lib/stackdepot.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * Stack depot - a stack trace storage that avoids duplication.
   4  *
   5  * Internally, stack depot maintains a hash table of unique stacktraces. The
   6  * stack traces themselves are stored contiguously one after another in a set
   7  * of separate page allocations.
   8  *
   9  * Author: Alexander Potapenko <[email protected]>
  10  * Copyright (C) 2016 Google, Inc.
  11  *
  12  * Based on the code by Dmitry Chernenkov.
  13  */
  14
  15 #define pr_fmt(fmt) "stackdepot: " fmt
  16
  17 #include <linux/debugfs.h>
  18 #include <linux/gfp.h>
  19 #include <linux/jhash.h>
  20 #include <linux/kernel.h>
  21 #include <linux/kmsan.h>
  22 #include <linux/list.h>
  23 #include <linux/mm.h>
  24 #include <linux/mutex.h>
  25 #include <linux/poison.h>
  26 #include <linux/printk.h>
  27 #include <linux/rculist.h>
  28 #include <linux/rcupdate.h>
  29 #include <linux/refcount.h>
  30 #include <linux/slab.h>
  31 #include <linux/spinlock.h>
  32 #include <linux/stacktrace.h>
  33 #include <linux/stackdepot.h>
  34 #include <linux/string.h>
  35 #include <linux/types.h>
  36 #include <linux/memblock.h>
  37 #include <linux/kasan-enabled.h>
  38
  39 #define DEPOT_POOLS_CAP 8192
  40 /* The pool_index is offset by 1 so the first record does not have a 0 handle. */
  41 #define DEPOT_MAX_POOLS \
  42         (((1LL << (DEPOT_POOL_INDEX_BITS)) - 1 < DEPOT_POOLS_CAP) ? \
  43          (1LL << (DEPOT_POOL_INDEX_BITS)) - 1 : DEPOT_POOLS_CAP)
  44
  45 static bool stack_depot_disabled;
  46 static bool __stack_depot_early_init_requested __initdata = IS_ENABLED(CONFIG_STACKDEPOT_ALWAYS_INIT);
  47 static bool __stack_depot_early_init_passed __initdata;
  48
  49 /* Use one hash table bucket per 16 KB of memory. */
  50 #define STACK_HASH_TABLE_SCALE 14
  51 /* Limit the number of buckets between 4K and 1M. */
  52 #define STACK_BUCKET_NUMBER_ORDER_MIN 12
  53 #define STACK_BUCKET_NUMBER_ORDER_MAX 20
  54 /* Initial seed for jhash2. */
  55 #define STACK_HASH_SEED 0x9747b28c
  56
  57 /* Hash table of stored stack records. */
  58 static struct list_head *stack_table;
  59 /* Fixed order of the number of table buckets. Used when KASAN is enabled. */
  60 static unsigned int stack_bucket_number_order;
  61 /* Hash mask for indexing the table. */
  62 static unsigned int stack_hash_mask;
  63
  64 /* Array of memory regions that store stack records. */
  65 static void *stack_pools[DEPOT_MAX_POOLS];
  66 /* Newly allocated pool that is not yet added to stack_pools. */
  67 static void *new_pool;
  68 /* Number of pools in stack_pools. */
  69 static int pools_num;
  70 /* Offset to the unused space in the currently used pool. */
  71 static size_t pool_offset = DEPOT_POOL_SIZE;
  72 /* Freelist of stack records within stack_pools. */
  73 static LIST_HEAD(free_stacks);
  74 /* The lock must be held when performing pool or freelist modifications. */
  75 static DEFINE_RAW_SPINLOCK(pool_lock);
  76
  77 /* Statistics counters for debugfs. */
  78 enum depot_counter_id {
  79         DEPOT_COUNTER_REFD_ALLOCS,
  80         DEPOT_COUNTER_REFD_FREES,
  81         DEPOT_COUNTER_REFD_INUSE,
  82         DEPOT_COUNTER_FREELIST_SIZE,
  83         DEPOT_COUNTER_PERSIST_COUNT,
  84         DEPOT_COUNTER_PERSIST_BYTES,
  85         DEPOT_COUNTER_COUNT,
  86 };
  87 static long counters[DEPOT_COUNTER_COUNT];
  88 static const char *const counter_names[] = {
  89         [DEPOT_COUNTER_REFD_ALLOCS]     = "refcounted_allocations",
  90         [DEPOT_COUNTER_REFD_FREES]      = "refcounted_frees",
  91         [DEPOT_COUNTER_REFD_INUSE]      = "refcounted_in_use",
  92         [DEPOT_COUNTER_FREELIST_SIZE]   = "freelist_size",
