/*
   *  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/sched/mm.h>

  #include <linux/sched/coredump.h>

  #include <linux/sched/task.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>

  #include <linux/kthread.h>
  #include <linux/init.h>

  #include <linux/mmu_notifier.h>

  
  #include <asm/tlb.h>
  #include "internal.h"

  #include "slab.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;

  /*
   * Serializes oom killer invocations (out_of_memory()) from all contexts to
   * prevent from over eager oom killing (e.g. when the oom killer is invoked
   * from different domains).
   *
   * oom_killer_disable() relies on this lock to stabilize oom_killer_disabled
   * and mark_oom_victim
   */

  DEFINE_MUTEX(oom_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;

  }

  /*
   * order == -1 means the oom kill is required by sysrq, otherwise only
   * for display purposes.
   */
  static inline bool is_sysrq_oom(struct oom_control *oc)
  {
  	return oc->order == -1;
  }

  static inline bool is_memcg_oom(struct oom_control *oc)
  {
  	return oc->memcg != NULL;
  }

  /* return true if the task is not adequate as candidate victim task. */

  static bool oom_unkillable_task(struct task_struct *p,

  		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;
  }

  /*
   * Print out unreclaimble slabs info when unreclaimable slabs amount is greater
   * than all user memory (LRU pages)
   */
  static bool is_dump_unreclaim_slabs(void)
  {
  	unsigned long nr_lru;
  
  	nr_lru = global_node_page_state(NR_ACTIVE_ANON) +
  		 global_node_page_state(NR_INACTIVE_ANON) +
  		 global_node_page_state(NR_ACTIVE_FILE) +
  		 global_node_page_state(NR_INACTIVE_FILE) +
  		 global_node_page_state(NR_ISOLATED_ANON) +
  		 global_node_page_state(NR_ISOLATED_FILE) +
  		 global_node_page_state(NR_UNEVICTABLE);
  
  	return (global_node_page_state(NR_SLAB_UNRECLAIMABLE) > nr_lru);
  }

  /**

   * 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

   * @memcg: task's memory controller, if constrained
   * @nodemask: nodemask passed to page allocator for mempolicy ooms

   *

   * 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;

  	/*
  	 * Do not even consider tasks which are explicitly marked oom

  	 * unkillable or have been already oom reaped or the are in
  	 * the middle of vfork

  	 */

  	adj = (long)p->signal->oom_score_adj;

  	if (adj == OOM_SCORE_ADJ_MIN ||

  			test_bit(MMF_OOM_SKIP, &p->mm->flags) ||

  			in_vfork(p)) {

  		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) + get_mm_counter(p->mm, MM_SWAPENTS) +

  		mm_pgtables_bytes(p->mm) / PAGE_SIZE;

  	task_unlock(p);

  	/* 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;

  }

  enum oom_constraint {
  	CONSTRAINT_NONE,
  	CONSTRAINT_CPUSET,
  	CONSTRAINT_MEMORY_POLICY,
  	CONSTRAINT_MEMCG,
  };

  /*

   * Determine the type of allocation constraint.
   */

  static enum oom_constraint constrained_alloc(struct oom_control *oc)

  {

  	struct zone *zone;

  	struct zoneref *z;

  	enum zone_type high_zoneidx = gfp_zone(oc->gfp_mask);

  	bool cpuset_limited = false;
  	int nid;

  	if (is_memcg_oom(oc)) {

  		oc->totalpages = mem_cgroup_get_max(oc->memcg) ?: 1;

  		return CONSTRAINT_MEMCG;
  	}

  	/* Default to all available memory */

  	oc->totalpages = totalram_pages + total_swap_pages;
  
  	if (!IS_ENABLED(CONFIG_NUMA))
  		return CONSTRAINT_NONE;

  	if (!oc->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 (oc->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 (oc->nodemask &&
  	    !nodes_subset(node_states[N_MEMORY], *oc->nodemask)) {

  		oc->totalpages = total_swap_pages;

  		for_each_node_mask(nid, *oc->nodemask)

  			oc->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, oc->zonelist,
  			high_zoneidx, oc->nodemask)
  		if (!cpuset_zone_allowed(zone, oc->gfp_mask))

  			cpuset_limited = true;

  	if (cpuset_limited) {

  		oc->totalpages = total_swap_pages;

  		for_each_node_mask(nid, cpuset_current_mems_allowed)

  			oc->totalpages += node_spanned_pages(nid);

  		return CONSTRAINT_CPUSET;
  	}

  	return CONSTRAINT_NONE;
  }

  static int oom_evaluate_task(struct task_struct *task, void *arg)

  {

  	struct oom_control *oc = arg;
  	unsigned long points;

  	if (oom_unkillable_task(task, NULL, oc->nodemask))

  		goto next;

  
  	/*
  	 * This task already has access to memory reserves and is being killed.

