linux/mm/oom_kill.c
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   1/*
   2 *  linux/mm/oom_kill.c
   3 * 
   4 *  Copyright (C)  1998,2000  Rik van Riel
   5 *      Thanks go out to Claus Fischer for some serious inspiration and
   6 *      for goading me into coding this file...
   7 *  Copyright (C)  2010  Google, Inc.
   8 *      Rewritten by David Rientjes
   9 *
  10 *  The routines in this file are used to kill a process when
  11 *  we're seriously out of memory. This gets called from __alloc_pages()
  12 *  in mm/page_alloc.c when we really run out of memory.
  13 *
  14 *  Since we won't call these routines often (on a well-configured
  15 *  machine) this file will double as a 'coding guide' and a signpost
  16 *  for newbie kernel hackers. It features several pointers to major
  17 *  kernel subsystems and hints as to where to find out what things do.
  18 */
  19
  20#include <linux/oom.h>
  21#include <linux/mm.h>
  22#include <linux/err.h>
  23#include <linux/gfp.h>
  24#include <linux/sched.h>
  25#include <linux/swap.h>
  26#include <linux/timex.h>
  27#include <linux/jiffies.h>
  28#include <linux/cpuset.h>
  29#include <linux/module.h>
  30#include <linux/notifier.h>
  31#include <linux/memcontrol.h>
  32#include <linux/mempolicy.h>
  33#include <linux/security.h>
  34#include <linux/ptrace.h>
  35
  36int sysctl_panic_on_oom;
  37int sysctl_oom_kill_allocating_task;
  38int sysctl_oom_dump_tasks = 1;
  39static DEFINE_SPINLOCK(zone_scan_lock);
  40
  41/**
  42 * test_set_oom_score_adj() - set current's oom_score_adj and return old value
  43 * @new_val: new oom_score_adj value
  44 *
  45 * Sets the oom_score_adj value for current to @new_val with proper
  46 * synchronization and returns the old value.  Usually used to temporarily
  47 * set a value, save the old value in the caller, and then reinstate it later.
  48 */
  49int test_set_oom_score_adj(int new_val)
  50{
  51        struct sighand_struct *sighand = current->sighand;
  52        int old_val;
  53
  54        spin_lock_irq(&sighand->siglock);
  55        old_val = current->signal->oom_score_adj;
  56        if (new_val != old_val) {
  57                if (new_val == OOM_SCORE_ADJ_MIN)
  58                        atomic_inc(&current->mm->oom_disable_count);
  59                else if (old_val == OOM_SCORE_ADJ_MIN)
  60                        atomic_dec(&current->mm->oom_disable_count);
  61                current->signal->oom_score_adj = new_val;
  62        }
  63        spin_unlock_irq(&sighand->siglock);
  64
  65        return old_val;
  66}
  67
  68#ifdef CONFIG_NUMA
  69/**
  70 * has_intersects_mems_allowed() - check task eligiblity for kill
  71 * @tsk: task struct of which task to consider
  72 * @mask: nodemask passed to page allocator for mempolicy ooms
  73 *
  74 * Task eligibility is determined by whether or not a candidate task, @tsk,
  75 * shares the same mempolicy nodes as current if it is bound by such a policy
  76 * and whether or not it has the same set of allowed cpuset nodes.
  77 */
  78static bool has_intersects_mems_allowed(struct task_struct *tsk,
  79                                        const nodemask_t *mask)
  80{
  81        struct task_struct *start = tsk;
  82
  83        do {
  84                if (mask) {
  85                        /*
  86                         * If this is a mempolicy constrained oom, tsk's
  87                         * cpuset is irrelevant.  Only return true if its
  88                         * mempolicy intersects current, otherwise it may be
  89                         * needlessly killed.
  90                         */
  91                        if (mempolicy_nodemask_intersects(tsk, mask))
  92                                return true;
  93                } else {
  94                        /*
  95                         * This is not a mempolicy constrained oom, so only
  96                         * check the mems of tsk's cpuset.
