Add linux-next specific files for 20110716

[linux-2.6/next.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/module.h>
#include <linux/notifier.h>
#include <linux/memcontrol.h>
#include <linux/mempolicy.h>
#include <linux/security.h>
#include <linux/ptrace.h>

int sysctl_panic_on_oom;
int sysctl_oom_kill_allocating_task;
int sysctl_oom_dump_tasks = 1;
static DEFINE_SPINLOCK(zone_scan_lock);

/**
 * test_set_oom_score_adj() - set current's oom_score_adj and return old value
 * @new_val: new oom_score_adj value
 *
 * Sets the oom_score_adj value for current to @new_val with proper
 * synchronization and returns the old value.  Usually used to temporarily
 * set a value, save the old value in the caller, and then reinstate it later.
 */
int test_set_oom_score_adj(int new_val)
{
        struct sighand_struct *sighand = current->sighand;
        int old_val;

        spin_lock_irq(&sighand->siglock);
        old_val = current->signal->oom_score_adj;
        if (new_val != old_val) {
                if (new_val == OOM_SCORE_ADJ_MIN)
                        atomic_inc(&current->mm->oom_disable_count);
                else if (old_val == OOM_SCORE_ADJ_MIN)
                        atomic_dec(&current->mm->oom_disable_count);
                current->signal->oom_score_adj = new_val;
        }
        spin_unlock_irq(&sighand->siglock);

        return old_val;
}

#ifdef CONFIG_NUMA
/**
 * has_intersects_mems_allowed() - check task eligiblity for kill
 * @tsk: 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 *tsk,
                                        const nodemask_t *mask)
{
        struct task_struct *start = tsk;

        do {
                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.
                         */
                        if (mempolicy_nodemask_intersects(tsk, mask))
                                return true;
                } else {
                        /*
                         * This is not a mempolicy constrained oom, so only
                         * check the mems of tsk's cpuset.
                         */
                        if (cpuset_mems_allowed_intersects(current, tsk))
                                return true;
                }
        } while_each_thread(start, tsk);

        return false;
}
#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 = p;

        do {
                task_lock(t);
                if (likely(t->mm))
                        return t;
                task_unlock(t);
        } while_each_thread(p, t);

        return NULL;
}

/* 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 *mem, 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 (mem && !task_in_mem_cgroup(p, mem))
                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 int oom_badness(struct task_struct *p, struct mem_cgroup *mem,
                      const nodemask_t *nodemask, unsigned long totalpages)
{
        int points;

        if (oom_unkillable_task(p, mem, nodemask))
                return 0;

        p = find_lock_task_mm(p);
        if (!p)
                return 0;

        /*
         * Shortcut check for a thread sharing p->mm that is OOM_SCORE_ADJ_MIN
         * so the entire heuristic doesn't need to be executed for something
         * that cannot be killed.
         */
        if (atomic_read(&p->mm->oom_disable_count)) {
                task_unlock(p);
                return 0;
        }

        /*
         * The memory controller may have a limit of 0 bytes, so avoid a divide
         * by zero, if necessary.
         */
        if (!totalpages)
                totalpages = 1;

        /*
         * 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) + p->mm->nr_ptes;
        points += get_mm_counter(p->mm, MM_SWAPENTS);

        points *= 1000;
        points /= totalpages;
        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 -= 30;

        /*
         * /proc/pid/oom_score_adj ranges from -1000 to +1000 such that it may
         * either completely disable oom killing or always prefer a certain
         * task.
         */
        points += p->signal->oom_score_adj;

        /*
         * Never return 0 for an eligible task that may be killed since it's
         * possible that no single user task uses more than 0.1% of memory and
         * no single admin tasks uses more than 3.0%.
         */
        if (points <= 0)
                return 1;
        return (points < 1000) ? points : 1000;
}

/*
 * 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_HIGH_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

/*
 * Simple selection loop. We chose the process with the highest
 * number of 'points'. We expect the caller will lock the tasklist.
 *
 * (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, struct mem_cgroup *mem,
                const nodemask_t *nodemask)
{
        struct task_struct *g, *p;
        struct task_struct *chosen = NULL;
        *ppoints = 0;

        do_each_thread(g, p) {
                unsigned int points;

                if (!p->mm)
                        continue;
                if (oom_unkillable_task(p, mem, nodemask))
                        continue;