  	 * Don't allow any other task to have access to the reserves unless

  	 * the task has MMF_OOM_SKIP because chances that it would release

  	 * any memory is quite low.

  	 */

  	if (!is_sysrq_oom(oc) && tsk_is_oom_victim(task)) {
  		if (test_bit(MMF_OOM_SKIP, &task->signal->oom_mm->flags))

  			goto next;
  		goto abort;

  	}

  	/*
  	 * 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)) {
  		points = ULONG_MAX;
  		goto select;
  	}

  	points = oom_badness(task, NULL, oc->nodemask, oc->totalpages);
  	if (!points || points < oc->chosen_points)
  		goto next;
  
  	/* Prefer thread group leaders for display purposes */
  	if (points == oc->chosen_points && thread_group_leader(oc->chosen))
  		goto next;
  select:
  	if (oc->chosen)
  		put_task_struct(oc->chosen);
  	get_task_struct(task);
  	oc->chosen = task;
  	oc->chosen_points = points;
  next:
  	return 0;
  abort:
  	if (oc->chosen)
  		put_task_struct(oc->chosen);
  	oc->chosen = (void *)-1UL;
  	return 1;

  }

  /*

   * Simple selection loop. We choose the process with the highest number of
   * 'points'. In case scan was aborted, oc->chosen is set to -1.

   */

  static void select_bad_process(struct oom_control *oc)

  {

  	if (is_memcg_oom(oc))
  		mem_cgroup_scan_tasks(oc->memcg, oom_evaluate_task, oc);
  	else {
  		struct task_struct *p;

  		rcu_read_lock();
  		for_each_process(p)
  			if (oom_evaluate_task(p, oc))
  				break;
  		rcu_read_unlock();

  	}

  	oc->chosen_points = oc->chosen_points * 1000 / oc->totalpages;

  }
  
  /**

   * 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,
   * pgtables_bytes, swapents, oom_score_adj value, and name.

   */

  static void dump_tasks(struct mem_cgroup *memcg, const nodemask_t *nodemask)

  {

  	struct task_struct *p;
  	struct task_struct *task;

  	pr_info("Tasks state (memory values in pages):
  ");
  	pr_info("[  pid  ]   uid  tgid total_vm      rss pgtables_bytes 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("[%7d] %5d %5d %8lu %8lu %8ld %8lu         %5hd %s
  ",

  			task->pid, from_kuid(&init_user_ns, task_uid(task)),
  			task->tgid, task->mm->total_vm, get_mm_rss(task->mm),

  			mm_pgtables_bytes(task->mm),

  			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 oom_control *oc, struct task_struct *p)

  {

  	pr_warn("%s invoked oom-killer: gfp_mask=%#x(%pGg), nodemask=%*pbl, order=%d, oom_score_adj=%hd
  ",
  		current->comm, oc->gfp_mask, &oc->gfp_mask,
  		nodemask_pr_args(oc->nodemask), oc->order,
  			current->signal->oom_score_adj);

  	if (!IS_ENABLED(CONFIG_COMPACTION) && oc->order)
  		pr_warn("COMPACTION is disabled!!!
  ");

  	cpuset_print_current_mems_allowed();

  	dump_stack();

  	if (is_memcg_oom(oc))

  		mem_cgroup_print_oom_info(oc->memcg, p);

  	else {

  		show_mem(SHOW_MEM_FILTER_NODES, oc->nodemask);

  		if (is_dump_unreclaim_slabs())
  			dump_unreclaimable_slab();
  	}

  	if (sysctl_oom_dump_tasks)

  		dump_tasks(oc->memcg, oc->nodemask);