  97                         */
  98                        if (cpuset_mems_allowed_intersects(current, tsk))
  99                                return true;
 100                }
 101        } while_each_thread(start, tsk);
 102
 103        return false;
 104}
 105#else
 106static bool has_intersects_mems_allowed(struct task_struct *tsk,
 107                                        const nodemask_t *mask)
 108{
 109        return true;
 110}
 111#endif /* CONFIG_NUMA */
 112
 113/*
 114 * The process p may have detached its own ->mm while exiting or through
 115 * use_mm(), but one or more of its subthreads may still have a valid
 116 * pointer.  Return p, or any of its subthreads with a valid ->mm, with
 117 * task_lock() held.
 118 */
 119struct task_struct *find_lock_task_mm(struct task_struct *p)
 120{
 121        struct task_struct *t = p;
 122
 123        do {
 124                task_lock(t);
 125                if (likely(t->mm))
 126                        return t;
 127                task_unlock(t);
 128        } while_each_thread(p, t);
 129
 130        return NULL;
 131}
 132
 133/* return true if the task is not adequate as candidate victim task. */
 134static bool oom_unkillable_task(struct task_struct *p,
 135                const struct mem_cgroup *mem, const nodemask_t *nodemask)
 136{
 137        if (is_global_init(p))
 138                return true;
 139        if (p->flags & PF_KTHREAD)
 140                return true;
 141
 142        /* When mem_cgroup_out_of_memory() and p is not member of the group */
 143        if (mem && !task_in_mem_cgroup(p, mem))
 144                return true;
 145
 146        /* p may not have freeable memory in nodemask */
 147        if (!has_intersects_mems_allowed(p, nodemask))
 148                return true;
 149
 150        return false;
 151}
 152
 153/**
 154 * oom_badness - heuristic function to determine which candidate task to kill
 155 * @p: task struct of which task we should calculate
 156 * @totalpages: total present RAM allowed for page allocation
 157 *
 158 * The heuristic for determining which task to kill is made to be as simple and
 159 * predictable as possible.  The goal is to return the highest value for the
 160 * task consuming the most memory to avoid subsequent oom failures.
 161 */
 162unsigned int oom_badness(struct task_struct *p, struct mem_cgroup *mem,
 163                      const nodemask_t *nodemask, unsigned long totalpages)
 164{
 165        int points;
 166
 167        if (oom_unkillable_task(p, mem, nodemask))
 168                return 0;
 169
 170        p = find_lock_task_mm(p);
 171        if (!p)
 172                return 0;
 173
 174        /*
 175         * Shortcut check for a thread sharing p->mm that is OOM_SCORE_ADJ_MIN
 176         * so the entire heuristic doesn't need to be executed for something
 177         * that cannot be killed.
 178         */
 179        if (atomic_read(&p->mm->oom_disable_count)) {
 180                task_unlock(p);
 181                return 0;
 182        }
 183
 184        /*
 185         * The memory controller may have a limit of 0 bytes, so avoid a divide
 186         * by zero, if necessary.
 187         */
 188        if (!totalpages)
 189                totalpages = 1;
 190
 191        /*
 192         * The baseline for the badness score is the proportion of RAM that each
 193         * task's rss, pagetable and swap space use.
 194         */
 195        points = get_mm_rss(p->mm) + p->mm->nr_ptes;
 196        points += get_mm_counter(p->mm, MM_SWAPENTS);
 197
 198        points *= 1000;
 199        points /= totalpages;
 200        task_unlock(p);
 201
 202        /*
 203         * Root processes get 3% bonus, just like the __vm_enough_memory()
 204         * implementation used by LSMs.
 205         */
 206        if (has_capability_noaudit(p, CAP_SYS_ADMIN))
 207                points -= 30;
 208
 209        /*
 210         * /proc/pid/oom_score_adj ranges from -1000 to +1000 such that it may
 211         * either completely disable oom killing or always prefer a certain
 212         * task.
 213         */
 214        points += p->signal->oom_score_adj;
 215
 216        /*
 217         * Never return 0 for an eligible task that may be killed since it's
 218         * possible that no single user task uses more than 0.1% of memory and
 219         * no single admin tasks uses more than 3.0%.
 220         */
 221        if (points <= 0)
 222                return 1;
 223        return (points < 1000) ? points : 1000;
 224}
 225
 226/*
 227 * Determine the type of allocation constraint.
