
  
  void note_oom_kill(void)
  {
  	atomic_inc(&oom_kills);
  }
  
  #define K(x) ((x) << (PAGE_SHIFT-10))
  /*
   * Must be called while holding a reference to p, which will be released upon
   * returning.
   */
  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 the task is already exiting, don't alarm the sysadmin or kill
  	 * its children or threads, just set TIF_MEMDIE so it can die quickly
  	 */
  	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
  ",
  		message, task_pid_nr(p), p->comm, points);
  	task_unlock(p);
  
  	/*
  	 * If any of p's children has a different mm and is eligible for kill,
  	 * the one with the highest oom_badness() score is sacrificed for its
  	 * parent.  This attempts to lose the minimal amount of work done while
  	 * still freeing memory.
  	 */
  	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;
  			/*
  			 * oom_badness() returns 0 if the thread is unkillable
  			 */
  			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 cannot safely be dereferenced after task_unlock(victim) */
  	mm = victim->mm;
  	pr_err("Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB
  ",
  		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);
  
  	/*
  	 * Kill all user processes sharing victim->mm in other thread groups, if
  	 * any.  They don't get access to memory reserves, though, to avoid
  	 * depletion of all memory.  This prevents mm->mmap_sem livelock when an
  	 * oom killed thread cannot exit because it requires the semaphore and
  	 * its contended by another thread trying to allocate memory itself.
  	 * That thread will now get access to memory reserves since it has a
  	 * pending fatal signal.
  	 */
  	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);	/* Protect ->comm from prctl() */
  			pr_err("Kill process %d (%s) sharing same memory
  ",
  				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);
  }
  #undef K
  
  /*
   * Determines whether the kernel must panic because of the panic_on_oom sysctl.
   */
  void check_panic_on_oom(enum oom_constraint constraint, gfp_t gfp_mask,
  			int order, const nodemask_t *nodemask)
  {
  	if (likely(!sysctl_panic_on_oom))
  		return;
  	if (sysctl_panic_on_oom != 2) {
  		/*
  		 * panic_on_oom == 1 only affects CONSTRAINT_NONE, the kernel
  		 * does not panic for cpuset, mempolicy, or memcg allocation
  		 * failures.
  		 */
  		if (constraint != CONSTRAINT_NONE)
  			return;
  	}
  	dump_header(NULL, gfp_mask, order, NULL, nodemask);
  	panic("Out of memory: %s panic_on_oom is enabled
  ",
  		sysctl_panic_on_oom == 2 ? "compulsory" : "system-wide");
  }
  
  static BLOCKING_NOTIFIER_HEAD(oom_notify_list);
  
  int register_oom_notifier(struct notifier_block *nb)
  {
  	return blocking_notifier_chain_register(&oom_notify_list, nb);
  }
  EXPORT_SYMBOL_GPL(register_oom_notifier);
  
  int unregister_oom_notifier(struct notifier_block *nb)
  {
  	return blocking_notifier_chain_unregister(&oom_notify_list, nb);
  }
  EXPORT_SYMBOL_GPL(unregister_oom_notifier);
  
  /*
   * Try to acquire the OOM killer lock for the zones in zonelist.  Returns zero
   * if a parallel OOM killing is already taking place that includes a zone in
   * the zonelist.  Otherwise, locks all zones in the zonelist and returns 1.
   */
  int try_set_zonelist_oom(struct zonelist *zonelist, gfp_t gfp_mask)
  {
  	struct zoneref *z;
  	struct zone *zone;
  	int ret = 1;
  
  	spin_lock(&zone_scan_lock);
  	for_each_zone_zonelist(zone, z, zonelist, gfp_zone(gfp_mask)) {
  		if (zone_is_oom_locked(zone)) {
  			ret = 0;
  			goto out;
  		}
  	}
  
  	for_each_zone_zonelist(zone, z, zonelist, gfp_zone(gfp_mask)) {
  		/*
  		 * Lock each zone in the zonelist under zone_scan_lock so a
  		 * parallel invocation of try_set_zonelist_oom() doesn't succeed
  		 * when it shouldn't.
  		 */
  		zone_set_flag(zone, ZONE_OOM_LOCKED);
  	}
  
  out:
  	spin_unlock(&zone_scan_lock);
  	return ret;
  }
  
  /*
   * Clears the ZONE_OOM_LOCKED flag for all zones in the zonelist so that failed
   * allocation attempts with zonelists containing them may now recall the OOM
   * killer, if necessary.
   */
  void clear_zonelist_oom(struct zonelist *zonelist, gfp_t gfp_mask)
  {
  	struct zoneref *z;
  	struct zone *zone;
  
  	spin_lock(&zone_scan_lock);
  	for_each_zone_zonelist(zone, z, zonelist, gfp_zone(gfp_mask)) {
  		zone_clear_flag(zone, ZONE_OOM_LOCKED);
  	}
  	spin_unlock(&zone_scan_lock);
  }
  
  /**
   * out_of_memory - kill the "best" process when we run out of memory
   * @zonelist: zonelist pointer
   * @gfp_mask: memory allocation flags
   * @order: amount of memory being requested as a power of 2
   * @nodemask: nodemask passed to page allocator
   * @force_kill: true if a task must be killed, even if others are exiting
   *
   * If we run out of memory, we have the choice between either
   * killing a random task (bad), letting the system crash (worse)
   * OR try to be smart about which process to kill. Note that we
   * don't have to be perfect here, we just have to be good.
   */
  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)
  		/* Got some memory back in the last second. */
  		return;
  
  	/*
  	 * If current has a pending SIGKILL or is exiting, then automatically
  	 * select it.  The goal is to allow it to allocate so that it may
  	 * quickly exit and free its memory.
  	 */
  	if (fatal_signal_pending(current) || current->flags & PF_EXITING) {
  		set_thread_flag(TIF_MEMDIE);
  		return;
  	}
  
  	/*
  	 * Check if there were limitations on the allocation (only relevant for
  	 * NUMA) that may require different handling.
  	 */
  	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);
  	/* Found nothing?!?! Either we hang forever, or we panic. */
  	if (!p) {
  		dump_header(NULL, gfp_mask, order, NULL, mpol_mask);
  		panic("Out of memory and no killable processes...
  ");
  	}
  	if (p != (void *)-1UL) {
  		oom_kill_process(p, gfp_mask, order, points, totalpages, NULL,
  				 nodemask, "Out of memory");
  		killed = 1;
  	}
  out:
  	/*
  	 * Give the killed threads a good chance of exiting before trying to
  	 * allocate memory again.
  	 */
  	if (killed)
  		schedule_timeout_killable(1);
  }
  
  /*
   * The pagefault handler calls here because it is out of memory, so kill a
   * memory-hogging task.  If any populated zone has ZONE_OOM_LOCKED set, a
   * parallel oom killing is already in progress so do nothing.
   */
  void pagefault_out_of_memory(void)
  {
  	struct zonelist *zonelist;
  
  	if (mem_cgroup_oom_synchronize(true))
  		return;
  
  	zonelist = node_zonelist(first_online_node, GFP_KERNEL);
  	if (try_set_zonelist_oom(zonelist, GFP_KERNEL)) {
  		out_of_memory(NULL, 0, 0, NULL, false);
  		clear_zonelist_oom(zonelist, GFP_KERNEL);
  	}
  }