  }

  /*

   * Number of OOM victims in flight

   */

  static atomic_t oom_victims = ATOMIC_INIT(0);
  static DECLARE_WAIT_QUEUE_HEAD(oom_victims_wait);

  static bool oom_killer_disabled __read_mostly;

  #define K(x) ((x) << (PAGE_SHIFT-10))

  /*
   * task->mm can be NULL if the task is the exited group leader.  So to
   * determine whether the task is using a particular mm, we examine all the
   * task's threads: if one of those is using this mm then this task was also
   * using it.
   */

  bool process_shares_mm(struct task_struct *p, struct mm_struct *mm)

  {
  	struct task_struct *t;
  
  	for_each_thread(p, t) {
  		struct mm_struct *t_mm = READ_ONCE(t->mm);
  		if (t_mm)
  			return t_mm == mm;
  	}
  	return false;
  }

  #ifdef CONFIG_MMU
  /*
   * OOM Reaper kernel thread which tries to reap the memory used by the OOM
   * victim (if that is possible) to help the OOM killer to move on.
   */
  static struct task_struct *oom_reaper_th;

  static DECLARE_WAIT_QUEUE_HEAD(oom_reaper_wait);

  static struct task_struct *oom_reaper_list;

  static DEFINE_SPINLOCK(oom_reaper_lock);

  bool __oom_reap_task_mm(struct mm_struct *mm)

  {

  	struct vm_area_struct *vma;

  	bool ret = true;

  
  	/*
  	 * Tell all users of get_user/copy_from_user etc... that the content
  	 * is no longer stable. No barriers really needed because unmapping
  	 * should imply barriers already and the reader would hit a page fault
  	 * if it stumbled over a reaped memory.
  	 */
  	set_bit(MMF_UNSTABLE, &mm->flags);
  
  	for (vma = mm->mmap ; vma; vma = vma->vm_next) {
  		if (!can_madv_dontneed_vma(vma))
  			continue;
  
  		/*
  		 * Only anonymous pages have a good chance to be dropped
  		 * without additional steps which we cannot afford as we
  		 * are OOM already.
  		 *
  		 * We do not even care about fs backed pages because all
  		 * which are reclaimable have already been reclaimed and
  		 * we do not want to block exit_mmap by keeping mm ref
  		 * count elevated without a good reason.
  		 */
  		if (vma_is_anonymous(vma) || !(vma->vm_flags & VM_SHARED)) {
  			const unsigned long start = vma->vm_start;
  			const unsigned long end = vma->vm_end;
  			struct mmu_gather tlb;
  
  			tlb_gather_mmu(&tlb, mm, start, end);

  			if (mmu_notifier_invalidate_range_start_nonblock(mm, start, end)) {

  				tlb_finish_mmu(&tlb, start, end);

  				ret = false;
  				continue;
  			}

  			unmap_page_range(&tlb, vma, start, end, NULL);
  			mmu_notifier_invalidate_range_end(mm, start, end);
  			tlb_finish_mmu(&tlb, start, end);
  		}
  	}

  
  	return ret;

  }

  /*
   * Reaps the address space of the give task.
   *
   * Returns true on success and false if none or part of the address space
   * has been reclaimed and the caller should retry later.
   */

  static bool oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
  {

  	bool ret = true;

  	if (!down_read_trylock(&mm->mmap_sem)) {

  		trace_skip_task_reaping(tsk->pid);

  		return false;

  	}
  
  	/*

  	 * MMF_OOM_SKIP is set by exit_mmap when the OOM reaper can't
  	 * work on the mm anymore. The check for MMF_OOM_SKIP must run
  	 * under mmap_sem for reading because it serializes against the
  	 * down_write();up_write() cycle in exit_mmap().