 228 */
 229#ifdef CONFIG_NUMA
 230static enum oom_constraint constrained_alloc(struct zonelist *zonelist,
 231                                gfp_t gfp_mask, nodemask_t *nodemask,
 232                                unsigned long *totalpages)
 233{
 234        struct zone *zone;
 235        struct zoneref *z;
 236        enum zone_type high_zoneidx = gfp_zone(gfp_mask);
 237        bool cpuset_limited = false;
 238        int nid;
 239
 240        /* Default to all available memory */
 241        *totalpages = totalram_pages + total_swap_pages;
 242
 243        if (!zonelist)
 244                return CONSTRAINT_NONE;
 245        /*
 246         * Reach here only when __GFP_NOFAIL is used. So, we should avoid
 247         * to kill current.We have to random task kill in this case.
 248         * Hopefully, CONSTRAINT_THISNODE...but no way to handle it, now.
 249         */
 250        if (gfp_mask & __GFP_THISNODE)
 251                return CONSTRAINT_NONE;
 252
 253        /*
 254         * This is not a __GFP_THISNODE allocation, so a truncated nodemask in
 255         * the page allocator means a mempolicy is in effect.  Cpuset policy
 256         * is enforced in get_page_from_freelist().
 257         */
 258        if (nodemask && !nodes_subset(node_states[N_HIGH_MEMORY], *nodemask)) {
 259                *totalpages = total_swap_pages;
 260                for_each_node_mask(nid, *nodemask)
 261                        *totalpages += node_spanned_pages(nid);
 262                return CONSTRAINT_MEMORY_POLICY;
 263        }
 264
 265        /* Check this allocation failure is caused by cpuset's wall function */
 266        for_each_zone_zonelist_nodemask(zone, z, zonelist,
 267                        high_zoneidx, nodemask)
 268                if (!cpuset_zone_allowed_softwall(zone, gfp_mask))
 269                        cpuset_limited = true;
 270
 271        if (cpuset_limited) {
 272                *totalpages = total_swap_pages;
 273                for_each_node_mask(nid, cpuset_current_mems_allowed)
 274                        *totalpages += node_spanned_pages(nid);
 275                return CONSTRAINT_CPUSET;
 276        }
 277        return CONSTRAINT_NONE;
 278}
 279#else
 280static enum oom_constraint constrained_alloc(struct zonelist *zonelist,
 281                                gfp_t gfp_mask, nodemask_t *nodemask,
 282                                unsigned long *totalpages)
 283{
 284        *totalpages = totalram_pages + total_swap_pages;
 285        return CONSTRAINT_NONE;
 286}
 287#endif
 288
 289/*
 290 * Simple selection loop. We chose the process with the highest
 291 * number of 'points'. We expect the caller will lock the tasklist.
 292 *
 293 * (not docbooked, we don't want this one cluttering up the manual)
 294 */
 295static struct task_struct *select_bad_process(unsigned int *ppoints,
 296                unsigned long totalpages, struct mem_cgroup *mem,
 297                const nodemask_t *nodemask)
 298{
 299        struct task_struct *g, *p;
 300        struct task_struct *chosen = NULL;
 301        *ppoints = 0;
 302
 303        do_each_thread(g, p) {
 304                unsigned int points;
 305
 306                if (p->exit_state)
 307                        continue;
 308                if (oom_unkillable_task(p, mem, nodemask))
 309                        continue;
 310
 311                /*
 312                 * This task already has access to memory reserves and is
 313                 * being killed. Don't allow any other task access to the
 314                 * memory reserve.
 315                 *
 316                 * Note: this may have a chance of deadlock if it gets
 317                 * blocked waiting for another task which itself is waiting
 318                 * for memory. Is there a better alternative?
 319                 */
 320                if (test_tsk_thread_flag(p, TIF_MEMDIE))
 321                        return ERR_PTR(-1UL);
 322                if (!p->mm)
 323                        continue;
 324
 325                if (p->flags & PF_EXITING) {
 326                        /*
 327                         * If p is the current task and is in the process of
 328                         * releasing memory, we allow the "kill" to set
 329                         * TIF_MEMDIE, which will allow it to gain access to
 330                         * memory reserves.  Otherwise, it may stall forever.
 331                         *
 332                         * The loop isn't broken here, however, in case other
 333                         * threads are found to have already been oom killed.
 334                         */
 335                        if (p == current) {
 336                                chosen = p;
 337                                *ppoints = 1000;
 338                        } else {
 339                                /*
 340                                 * If this task is not being ptraced on exit,
 341                                 * then wait for it to finish before killing
 342                                 * some other task unnecessarily.