  93         [DEPOT_COUNTER_PERSIST_COUNT]   = "persistent_count",
  94         [DEPOT_COUNTER_PERSIST_BYTES]   = "persistent_bytes",
  95 };
  96 static_assert(ARRAY_SIZE(counter_names) == DEPOT_COUNTER_COUNT);
  97
  98 static int __init disable_stack_depot(char *str)
  99 {
 100         return kstrtobool(str, &stack_depot_disabled);
 101 }
 102 early_param("stack_depot_disable", disable_stack_depot);
 103
 104 void __init stack_depot_request_early_init(void)
 105 {
 106         /* Too late to request early init now. */
 107         WARN_ON(__stack_depot_early_init_passed);
 108
 109         __stack_depot_early_init_requested = true;
 110 }
 111
 112 /* Initialize list_head's within the hash table. */
 113 static void init_stack_table(unsigned long entries)
 114 {
 115         unsigned long i;
 116
 117         for (i = 0; i < entries; i++)
 118                 INIT_LIST_HEAD(&stack_table[i]);
 119 }
 120
 121 /* Allocates a hash table via memblock. Can only be used during early boot. */
 122 int __init stack_depot_early_init(void)
 123 {
 124         unsigned long entries = 0;
 125
 126         /* This function must be called only once, from mm_init(). */
 127         if (WARN_ON(__stack_depot_early_init_passed))
 128                 return 0;
 129         __stack_depot_early_init_passed = true;
 130
 131         /*
 132          * Print disabled message even if early init has not been requested:
 133          * stack_depot_init() will not print one.
 134          */
 135         if (stack_depot_disabled) {
 136                 pr_info("disabled\n");
 137                 return 0;
 138         }
 139
 140         /*
 141          * If KASAN is enabled, use the maximum order: KASAN is frequently used
 142          * in fuzzing scenarios, which leads to a large number of different
 143          * stack traces being stored in stack depot.
 144          */
 145         if (kasan_enabled() && !stack_bucket_number_order)
 146                 stack_bucket_number_order = STACK_BUCKET_NUMBER_ORDER_MAX;
 147
 148         /*
 149          * Check if early init has been requested after setting
 150          * stack_bucket_number_order: stack_depot_init() uses its value.
 151          */
 152         if (!__stack_depot_early_init_requested)
 153                 return 0;
 154
 155         /*
 156          * If stack_bucket_number_order is not set, leave entries as 0 to rely
 157          * on the automatic calculations performed by alloc_large_system_hash().
 158          */
 159         if (stack_bucket_number_order)
 160                 entries = 1UL << stack_bucket_number_order;
 161         pr_info("allocating hash table via alloc_large_system_hash\n");
 162         stack_table = alloc_large_system_hash("stackdepot",
 163                                                 sizeof(struct list_head),
 164                                                 entries,
 165                                                 STACK_HASH_TABLE_SCALE,
 166                                                 HASH_EARLY,
 167                                                 NULL,
 168                                                 &stack_hash_mask,
 169                                                 1UL << STACK_BUCKET_NUMBER_ORDER_MIN,
 170                                                 1UL << STACK_BUCKET_NUMBER_ORDER_MAX);
 171         if (!stack_table) {
 172                 pr_err("hash table allocation failed, disabling\n");
 173                 stack_depot_disabled = true;
 174                 return -ENOMEM;
 175         }
 176         if (!entries) {
 177                 /*
 178                  * Obtain the number of entries that was calculated by
 179                  * alloc_large_system_hash().
