oom: clear TIF_MEMDIE after oom_reaper managed to unmap the address space

[deliverable/linux.git] / mm / oom_kill.c

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

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

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;
}

/* 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;
}

/**
 * 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) + get_mm_counter(p->mm, MM_SWAPENTS) +
                atomic_long_read(&p->mm->nr_ptes) + mm_nr_pmds(p->mm);
        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 oom_control *oc,
                                             unsigned long *totalpages)
{
        struct zone *zone;
        struct zoneref *z;
        enum zone_type high_zoneidx = gfp_zone(oc->gfp_mask);
        bool cpuset_limited = false;
        int nid;

        /* Default to all available memory */
        *totalpages = totalram_pages + total_swap_pages;

        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)) {
                *totalpages = total_swap_pages;
                for_each_node_mask(nid, *oc->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, oc->zonelist,
                        high_zoneidx, oc->nodemask)
                if (!cpuset_zone_allowed(zone, oc->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 oom_control *oc,
                                             unsigned long *totalpages)
{
        *totalpages = totalram_pages + total_swap_pages;
        return CONSTRAINT_NONE;
}
#endif

enum oom_scan_t oom_scan_process_thread(struct oom_control *oc,
                        struct task_struct *task, unsigned long totalpages)
{
        if (oom_unkillable_task(task, NULL, oc->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 (!is_sysrq_oom(oc))
                        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;

        return OOM_SCAN_OK;
}

/*
 * Simple selection loop. We chose the process with the highest
 * number of 'points'.  Returns -1 on scan abort.
 */
static struct task_struct *select_bad_process(struct oom_control *oc,
                unsigned int *ppoints, unsigned long totalpages)
{
        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(oc, p, totalpages)) {
                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, oc->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(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 nr_pmds swapents oom_score_adj name\n");
        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 %7ld %8lu         %5hd %s\n",
                        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),
                        mm_nr_pmds(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,
                        struct mem_cgroup *memcg)
{
        pr_warn("%s invoked oom-killer: gfp_mask=%#x(%pGg), order=%d, oom_score_adj=%hd\n",
                current->comm, oc->gfp_mask, &oc->gfp_mask, oc->order,
                current->signal->oom_score_adj);

        cpuset_print_current_mems_allowed();
        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, oc->nodemask);
}

/*
 * Number of OOM victims in flight
 */
static atomic_t oom_victims = ATOMIC_INIT(0);
static DECLARE_WAIT_QUEUE_HEAD(oom_victims_wait);

bool oom_killer_disabled __read_mostly;

#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 struct task_struct *task_to_reap;
static DECLARE_WAIT_QUEUE_HEAD(oom_reaper_wait);

static bool __oom_reap_task(struct task_struct *tsk)
{
        struct mmu_gather tlb;
        struct vm_area_struct *vma;
        struct mm_struct *mm;
        struct task_struct *p;
        struct zap_details details = {.check_swap_entries = true,
                                      .ignore_dirty = true};
        bool ret = true;

        /*
         * Make sure we find the associated mm_struct even when the particular
         * thread has already terminated and cleared its mm.
         * We might have race with exit path so consider our work done if there
         * is no mm.
         */
        p = find_lock_task_mm(tsk);
        if (!p)
                return true;

        mm = p->mm;
        if (!atomic_inc_not_zero(&mm->mm_users)) {
                task_unlock(p);
                return true;
        }

        task_unlock(p);

        if (!down_read_trylock(&mm->mmap_sem)) {
                ret = false;
                goto out;
        }

        tlb_gather_mmu(&tlb, mm, 0, -1);
        for (vma = mm->mmap ; vma; vma = vma->vm_next) {
                if (is_vm_hugetlb_page(vma))
                        continue;

                /*
                 * mlocked VMAs require explicit munlocking before unmap.
                 * Let's keep it simple here and skip such VMAs.
                 */
                if (vma->vm_flags & VM_LOCKED)
                        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))
                        unmap_page_range(&tlb, vma, vma->vm_start, vma->vm_end,
                                         &details);
        }
        tlb_finish_mmu(&tlb, 0, -1);
        up_read(&mm->mmap_sem);

        /*
         * Clear TIF_MEMDIE because the task shouldn't be sitting on a
         * reasonably reclaimable memory anymore. OOM killer can continue
         * by selecting other victim if unmapping hasn't led to any
         * improvements. This also means that selecting this task doesn't
         * make any sense.
         */
        tsk->signal->oom_score_adj = OOM_SCORE_ADJ_MIN;
        exit_oom_victim(tsk);
out:
        mmput(mm);
        return ret;
}

static void oom_reap_task(struct task_struct *tsk)
{
        int attempts = 0;

        /* Retry the down_read_trylock(mmap_sem) a few times */
        while (attempts++ < 10 && !__oom_reap_task(tsk))
                schedule_timeout_idle(HZ/10);

        /* Drop a reference taken by wake_oom_reaper */
        put_task_struct(tsk);
}

static int oom_reaper(void *unused)
{
        while (true) {
                struct task_struct *tsk;

                wait_event_freezable(oom_reaper_wait,
                                     (tsk = READ_ONCE(task_to_reap)));
                oom_reap_task(tsk);
                WRITE_ONCE(task_to_reap, NULL);
        }

        return 0;
}

static void wake_oom_reaper(struct task_struct *tsk)
{
        struct task_struct *old_tsk;

        if (!oom_reaper_th)
                return;

        get_task_struct(tsk);