 343                                 */
 344                                if (!(p->group_leader->ptrace & PT_TRACE_EXIT))
 345                                        return ERR_PTR(-1UL);
 346                        }
 347                }
 348
 349                points = oom_badness(p, mem, nodemask, totalpages);
 350                if (points > *ppoints) {
 351                        chosen = p;
 352                        *ppoints = points;
 353                }
 354        } while_each_thread(g, p);
 355
 356        return chosen;
 357}
 358
 359/**
 360 * dump_tasks - dump current memory state of all system tasks
 361 * @mem: current's memory controller, if constrained
 362 * @nodemask: nodemask passed to page allocator for mempolicy ooms
 363 *
 364 * Dumps the current memory state of all eligible tasks.  Tasks not in the same
 365 * memcg, not in the same cpuset, or bound to a disjoint set of mempolicy nodes
 366 * are not shown.
 367 * State information includes task's pid, uid, tgid, vm size, rss, cpu, oom_adj
 368 * value, oom_score_adj value, and name.
 369 *
 370 * Call with tasklist_lock read-locked.
 371 */
 372static void dump_tasks(const struct mem_cgroup *mem, const nodemask_t *nodemask)
 373{
 374        struct task_struct *p;
 375        struct task_struct *task;
 376
 377        pr_info("[ pid ]   uid  tgid total_vm      rss cpu oom_adj oom_score_adj name\n");
 378        for_each_process(p) {
 379                if (oom_unkillable_task(p, mem, nodemask))
 380                        continue;
 381
 382                task = find_lock_task_mm(p);
 383                if (!task) {
 384                        /*
 385                         * This is a kthread or all of p's threads have already
 386                         * detached their mm's.  There's no need to report
 387                         * them; they can't be oom killed anyway.
 388                         */
 389                        continue;
 390                }
 391
 392                pr_info("[%5d] %5d %5d %8lu %8lu %3u     %3d         %5d %s\n",
 393                        task->pid, task_uid(task), task->tgid,
 394                        task->mm->total_vm, get_mm_rss(task->mm),
 395                        task_cpu(task), task->signal->oom_adj,
 396                        task->signal->oom_score_adj, task->comm);
 397                task_unlock(task);
 398        }
 399}
 400
 401static void dump_header(struct task_struct *p, gfp_t gfp_mask, int order,
 402                        struct mem_cgroup *mem, const nodemask_t *nodemask)
 403{
 404        task_lock(current);
 405        pr_warning("%s invoked oom-killer: gfp_mask=0x%x, order=%d, "
 406                "oom_adj=%d, oom_score_adj=%d\n",
 407                current->comm, gfp_mask, order, current->signal->oom_adj,
 408                current->signal->oom_score_adj);
 409        cpuset_print_task_mems_allowed(current);
 410        task_unlock(current);
 411        dump_stack();
 412        mem_cgroup_print_oom_info(mem, p);
 413        show_mem(SHOW_MEM_FILTER_NODES);
 414        if (sysctl_oom_dump_tasks)
 415                dump_tasks(mem, nodemask);
 416}
 417
 418#define K(x) ((x) << (PAGE_SHIFT-10))
 419static int oom_kill_task(struct task_struct *p, struct mem_cgroup *mem)
 420{
 421        struct task_struct *q;
 422        struct mm_struct *mm;
 423
 424        p = find_lock_task_mm(p);
 425        if (!p)
 426                return 1;
 427
 428        /* mm cannot be safely dereferenced after task_unlock(p) */
 429        mm = p->mm;
 430
 431        pr_err("Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB\n",
 432                task_pid_nr(p), p->comm, K(p->mm->total_vm),
 433                K(get_mm_counter(p->mm, MM_ANONPAGES)),
 434                K(get_mm_counter(p->mm, MM_FILEPAGES)));
 435        task_unlock(p);
 436
 437        /*
 438         * Kill all processes sharing p->mm in other thread groups, if any.
 439         * They don't get access to memory reserves or a higher scheduler
 440         * priority, though, to avoid depletion of all memory or task
 441         * starvation.  This prevents mm->mmap_sem livelock when an oom killed
 442         * task cannot exit because it requires the semaphore and its contended
 443         * by another thread trying to allocate memory itself.  That thread will
 444         * now get access to memory reserves since it has a pending fatal
 445         * signal.