 180                  */
 181                 entries = stack_hash_mask + 1;
 182         }
 183         init_stack_table(entries);
 184
 185         return 0;
 186 }
 187
 188 /* Allocates a hash table via kvcalloc. Can be used after boot. */
 189 int stack_depot_init(void)
 190 {
 191         static DEFINE_MUTEX(stack_depot_init_mutex);
 192         unsigned long entries;
 193         int ret = 0;
 194
 195         mutex_lock(&stack_depot_init_mutex);
 196
 197         if (stack_depot_disabled || stack_table)
 198                 goto out_unlock;
 199
 200         /*
 201          * Similarly to stack_depot_early_init, use stack_bucket_number_order
 202          * if assigned, and rely on automatic scaling otherwise.
 203          */
 204         if (stack_bucket_number_order) {
 205                 entries = 1UL << stack_bucket_number_order;
 206         } else {
 207                 int scale = STACK_HASH_TABLE_SCALE;
 208
 209                 entries = nr_free_buffer_pages();
 210                 entries = roundup_pow_of_two(entries);
 211
 212                 if (scale > PAGE_SHIFT)
 213                         entries >>= (scale - PAGE_SHIFT);
 214                 else
 215                         entries <<= (PAGE_SHIFT - scale);
 216         }
 217
 218         if (entries < 1UL << STACK_BUCKET_NUMBER_ORDER_MIN)
 219                 entries = 1UL << STACK_BUCKET_NUMBER_ORDER_MIN;
 220         if (entries > 1UL << STACK_BUCKET_NUMBER_ORDER_MAX)
 221                 entries = 1UL << STACK_BUCKET_NUMBER_ORDER_MAX;
 222
 223         pr_info("allocating hash table of %lu entries via kvcalloc\n", entries);
 224         stack_table = kvcalloc(entries, sizeof(struct list_head), GFP_KERNEL);
 225         if (!stack_table) {
 226                 pr_err("hash table allocation failed, disabling\n");
 227                 stack_depot_disabled = true;
 228                 ret = -ENOMEM;
 229                 goto out_unlock;
 230         }
 231         stack_hash_mask = entries - 1;
 232         init_stack_table(entries);
 233
 234 out_unlock:
 235         mutex_unlock(&stack_depot_init_mutex);
 236
 237         return ret;
 238 }
 239 EXPORT_SYMBOL_GPL(stack_depot_init);
 240
 241 /*
 242  * Initializes new stack pool, and updates the list of pools.
 243  */
 244 static bool depot_init_pool(void **prealloc)
 245 {
 246         lockdep_assert_held(&pool_lock);
 247
 248         if (unlikely(pools_num >= DEPOT_MAX_POOLS)) {
 249                 /* Bail out if we reached the pool limit. */
 250                 WARN_ON_ONCE(pools_num > DEPOT_MAX_POOLS); /* should never happen */
 251                 WARN_ON_ONCE(!new_pool); /* to avoid unnecessary pre-allocation */
 252                 WARN_ONCE(1, "Stack depot reached limit capacity");
 253                 return false;
 254         }
 255
 256         if (!new_pool && *prealloc) {
 257                 /* We have preallocated memory, use it. */
 258                 WRITE_ONCE(new_pool, *prealloc);
 259                 *prealloc = NULL;
 260         }
 261
 262         if (!new_pool)
 263                 return false; /* new_pool and *prealloc are NULL */
 264
 265         /* Save reference to the pool to be used by depot_fetch_stack(). */
 266         stack_pools[pools_num] = new_pool;
 267
 268         /*
 269          * Stack depot tries to keep an extra pool allocated even before it runs
 270          * out of space in the currently used pool.
 271          *
 272          * To indicate that a new preallocation is needed new_pool is reset to
 273          * NULL; do not reset to NULL if we have reached the maximum number of
 274          * pools.