  	 */

  	if (test_bit(MMF_OOM_SKIP, &mm->flags)) {

  		trace_skip_task_reaping(tsk->pid);

  		goto out_unlock;

  	}

  	trace_start_task_reaping(tsk->pid);

  	/* failed to reap part of the address space. Try again later */

  	ret = __oom_reap_task_mm(mm);
  	if (!ret)
  		goto out_finish;

  	pr_info("oom_reaper: reaped process %d (%s), now anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB
  ",
  			task_pid_nr(tsk), tsk->comm,
  			K(get_mm_counter(mm, MM_ANONPAGES)),
  			K(get_mm_counter(mm, MM_FILEPAGES)),
  			K(get_mm_counter(mm, MM_SHMEMPAGES)));

  out_finish:
  	trace_finish_task_reaping(tsk->pid);
  out_unlock:

  	up_read(&mm->mmap_sem);

  	return ret;
  }

  #define MAX_OOM_REAP_RETRIES 10

  static void oom_reap_task(struct task_struct *tsk)

  {
  	int attempts = 0;

  	struct mm_struct *mm = tsk->signal->oom_mm;

  
  	/* Retry the down_read_trylock(mmap_sem) a few times */

  	while (attempts++ < MAX_OOM_REAP_RETRIES && !oom_reap_task_mm(tsk, mm))

  		schedule_timeout_idle(HZ/10);

  	if (attempts <= MAX_OOM_REAP_RETRIES ||
  	    test_bit(MMF_OOM_SKIP, &mm->flags))

  		goto done;

  	pr_info("oom_reaper: unable to reap pid:%d (%s)
  ",
  		task_pid_nr(tsk), tsk->comm);

  	debug_show_all_locks();

  done:

  	tsk->oom_reaper_list = NULL;

  	/*
  	 * Hide this mm from OOM killer because it has been either reaped or
  	 * somebody can't call up_write(mmap_sem).
  	 */

  	set_bit(MMF_OOM_SKIP, &mm->flags);

  	/* Drop a reference taken by wake_oom_reaper */

  	put_task_struct(tsk);

  }
  
  static int oom_reaper(void *unused)
  {
  	while (true) {

  		struct task_struct *tsk = NULL;

  		wait_event_freezable(oom_reaper_wait, oom_reaper_list != NULL);

  		spin_lock(&oom_reaper_lock);

  		if (oom_reaper_list != NULL) {
  			tsk = oom_reaper_list;
  			oom_reaper_list = tsk->oom_reaper_list;

  		}
  		spin_unlock(&oom_reaper_lock);
  
  		if (tsk)
  			oom_reap_task(tsk);

  	}
  
  	return 0;
  }

  static void wake_oom_reaper(struct task_struct *tsk)

  {

  	/* mm is already queued? */
  	if (test_and_set_bit(MMF_OOM_REAP_QUEUED, &tsk->signal->oom_mm->flags))

  		return;

  	get_task_struct(tsk);

  	spin_lock(&oom_reaper_lock);

  	tsk->oom_reaper_list = oom_reaper_list;
  	oom_reaper_list = tsk;

  	spin_unlock(&oom_reaper_lock);

  	trace_wake_reaper(tsk->pid);

  	wake_up(&oom_reaper_wait);

  }
  
  static int __init oom_init(void)
  {
  	oom_reaper_th = kthread_run(oom_reaper, NULL, "oom_reaper");

  	return 0;
  }
  subsys_initcall(oom_init)

  #else
  static inline void wake_oom_reaper(struct task_struct *tsk)
  {
  }
  #endif /* CONFIG_MMU */

  /**

   * mark_oom_victim - mark the given task as OOM victim

   * @tsk: task to mark

   *

   * Has to be called with oom_lock held and never after

   * oom has been disabled already.

   *
   * tsk->mm has to be non NULL and caller has to guarantee it is stable (either
   * under task_lock or operate on the current).

   */

  static void mark_oom_victim(struct task_struct *tsk)

  {

  	struct mm_struct *mm = tsk->mm;

  	WARN_ON(oom_killer_disabled);
  	/* OOM killer might race with memcg OOM */
  	if (test_and_set_tsk_thread_flag(tsk, TIF_MEMDIE))
  		return;

  	/* oom_mm is bound to the signal struct life time. */

  	if (!cmpxchg(&tsk->signal->oom_mm, NULL, mm)) {

  		mmgrab(tsk->signal->oom_mm);

  		set_bit(MMF_OOM_VICTIM, &mm->flags);
  	}

  	/*
  	 * Make sure that the task is woken up from uninterruptible sleep
  	 * if it is frozen because OOM killer wouldn't be able to free
  	 * any memory and livelock. freezing_slow_path will tell the freezer
  	 * that TIF_MEMDIE tasks should be ignored.
  	 */
  	__thaw_task(tsk);

  	atomic_inc(&oom_victims);

  	trace_mark_victim(tsk->pid);