                /*
                 * This task already has access to memory reserves and is
                 * being killed. Don't allow any other task access to the
                 * memory reserve.
                 *
                 * Note: this may have a chance of deadlock if it gets
                 * blocked waiting for another task which itself is waiting
                 * for memory. Is there a better alternative?
                 */
                if (test_tsk_thread_flag(p, TIF_MEMDIE))
                        return ERR_PTR(-1UL);

                if (p->flags & PF_EXITING) {
                        /*
                         * If p is the current task and is in the process of
                         * releasing memory, we allow the "kill" to set
                         * TIF_MEMDIE, which will allow it to gain access to
                         * memory reserves.  Otherwise, it may stall forever.
                         *
                         * The loop isn't broken here, however, in case other
                         * threads are found to have already been oom killed.
                         */
                        if (p == current) {
                                chosen = p;
                                *ppoints = 1000;
                        } else {
                                /*
                                 * If this task is not being ptraced on exit,
                                 * then wait for it to finish before killing
                                 * some other task unnecessarily.
                                 */
                                if (!(p->group_leader->ptrace & PT_TRACE_EXIT))
                                        return ERR_PTR(-1UL);
                        }
                }

                points = oom_badness(p, mem, nodemask, totalpages);
                if (points > *ppoints) {
                        chosen = p;
                        *ppoints = points;
                }
        } while_each_thread(g, p);

        return chosen;
}

/**
 * dump_tasks - dump current memory state of all system tasks
 * @mem: 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, cpu, oom_adj
 * value, oom_score_adj value, and name.
 *
 * Call with tasklist_lock read-locked.
 */
static void dump_tasks(const struct mem_cgroup *mem, const nodemask_t *nodemask)
{
        struct task_struct *p;
        struct task_struct *task;

        pr_info("[ pid ]   uid  tgid total_vm      rss cpu oom_adj oom_score_adj name\n");
        for_each_process(p) {
                if (oom_unkillable_task(p, mem, 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 %3u     %3d         %5d %s\n",
                        task->pid, task_uid(task), task->tgid,
                        task->mm->total_vm, get_mm_rss(task->mm),
                        task_cpu(task), task->signal->oom_adj,
                        task->signal->oom_score_adj, task->comm);
                task_unlock(task);
        }
}

static void dump_header(struct task_struct *p, gfp_t gfp_mask, int order,
                        struct mem_cgroup *mem, const nodemask_t *nodemask)
{
        task_lock(current);
        pr_warning("%s invoked oom-killer: gfp_mask=0x%x, order=%d, "
                "oom_adj=%d, oom_score_adj=%d\n",
                current->comm, gfp_mask, order, current->signal->oom_adj,
                current->signal->oom_score_adj);
        cpuset_print_task_mems_allowed(current);
        task_unlock(current);
        dump_stack();
        mem_cgroup_print_oom_info(mem, p);
        show_mem(SHOW_MEM_FILTER_NODES);
        if (sysctl_oom_dump_tasks)
                dump_tasks(mem, nodemask);
}

#define K(x) ((x) << (PAGE_SHIFT-10))
static int oom_kill_task(struct task_struct *p, struct mem_cgroup *mem)
{
        struct task_struct *q;
        struct mm_struct *mm;

        p = find_lock_task_mm(p);
        if (!p)
                return 1;

        /* mm cannot be safely dereferenced after task_unlock(p) */
        mm = p->mm;

        pr_err("Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB\n",
                task_pid_nr(p), p->comm, K(p->mm->total_vm),
                K(get_mm_counter(p->mm, MM_ANONPAGES)),
                K(get_mm_counter(p->mm, MM_FILEPAGES)));
        task_unlock(p);

        /*
         * Kill all processes sharing p->mm in other thread groups, if any.
         * They don't get access to memory reserves or a higher scheduler
         * priority, though, to avoid depletion of all memory or task
         * starvation.  This prevents mm->mmap_sem livelock when an oom killed
         * task 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.
         */
        for_each_process(q)
                if (q->mm == mm && !same_thread_group(q, p)) {
                        task_lock(q);   /* Protect ->comm from prctl() */
                        pr_err("Kill process %d (%s) sharing same memory\n",
                                task_pid_nr(q), q->comm);
                        task_unlock(q);
                        force_sig(SIGKILL, q);
                }

        set_tsk_thread_flag(p, TIF_MEMDIE);
        force_sig(SIGKILL, p);

        return 0;
}
#undef K

static int oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
                            unsigned int points, unsigned long totalpages,
                            struct mem_cgroup *mem, nodemask_t *nodemask,
                            const char *message)
{
        struct task_struct *victim = p;
        struct task_struct *child;
        struct task_struct *t = p;
        unsigned int victim_points = 0;

        if (printk_ratelimit())
                dump_header(p, gfp_mask, order, mem, nodemask);

        /*
         * 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);
                return 0;
        }

        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);