 275          */
 276         if (pools_num < DEPOT_MAX_POOLS)
 277                 WRITE_ONCE(new_pool, NULL);
 278         else
 279                 WRITE_ONCE(new_pool, STACK_DEPOT_POISON);
 280
 281         /* Pairs with concurrent READ_ONCE() in depot_fetch_stack(). */
 282         WRITE_ONCE(pools_num, pools_num + 1);
 283         ASSERT_EXCLUSIVE_WRITER(pools_num);
 284
 285         pool_offset = 0;
 286
 287         return true;
 288 }
 289
 290 /* Keeps the preallocated memory to be used for a new stack depot pool. */
 291 static void depot_keep_new_pool(void **prealloc)
 292 {
 293         lockdep_assert_held(&pool_lock);
 294
 295         /*
 296          * If a new pool is already saved or the maximum number of
 297          * pools is reached, do not use the preallocated memory.
 298          */
 299         if (new_pool)
 300                 return;
 301
 302         WRITE_ONCE(new_pool, *prealloc);
 303         *prealloc = NULL;
 304 }
 305
 306 /*
 307  * Try to initialize a new stack record from the current pool, a cached pool, or
 308  * the current pre-allocation.
 309  */
 310 static struct stack_record *depot_pop_free_pool(void **prealloc, size_t size)
 311 {
 312         struct stack_record *stack;
 313         void *current_pool;
 314         u32 pool_index;
 315
 316         lockdep_assert_held(&pool_lock);
 317
 318         if (pool_offset + size > DEPOT_POOL_SIZE) {
 319                 if (!depot_init_pool(prealloc))
 320                         return NULL;
 321         }
 322
 323         if (WARN_ON_ONCE(pools_num < 1))
 324                 return NULL;
 325         pool_index = pools_num - 1;
 326         current_pool = stack_pools[pool_index];
 327         if (WARN_ON_ONCE(!current_pool))
 328                 return NULL;
 329
 330         stack = current_pool + pool_offset;
 331
 332         /* Pre-initialize handle once. */
 333         stack->handle.pool_index_plus_1 = pool_index + 1;
 334         stack->handle.offset = pool_offset >> DEPOT_STACK_ALIGN;
 335         stack->handle.extra = 0;
 336         INIT_LIST_HEAD(&stack->hash_list);
 337
 338         pool_offset += size;
 339
 340         return stack;
 341 }
 342
 343 /* Try to find next free usable entry from the freelist. */
 344 static struct stack_record *depot_pop_free(void)
 345 {
 346         struct stack_record *stack;
 347
 348         lockdep_assert_held(&pool_lock);
 349
 350         if (list_empty(&free_stacks))
 351                 return NULL;
 352
 353         /*
 354          * We maintain the invariant that the elements in front are least
 355          * recently used, and are therefore more likely to be associated with an
 356          * RCU grace period in the past. Consequently it is sufficient to only
 357          * check the first entry.
 358          */
 359         stack = list_first_entry(&free_stacks, struct stack_record, free_list);
 360         if (!poll_state_synchronize_rcu(stack->rcu_state))
 361                 return NULL;
 362
 363         list_del(&stack->free_list);
 364         counters[DEPOT_COUNTER_FREELIST_SIZE]--;
 365
 366         return stack;
 367 }
 368
 369 static inline size_t depot_stack_record_size(struct stack_record *s, unsigned int nr_entries)
 370 {
 371         const size_t used = flex_array_size(s, entries, nr_entries);
 372         const size_t unused = sizeof(s->entries) - used;
 373
 374         WARN_ON_ONCE(sizeof(s->entries) < used);
 375
 376         return ALIGN(sizeof(struct stack_record) - unused, 1 << DEPOT_STACK_ALIGN);
 377 }
 378
 379 /* Allocates a new stack in a stack depot pool. */
 380 static struct stack_record *
 381 depot_alloc_stack(unsigned long *entries, unsigned int nr_entries, u32 hash, depot_flags_t flags, void **prealloc)
 382 {
 383         struct stack_record *stack = NULL;
 384         size_t record_size;
 385
 386         lockdep_assert_held(&pool_lock);
 387
 388         /* This should already be checked by public API entry points. */
 389         if (WARN_ON_ONCE(!nr_entries))
 390                 return NULL;
 391
 392         /* Limit number of saved frames to CONFIG_STACKDEPOT_MAX_FRAMES. */
 393         if (nr_entries > CONFIG_STACKDEPOT_MAX_FRAMES)
 394                 nr_entries = CONFIG_STACKDEPOT_MAX_FRAMES;
 395
 396         if (flags & STACK_DEPOT_FLAG_GET) {
 397                 /*
 398                  * Evictable entries have to allocate the max. size so they may
 399                  * safely be re-used by differently sized allocations.