  }
  
  /**

   * exit_oom_victim - note the exit of an OOM victim

   */

  void exit_oom_victim(void)

  {

  	clear_thread_flag(TIF_MEMDIE);

  	if (!atomic_dec_return(&oom_victims))

  		wake_up_all(&oom_victims_wait);

  }
  
  /**

   * oom_killer_enable - enable OOM killer
   */
  void oom_killer_enable(void)
  {
  	oom_killer_disabled = false;

  	pr_info("OOM killer enabled.
  ");

  }
  
  /**

   * oom_killer_disable - disable OOM killer

   * @timeout: maximum timeout to wait for oom victims in jiffies

   *
   * Forces all page allocations to fail rather than trigger OOM killer.

   * Will block and wait until all OOM victims are killed or the given
   * timeout expires.

   *
   * The function cannot be called when there are runnable user tasks because
   * the userspace would see unexpected allocation failures as a result. Any
   * new usage of this function should be consulted with MM people.
   *
   * Returns true if successful and false if the OOM killer cannot be
   * disabled.
   */

  bool oom_killer_disable(signed long timeout)

  {

  	signed long ret;

  	/*

  	 * Make sure to not race with an ongoing OOM killer. Check that the
  	 * current is not killed (possibly due to sharing the victim's memory).

  	 */

  	if (mutex_lock_killable(&oom_lock))

  		return false;

  	oom_killer_disabled = true;

  	mutex_unlock(&oom_lock);

  	ret = wait_event_interruptible_timeout(oom_victims_wait,
  			!atomic_read(&oom_victims), timeout);
  	if (ret <= 0) {
  		oom_killer_enable();
  		return false;
  	}

  	pr_info("OOM killer disabled.
  ");

  
  	return true;
  }

  static inline bool __task_will_free_mem(struct task_struct *task)
  {
  	struct signal_struct *sig = task->signal;
  
  	/*
  	 * A coredumping process may sleep for an extended period in exit_mm(),
  	 * so the oom killer cannot assume that the process will promptly exit
  	 * and release memory.
  	 */
  	if (sig->flags & SIGNAL_GROUP_COREDUMP)
  		return false;
  
  	if (sig->flags & SIGNAL_GROUP_EXIT)
  		return true;
  
  	if (thread_group_empty(task) && (task->flags & PF_EXITING))
  		return true;
  
  	return false;
  }
  
  /*
   * Checks whether the given task is dying or exiting and likely to
   * release its address space. This means that all threads and processes
   * sharing the same mm have to be killed or exiting.

   * Caller has to make sure that task->mm is stable (hold task_lock or
   * it operates on the current).

   */

  static bool task_will_free_mem(struct task_struct *task)

  {

  	struct mm_struct *mm = task->mm;

  	struct task_struct *p;

  	bool ret = true;

  	/*

  	 * Skip tasks without mm because it might have passed its exit_mm and
  	 * exit_oom_victim. oom_reaper could have rescued that but do not rely
  	 * on that for now. We can consider find_lock_task_mm in future.

  	 */

  	if (!mm)

  		return false;

  	if (!__task_will_free_mem(task))
  		return false;

  
  	/*
  	 * This task has already been drained by the oom reaper so there are
  	 * only small chances it will free some more
  	 */

  	if (test_bit(MMF_OOM_SKIP, &mm->flags))

  		return false;

  	if (atomic_read(&mm->mm_users) <= 1)

  		return true;

  
  	/*

  	 * Make sure that all tasks which share the mm with the given tasks
  	 * are dying as well to make sure that a) nobody pins its mm and
  	 * b) the task is also reapable by the oom reaper.