 446         */
 447        for_each_process(q)
 448                if (q->mm == mm && !same_thread_group(q, p)) {
 449                        task_lock(q);   /* Protect ->comm from prctl() */
 450                        pr_err("Kill process %d (%s) sharing same memory\n",
 451                                task_pid_nr(q), q->comm);
 452                        task_unlock(q);
 453                        force_sig(SIGKILL, q);
 454                }
 455
 456        set_tsk_thread_flag(p, TIF_MEMDIE);
 457        force_sig(SIGKILL, p);
 458
 459        return 0;
 460}
 461#undef K
 462
 463static int oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 464                            unsigned int points, unsigned long totalpages,
 465                            struct mem_cgroup *mem, nodemask_t *nodemask,
 466                            const char *message)
 467{
 468        struct task_struct *victim = p;
 469        struct task_struct *child;
 470        struct task_struct *t = p;
 471        unsigned int victim_points = 0;
 472
 473        if (printk_ratelimit())
 474                dump_header(p, gfp_mask, order, mem, nodemask);
 475
 476        /*
 477         * If the task is already exiting, don't alarm the sysadmin or kill
 478         * its children or threads, just set TIF_MEMDIE so it can die quickly
 479         */
 480        if (p->flags & PF_EXITING) {
 481                set_tsk_thread_flag(p, TIF_MEMDIE);
 482                return 0;
 483        }
 484
 485        task_lock(p);
 486        pr_err("%s: Kill process %d (%s) score %d or sacrifice child\n",
 487                message, task_pid_nr(p), p->comm, points);
 488        task_unlock(p);
 489
 490        /*
 491         * If any of p's children has a different mm and is eligible for kill,
 492         * the one with the highest oom_badness() score is sacrificed for its
 493         * parent.  This attempts to lose the minimal amount of work done while
 494         * still freeing memory.
 495         */
 496        do {
 497                list_for_each_entry(child, &t->children, sibling) {
 498                        unsigned int child_points;
 499
 500                        if (child->mm == p->mm)
 501                                continue;
 502                        /*
 503                         * oom_badness() returns 0 if the thread is unkillable
 504                         */
 505                        child_points = oom_badness(child, mem, nodemask,
 506                                                                totalpages);
 507                        if (child_points > victim_points) {
 508                                victim = child;
 509                                victim_points = child_points;
 510                        }
 511                }
 512        } while_each_thread(p, t);
 513
 514        return oom_kill_task(victim, mem);
 515}
 516
 517/*
 518 * Determines whether the kernel must panic because of the panic_on_oom sysctl.
 519 */
 520static void check_panic_on_oom(enum oom_constraint constraint, gfp_t gfp_mask,
 521                                int order, const nodemask_t *nodemask)
 522{
 523        if (likely(!sysctl_panic_on_oom))
 524                return;
 525        if (sysctl_panic_on_oom != 2) {
 526                /*
 527                 * panic_on_oom == 1 only affects CONSTRAINT_NONE, the kernel
 528                 * does not panic for cpuset, mempolicy, or memcg allocation
 529                 * failures.
 530                 */
 531                if (constraint != CONSTRAINT_NONE)
 532                        return;
 533        }
 534        read_lock(&tasklist_lock);
 535        dump_header(NULL, gfp_mask, order, NULL, nodemask);
 536        read_unlock(&tasklist_lock);
 537        panic("Out of memory: %s panic_on_oom is enabled\n",
 538                sysctl_panic_on_oom == 2 ? "compulsory" : "system-wide");
 539}
 540
 541#ifdef CONFIG_CGROUP_MEM_RES_CTLR
 542void mem_cgroup_out_of_memory(struct mem_cgroup *mem, gfp_t gfp_mask)
 543{
 544        unsigned long limit;
 545        unsigned int points = 0;
 546        struct task_struct *p;
 547
 548        /*
 549         * If current has a pending SIGKILL, then automatically select it.  The
 550         * goal is to allow it to allocate so that it may quickly exit and free
 551         * its memory.