        /*
         * If any of p's children has a different mm and is eligible for kill,
         * the one with the highest badness() score is sacrificed for its
         * parent.  This attempts to lose the minimal amount of work done while
         * still freeing memory.
         */
        do {
                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, mem, nodemask,
                                                                totalpages);
                        if (child_points > victim_points) {
                                victim = child;
                                victim_points = child_points;
                        }
                }
        } while_each_thread(p, t);

        return oom_kill_task(victim, mem);
}

/*
 * Determines whether the kernel must panic because of the panic_on_oom sysctl.
 */
static 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;
        }
        read_lock(&tasklist_lock);
        dump_header(NULL, gfp_mask, order, NULL, nodemask);
        read_unlock(&tasklist_lock);
        panic("Out of memory: %s panic_on_oom is enabled\n",
                sysctl_panic_on_oom == 2 ? "compulsory" : "system-wide");
}

#ifdef CONFIG_CGROUP_MEM_RES_CTLR
void mem_cgroup_out_of_memory(struct mem_cgroup *mem, gfp_t gfp_mask)
{
        unsigned long limit;
        unsigned int points = 0;
        struct task_struct *p;

        /*
         * If current has a pending SIGKILL, 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)) {
                set_thread_flag(TIF_MEMDIE);
                return;
        }

        check_panic_on_oom(CONSTRAINT_MEMCG, gfp_mask, 0, NULL);
        limit = mem_cgroup_get_limit(mem) >> PAGE_SHIFT;
        read_lock(&tasklist_lock);
retry:
        p = select_bad_process(&points, limit, mem, NULL);
        if (!p || PTR_ERR(p) == -1UL)
                goto out;

        if (oom_kill_process(p, gfp_mask, 0, points, limit, mem, NULL,
                                "Memory cgroup out of memory"))
                goto retry;
out:
        read_unlock(&tasklist_lock);
}
#endif

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);
}

/*
 * Try to acquire the oom killer lock for all system zones.  Returns zero if a
 * parallel oom killing is taking place, otherwise locks all zones and returns
 * non-zero.
 */
static int try_set_system_oom(void)
{
        struct zone *zone;
        int ret = 1;

        spin_lock(&zone_scan_lock);
        for_each_populated_zone(zone)
                if (zone_is_oom_locked(zone)) {
                        ret = 0;
                        goto out;
                }
        for_each_populated_zone(zone)
                zone_set_flag(zone, ZONE_OOM_LOCKED);
out:
        spin_unlock(&zone_scan_lock);
        return ret;
}

/*
 * Clears ZONE_OOM_LOCKED for all system zones so that failed allocation
 * attempts or page faults may now recall the oom killer, if necessary.
 */
static void clear_system_oom(void)
{
        struct zone *zone;

        spin_lock(&zone_scan_lock);
        for_each_populated_zone(zone)
                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
 *
 * 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)
{
        const nodemask_t *mpol_mask;
        struct task_struct *p;
        unsigned long totalpages;
        unsigned long freed = 0;
        unsigned int 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, 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)) {
                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);

        read_lock(&tasklist_lock);
        if (sysctl_oom_kill_allocating_task &&
            !oom_unkillable_task(current, NULL, nodemask) &&
            current->mm && !atomic_read(&current->mm->oom_disable_count)) {
                /*
                 * oom_kill_process() needs tasklist_lock held.  If it returns
                 * non-zero, current could not be killed so we must fallback to
                 * the tasklist scan.
                 */
                if (!oom_kill_process(current, gfp_mask, order, 0, totalpages,
                                NULL, nodemask,
                                "Out of memory (oom_kill_allocating_task)"))
                        goto out;
        }

retry:
        p = select_bad_process(&points, totalpages, NULL, mpol_mask);
        if (PTR_ERR(p) == -1UL)
                goto out;

        /* Found nothing?!?! Either we hang forever, or we panic. */
        if (!p) {
                dump_header(NULL, gfp_mask, order, NULL, mpol_mask);
                read_unlock(&tasklist_lock);
                panic("Out of memory and no killable processes...\n");
        }

        if (oom_kill_process(p, gfp_mask, order, points, totalpages, NULL,
                                nodemask, "Out of memory"))
                goto retry;
        killed = 1;
out:
        read_unlock(&tasklist_lock);

        /*
         * Give "p" a good chance of killing itself before we
         * retry to allocate memory unless "p" is current
         */
        if (killed && !test_thread_flag(TIF_MEMDIE))
                schedule_timeout_uninterruptible(1);
}

/*
 * The pagefault handler calls here because it is out of memory, so kill a
 * memory-hogging task.  If a populated zone has ZONE_OOM_LOCKED set, a parallel
 * oom killing is already in progress so do nothing.  If a task is found with
 * TIF_MEMDIE set, it has been killed so do nothing and allow it to exit.
 */
void pagefault_out_of_memory(void)
{
        if (try_set_system_oom()) {
                out_of_memory(NULL, 0, 0, NULL);
                clear_system_oom();
        }
        if (!test_thread_flag(TIF_MEMDIE))
                schedule_timeout_uninterruptible(1);
}