        /*
         * Make sure that only a single mm is ever queued for the reaper
         * because multiple are not necessary and the operation might be
         * disruptive so better reduce it to the bare minimum.
         */
        old_tsk = cmpxchg(&task_to_reap, NULL, tsk);
        if (!old_tsk)
                wake_up(&oom_reaper_wait);
        else
                put_task_struct(tsk);
}

static int __init oom_init(void)
{
        oom_reaper_th = kthread_run(oom_reaper, NULL, "oom_reaper");
        if (IS_ERR(oom_reaper_th)) {
                pr_err("Unable to start OOM reaper %ld. Continuing regardless\n",
                                PTR_ERR(oom_reaper_th));
                oom_reaper_th = NULL;
        }
        return 0;
}
subsys_initcall(oom_init)
#else
static void wake_oom_reaper(struct task_struct *tsk)
{
}
#endif

/**
 * 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.
 */
void mark_oom_victim(struct task_struct *tsk)
{
        WARN_ON(oom_killer_disabled);
        /* OOM killer might race with memcg OOM */
        if (test_and_set_tsk_thread_flag(tsk, TIF_MEMDIE))
                return;
        /*
         * 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);
}

/**
 * exit_oom_victim - note the exit of an OOM victim
 */
void exit_oom_victim(struct task_struct *tsk)
{
        if (!test_and_clear_tsk_thread_flag(tsk, TIF_MEMDIE))
                return;

        if (!atomic_dec_return(&oom_victims))
                wake_up_all(&oom_victims_wait);
}

/**
 * oom_killer_disable - disable OOM killer
 *
 * Forces all page allocations to fail rather than trigger OOM killer.
 * Will block and wait until all OOM victims are killed.
 *
 * 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(void)
{
        /*
         * 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);

        wait_event(oom_victims_wait, !atomic_read(&oom_victims));

        return true;
}

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

/*
 * 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.
 */
static 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;
}

#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 oom_control *oc, struct task_struct *p,
                      unsigned int points, unsigned long totalpages,
                      struct mem_cgroup *memcg, 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);
        bool can_oom_reap = true;

        /*
         * 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
         */
        task_lock(p);
        if (p->mm && task_will_free_mem(p)) {
                mark_oom_victim(p);
                task_unlock(p);
                put_task_struct(p);
                return;
        }
        task_unlock(p);

        if (__ratelimit(&oom_rs))
                dump_header(oc, p, memcg);

        pr_err("%s: Kill process %d (%s) score %u or sacrifice child\n",
                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);
        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, memcg, oc->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;
        }

        /* Get a reference to safely compare mm after task_unlock(victim) */
        mm = victim->mm;
        atomic_inc(&mm->mm_count);
        /*
         * We should send SIGKILL before setting TIF_MEMDIE 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, true);
        mark_oom_victim(victim);
        pr_err("Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB, shmem-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)),
                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 (unlikely(p->flags & PF_KTHREAD) || is_global_init(p) ||
                    p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN) {
                        /*
                         * We cannot use oom_reaper for the mm shared by this
                         * process because it wouldn't get killed and so the
                         * memory might be still used.
                         */
                        can_oom_reap = false;
                        continue;
                }
                do_send_sig_info(SIGKILL, SEND_SIG_FORCED, p, true);
        }
        rcu_read_unlock();

        if (can_oom_reap)
                wake_oom_reaper(victim);

        mmdrop(mm);
        put_task_struct(victim);
}
#undef K

/*
 * Determines whether the kernel must panic because of the panic_on_oom sysctl.
 */
void check_panic_on_oom(struct oom_control *oc, enum oom_constraint constraint,
                        struct mem_cgroup *memcg)
{
        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, memcg);
        panic("Out of memory: %s panic_on_oom is enabled\n",
                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)
{
        struct task_struct *p;
        unsigned long totalpages;
        unsigned long freed = 0;
        unsigned int uninitialized_var(points);
        enum oom_constraint constraint = CONSTRAINT_NONE;

        if (oom_killer_disabled)
                return false;

        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.
         *
         * But don't select if current has already released its mm and cleared
         * TIF_MEMDIE flag at exit_mm(), otherwise an OOM livelock may occur.
         */
        if (current->mm &&
            (fatal_signal_pending(current) || task_will_free_mem(current))) {
                mark_oom_victim(current);
                return true;
        }

        /*
         * Check if there were limitations on the allocation (only relevant for
         * NUMA) that may require different handling.
         */
        constraint = constrained_alloc(oc, &totalpages);
        if (constraint != CONSTRAINT_MEMORY_POLICY)
                oc->nodemask = NULL;
        check_panic_on_oom(oc, constraint, NULL);

        if (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);
                oom_kill_process(oc, current, 0, totalpages, NULL,
                                 "Out of memory (oom_kill_allocating_task)");
                return true;
        }

        p = select_bad_process(oc, &points, totalpages);
        /* Found nothing?!?! Either we hang forever, or we panic. */
        if (!p && !is_sysrq_oom(oc)) {
                dump_header(oc, NULL, NULL);
                panic("Out of memory and no killable processes...\n");
        }
        if (p && p != (void *)-1UL) {
                oom_kill_process(oc, p, points, totalpages, NULL,
                                 "Out of memory");
                /*
                 * Give the killed process a good chance to exit before trying
                 * to allocate memory again.
                 */
                schedule_timeout_killable(1);
        }
        return true;
}

/*
 * 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 oom_control oc = {
                .zonelist = NULL,
                .nodemask = NULL,
                .gfp_mask = 0,
                .order = 0,
        };

        if (mem_cgroup_oom_synchronize(true))
                return;

        if (!mutex_trylock(&oom_lock))
                return;

        if (!out_of_memory(&oc)) {
                /*
                 * There shouldn't be any user tasks runnable while the
                 * OOM killer is disabled, so the current task has to
                 * be a racing OOM victim for which oom_killer_disable()
                 * is waiting for.
                 */
                WARN_ON(test_thread_flag(TIF_MEMDIE));
        }

        mutex_unlock(&oom_lock);
}