  	 */
  	rcu_read_lock();
  	for_each_process(p) {
  		if (!process_shares_mm(p, mm))
  			continue;
  		if (same_thread_group(task, p))
  			continue;
  		ret = __task_will_free_mem(p);
  		if (!ret)
  			break;
  	}
  	rcu_read_unlock();

  
  	return ret;
  }

  static void __oom_kill_process(struct task_struct *victim)

  {

  	struct task_struct *p;

  	struct mm_struct *mm;

  	bool can_oom_reap = true;

  	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;
  	}

  	/* Get a reference to safely compare mm after task_unlock(victim) */

  	mm = victim->mm;

  	mmgrab(mm);

  
  	/* Raise event before sending signal: task reaper must see this */
  	count_vm_event(OOM_KILL);

  	memcg_memory_event_mm(mm, MEMCG_OOM_KILL);

  	/*

  	 * We should send SIGKILL before granting access to memory reserves
  	 * in order to prevent the OOM victim from depleting the memory
  	 * reserves from the user space under its control.

  	 */

  	do_send_sig_info(SIGKILL, SEND_SIG_FORCED, victim, PIDTYPE_TGID);

  	mark_oom_victim(victim);

  	pr_err("Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB, shmem-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)),
  		K(get_mm_counter(victim->mm, MM_SHMEMPAGES)));

  	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 (!process_shares_mm(p, mm))

  			continue;
  		if (same_thread_group(p, victim))
  			continue;

  		if (is_global_init(p)) {

  			can_oom_reap = false;

  			set_bit(MMF_OOM_SKIP, &mm->flags);

  			pr_info("oom killer %d (%s) has mm pinned by %d (%s)
  ",
  					task_pid_nr(victim), victim->comm,
  					task_pid_nr(p), p->comm);

  			continue;

  		}

  		/*
  		 * No use_mm() user needs to read from the userspace so we are
  		 * ok to reap it.
  		 */
  		if (unlikely(p->flags & PF_KTHREAD))
  			continue;

  		do_send_sig_info(SIGKILL, SEND_SIG_FORCED, p, PIDTYPE_TGID);

  	}

  	rcu_read_unlock();

  	if (can_oom_reap)

  		wake_oom_reaper(victim);

  	mmdrop(mm);

  	put_task_struct(victim);

  }

  #undef K

  /*

   * Kill provided task unless it's secured by setting
   * oom_score_adj to OOM_SCORE_ADJ_MIN.
   */
  static int oom_kill_memcg_member(struct task_struct *task, void *unused)
  {

  	if (task->signal->oom_score_adj != OOM_SCORE_ADJ_MIN &&
  	    !is_global_init(task)) {

  		get_task_struct(task);
  		__oom_kill_process(task);
  	}
  	return 0;
  }

  static void oom_kill_process(struct oom_control *oc, const char *message)
  {
  	struct task_struct *p = oc->chosen;
  	unsigned int points = oc->chosen_points;
  	struct task_struct *victim = p;
  	struct task_struct *child;
  	struct task_struct *t;

  	struct mem_cgroup *oom_group;

  	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 give it access to memory reserves
  	 * so it can die quickly
  	 */
  	task_lock(p);
  	if (task_will_free_mem(p)) {
  		mark_oom_victim(p);
  		wake_oom_reaper(p);
  		task_unlock(p);
  		put_task_struct(p);
  		return;
  	}
  	task_unlock(p);
  
  	if (__ratelimit(&oom_rs))
  		dump_header(oc, p);
  
  	pr_err("%s: Kill process %d (%s) score %u or sacrifice child
  ",
  		message, task_pid_nr(p), p->comm, points);
  
  	/*
  	 * 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);

  
  	/*
  	 * The task 'p' might have already exited before reaching here. The
  	 * put_task_struct() will free task_struct 'p' while the loop still try
  	 * to access the field of 'p', so, get an extra reference.
  	 */
  	get_task_struct(p);

  	for_each_thread(p, t) {
  		list_for_each_entry(child, &t->children, sibling) {
  			unsigned int child_points;
  
  			if (process_shares_mm(child, p->mm))
  				continue;
  			/*
  			 * oom_badness() returns 0 if the thread is unkillable
  			 */
  			child_points = oom_badness(child,
  				oc->memcg, oc->nodemask, oc->totalpages);
  			if (child_points > victim_points) {
  				put_task_struct(victim);
  				victim = child;
  				victim_points = child_points;
  				get_task_struct(victim);
  			}
  		}
  	}

  	put_task_struct(p);

  	read_unlock(&tasklist_lock);

  	/*
  	 * Do we need to kill the entire memory cgroup?
  	 * Or even one of the ancestor memory cgroups?
  	 * Check this out before killing the victim task.
  	 */
  	oom_group = mem_cgroup_get_oom_group(victim, oc->memcg);