 552         */
 553        if (fatal_signal_pending(current)) {
 554                set_thread_flag(TIF_MEMDIE);
 555                return;
 556        }
 557
 558        check_panic_on_oom(CONSTRAINT_MEMCG, gfp_mask, 0, NULL);
 559        limit = mem_cgroup_get_limit(mem) >> PAGE_SHIFT;
 560        read_lock(&tasklist_lock);
 561retry:
 562        p = select_bad_process(&points, limit, mem, NULL);
 563        if (!p || PTR_ERR(p) == -1UL)
 564                goto out;
 565
 566        if (oom_kill_process(p, gfp_mask, 0, points, limit, mem, NULL,
 567                                "Memory cgroup out of memory"))
 568                goto retry;
 569out:
 570        read_unlock(&tasklist_lock);
 571}
 572#endif
 573
 574static BLOCKING_NOTIFIER_HEAD(oom_notify_list);
 575
 576int register_oom_notifier(struct notifier_block *nb)
 577{
 578        return blocking_notifier_chain_register(&oom_notify_list, nb);
 579}
 580EXPORT_SYMBOL_GPL(register_oom_notifier);
 581
 582int unregister_oom_notifier(struct notifier_block *nb)
 583{
 584        return blocking_notifier_chain_unregister(&oom_notify_list, nb);
 585}
 586EXPORT_SYMBOL_GPL(unregister_oom_notifier);
 587
 588/*
 589 * Try to acquire the OOM killer lock for the zones in zonelist.  Returns zero
 590 * if a parallel OOM killing is already taking place that includes a zone in
 591 * the zonelist.  Otherwise, locks all zones in the zonelist and returns 1.
 592 */
 593int try_set_zonelist_oom(struct zonelist *zonelist, gfp_t gfp_mask)
 594{
 595        struct zoneref *z;
 596        struct zone *zone;
 597        int ret = 1;
 598
 599        spin_lock(&zone_scan_lock);
 600        for_each_zone_zonelist(zone, z, zonelist, gfp_zone(gfp_mask)) {
 601                if (zone_is_oom_locked(zone)) {
 602                        ret = 0;
 603                        goto out;
 604                }
 605        }
 606
 607        for_each_zone_zonelist(zone, z, zonelist, gfp_zone(gfp_mask)) {
 608                /*
 609                 * Lock each zone in the zonelist under zone_scan_lock so a
 610                 * parallel invocation of try_set_zonelist_oom() doesn't succeed
 611                 * when it shouldn't.
 612                 */
 613                zone_set_flag(zone, ZONE_OOM_LOCKED);
 614        }
 615
 616out:
 617        spin_unlock(&zone_scan_lock);
 618        return ret;
 619}
 620
 621/*
 622 * Clears the ZONE_OOM_LOCKED flag for all zones in the zonelist so that failed
 623 * allocation attempts with zonelists containing them may now recall the OOM
 624 * killer, if necessary.
 625 */
 626void clear_zonelist_oom(struct zonelist *zonelist, gfp_t gfp_mask)
 627{
 628        struct zoneref *z;
 629        struct zone *zone;
 630
 631        spin_lock(&zone_scan_lock);
 632        for_each_zone_zonelist(zone, z, zonelist, gfp_zone(gfp_mask)) {
 633                zone_clear_flag(zone, ZONE_OOM_LOCKED);
 634        }
 635        spin_unlock(&zone_scan_lock);
 636}
 637
 638/*
 639 * Try to acquire the oom killer lock for all system zones.  Returns zero if a
 640 * parallel oom killing is taking place, otherwise locks all zones and returns
 641 * non-zero.
 642 */
 643static int try_set_system_oom(void)
 644{
 645        struct zone *zone;
 646        int ret = 1;
 647
 648        spin_lock(&zone_scan_lock);
 649        for_each_populated_zone(zone)
 650                if (zone_is_oom_locked(zone)) {
 651                        ret = 0;
 652                        goto out;
 653                }
 654        for_each_populated_zone(zone)
 655                zone_set_flag(zone, ZONE_OOM_LOCKED);
 656out:
 657        spin_unlock(&zone_scan_lock);
 658        return ret;
 659}
 660
 661/*
 662 * Clears ZONE_OOM_LOCKED for all system zones so that failed allocation
 663 * attempts or page faults may now recall the oom killer, if necessary.