  	__oom_kill_process(victim);

  
  	/*
  	 * If necessary, kill all tasks in the selected memory cgroup.
  	 */
  	if (oom_group) {
  		mem_cgroup_print_oom_group(oom_group);
  		mem_cgroup_scan_tasks(oom_group, oom_kill_memcg_member, NULL);
  		mem_cgroup_put(oom_group);
  	}

  }

  /*
   * Determines whether the kernel must panic because of the panic_on_oom sysctl.
   */

  static void check_panic_on_oom(struct oom_control *oc,
  			       enum oom_constraint constraint)

  {
  	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;
  	}

  	/* Do not panic for oom kills triggered by sysrq */

  	if (is_sysrq_oom(oc))

  		return;

  	dump_header(oc, NULL);

  	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);

  /**

   * out_of_memory - kill the "best" process when we run out of memory
   * @oc: pointer to struct oom_control

   *
   * 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.
   */

  bool out_of_memory(struct oom_control *oc)

  {

  	unsigned long freed = 0;

  	enum oom_constraint constraint = CONSTRAINT_NONE;

  	if (oom_killer_disabled)
  		return false;

  	if (!is_memcg_oom(oc)) {
  		blocking_notifier_call_chain(&oom_notify_list, 0, &freed);
  		if (freed > 0)
  			/* Got some memory back in the last second. */
  			return true;
  	}

  	/*

  	 * 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 (task_will_free_mem(current)) {

  		mark_oom_victim(current);

  		wake_oom_reaper(current);

  		return true;

  	}

  	/*

  	 * The OOM killer does not compensate for IO-less reclaim.
  	 * pagefault_out_of_memory lost its gfp context so we have to
  	 * make sure exclude 0 mask - all other users should have at least
  	 * ___GFP_DIRECT_RECLAIM to get here.
  	 */

  	if (oc->gfp_mask && !(oc->gfp_mask & __GFP_FS))

  		return true;
  
  	/*

  	 * Check if there were limitations on the allocation (only relevant for

  	 * NUMA and memcg) that may require different handling.

  	 */

  	constraint = constrained_alloc(oc);

  	if (constraint != CONSTRAINT_MEMORY_POLICY)
  		oc->nodemask = NULL;

  	check_panic_on_oom(oc, constraint);

  	if (!is_memcg_oom(oc) && sysctl_oom_kill_allocating_task &&
  	    current->mm && !oom_unkillable_task(current, NULL, oc->nodemask) &&

  	    current->signal->oom_score_adj != OOM_SCORE_ADJ_MIN) {

  		get_task_struct(current);

  		oc->chosen = current;
  		oom_kill_process(oc, "Out of memory (oom_kill_allocating_task)");

  		return true;

  	}

  	select_bad_process(oc);

  	/* Found nothing?!?! */
  	if (!oc->chosen) {

  		dump_header(oc, NULL);

  		pr_warn("Out of memory and no killable processes...
  ");
  		/*
  		 * If we got here due to an actual allocation at the
  		 * system level, we cannot survive this and will enter
  		 * an endless loop in the allocator. Bail out now.
  		 */
  		if (!is_sysrq_oom(oc) && !is_memcg_oom(oc))
  			panic("System is deadlocked on memory
  ");

  	}

  	if (oc->chosen && oc->chosen != (void *)-1UL)

  		oom_kill_process(oc, !is_memcg_oom(oc) ? "Out of memory" :
  				 "Memory cgroup out of memory");

  	return !!oc->chosen;

  }

  /*
   * The pagefault handler calls here because it is out of memory, so kill a

   * memory-hogging task. If oom_lock is held by somebody else, a parallel oom
   * killing is already in progress so do nothing.

   */
  void pagefault_out_of_memory(void)
  {

  	struct oom_control oc = {
  		.zonelist = NULL,
  		.nodemask = NULL,

  		.memcg = NULL,

  		.gfp_mask = 0,
  		.order = 0,

  	};

  	if (mem_cgroup_oom_synchronize(true))

  		return;

  	if (!mutex_trylock(&oom_lock))
  		return;

  	out_of_memory(&oc);

  	mutex_unlock(&oom_lock);

  }