 664 */
 665static void clear_system_oom(void)
 666{
 667        struct zone *zone;
 668
 669        spin_lock(&zone_scan_lock);
 670        for_each_populated_zone(zone)
 671                zone_clear_flag(zone, ZONE_OOM_LOCKED);
 672        spin_unlock(&zone_scan_lock);
 673}
 674
 675/**
 676 * out_of_memory - kill the "best" process when we run out of memory
 677 * @zonelist: zonelist pointer
 678 * @gfp_mask: memory allocation flags
 679 * @order: amount of memory being requested as a power of 2
 680 * @nodemask: nodemask passed to page allocator
 681 *
 682 * If we run out of memory, we have the choice between either
 683 * killing a random task (bad), letting the system crash (worse)
 684 * OR try to be smart about which process to kill. Note that we
 685 * don't have to be perfect here, we just have to be good.
 686 */
 687void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
 688                int order, nodemask_t *nodemask)
 689{
 690        const nodemask_t *mpol_mask;
 691        struct task_struct *p;
 692        unsigned long totalpages;
 693        unsigned long freed = 0;
 694        unsigned int points;
 695        enum oom_constraint constraint = CONSTRAINT_NONE;
 696        int killed = 0;
 697
 698        blocking_notifier_call_chain(&oom_notify_list, 0, &freed);
 699        if (freed > 0)
 700                /* Got some memory back in the last second. */
 701                return;
 702
 703        /*
 704         * If current has a pending SIGKILL, then automatically select it.  The
 705         * goal is to allow it to allocate so that it may quickly exit and free
 706         * its memory.
 707         */
 708        if (fatal_signal_pending(current)) {
 709                set_thread_flag(TIF_MEMDIE);
 710                return;
 711        }
 712
 713        /*
 714         * Check if there were limitations on the allocation (only relevant for
 715         * NUMA) that may require different handling.
 716         */
 717        constraint = constrained_alloc(zonelist, gfp_mask, nodemask,
 718                                                &totalpages);
 719        mpol_mask = (constraint == CONSTRAINT_MEMORY_POLICY) ? nodemask : NULL;
 720        check_panic_on_oom(constraint, gfp_mask, order, mpol_mask);
 721
 722        read_lock(&tasklist_lock);
 723        if (sysctl_oom_kill_allocating_task &&
 724            !oom_unkillable_task(current, NULL, nodemask) &&
 725            current->mm && !atomic_read(&current->mm->oom_disable_count)) {
 726                /*
 727                 * oom_kill_process() needs tasklist_lock held.  If it returns
 728                 * non-zero, current could not be killed so we must fallback to
 729                 * the tasklist scan.
 730                 */
 731                if (!oom_kill_process(current, gfp_mask, order, 0, totalpages,
 732                                NULL, nodemask,
 733                                "Out of memory (oom_kill_allocating_task)"))
 734                        goto out;
 735        }
 736
 737retry:
 738        p = select_bad_process(&points, totalpages, NULL, mpol_mask);
 739        if (PTR_ERR(p) == -1UL)
 740                goto out;
 741
 742        /* Found nothing?!?! Either we hang forever, or we panic. */
 743        if (!p) {
 744                dump_header(NULL, gfp_mask, order, NULL, mpol_mask);
 745                read_unlock(&tasklist_lock);
 746                panic("Out of memory and no killable processes...\n");
 747        }
 748
 749        if (oom_kill_process(p, gfp_mask, order, points, totalpages, NULL,
 750                                nodemask, "Out of memory"))
 751                goto retry;
 752        killed = 1;
 753out:
 754        read_unlock(&tasklist_lock);
 755
 756        /*
 757         * Give "p" a good chance of killing itself before we
 758         * retry to allocate memory unless "p" is current
 759         */
 760        if (killed && !test_thread_flag(TIF_MEMDIE))
 761                schedule_timeout_uninterruptible(1);
 762}
 763
 764/*
 765 * The pagefault handler calls here because it is out of memory, so kill a
 766 * memory-hogging task.  If a populated zone has ZONE_OOM_LOCKED set, a parallel
 767 * oom killing is already in progress so do nothing.  If a task is found with
 768 * TIF_MEMDIE set, it has been killed so do nothing and allow it to exit.
 769 */
 770void pagefault_out_of_memory(void)
 771{
 772        if (try_set_system_oom()) {
 773                out_of_memory(NULL, 0, 0, NULL);
 774                clear_system_oom();
 775        }
 776        if (!test_thread_flag(TIF_MEMDIE))
 777                schedule_timeout_uninterruptible(1);
 778}
 779
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