 400                  */
 401                 record_size = depot_stack_record_size(stack, CONFIG_STACKDEPOT_MAX_FRAMES);
 402                 stack = depot_pop_free();
 403         } else {
 404                 record_size = depot_stack_record_size(stack, nr_entries);
 405         }
 406
 407         if (!stack) {
 408                 stack = depot_pop_free_pool(prealloc, record_size);
 409                 if (!stack)
 410                         return NULL;
 411         }
 412
 413         /* Save the stack trace. */
 414         stack->hash = hash;
 415         stack->size = nr_entries;
 416         /* stack->handle is already filled in by depot_pop_free_pool(). */
 417         memcpy(stack->entries, entries, flex_array_size(stack, entries, nr_entries));
 418
 419         if (flags & STACK_DEPOT_FLAG_GET) {
 420                 refcount_set(&stack->count, 1);
 421                 counters[DEPOT_COUNTER_REFD_ALLOCS]++;
 422                 counters[DEPOT_COUNTER_REFD_INUSE]++;
 423         } else {
 424                 /* Warn on attempts to switch to refcounting this entry. */
 425                 refcount_set(&stack->count, REFCOUNT_SATURATED);
 426                 counters[DEPOT_COUNTER_PERSIST_COUNT]++;
 427                 counters[DEPOT_COUNTER_PERSIST_BYTES] += record_size;
 428         }
 429
 430         /*
 431          * Let KMSAN know the stored stack record is initialized. This shall
 432          * prevent false positive reports if instrumented code accesses it.
 433          */
 434         kmsan_unpoison_memory(stack, record_size);
 435
 436         return stack;
 437 }
 438
 439 static struct stack_record *depot_fetch_stack(depot_stack_handle_t handle)
 440 {
 441         const int pools_num_cached = READ_ONCE(pools_num);
 442         union handle_parts parts = { .handle = handle };
 443         void *pool;
 444         u32 pool_index = parts.pool_index_plus_1 - 1;
 445         size_t offset = parts.offset << DEPOT_STACK_ALIGN;
 446         struct stack_record *stack;
 447
 448         lockdep_assert_not_held(&pool_lock);
 449
 450         if (pool_index >= pools_num_cached) {
 451                 WARN(1, "pool index %d out of bounds (%d) for stack id %08x\n",
 452                      pool_index, pools_num_cached, handle);
 453                 return NULL;
 454         }
 455
 456         pool = stack_pools[pool_index];
 457         if (WARN_ON(!pool))
 458                 return NULL;
 459
 460         stack = pool + offset;
 461         if (WARN_ON(!refcount_read(&stack->count)))
 462                 return NULL;
 463
 464         return stack;
 465 }
 466
 467 /* Links stack into the freelist. */
 468 static void depot_free_stack(struct stack_record *stack)
 469 {
 470         unsigned long flags;
 471
 472         lockdep_assert_not_held(&pool_lock);
 473
 474         raw_spin_lock_irqsave(&pool_lock, flags);
 475         printk_deferred_enter();
 476
 477         /*
 478          * Remove the entry from the hash list. Concurrent list traversal may
 479          * still observe the entry, but since the refcount is zero, this entry
 480          * will no longer be considered as valid.
 481          */
 482         list_del_rcu(&stack->hash_list);
 483
 484         /*
 485          * Due to being used from constrained contexts such as the allocators,
 486          * NMI, or even RCU itself, stack depot cannot rely on primitives that
 487          * would sleep (such as synchronize_rcu()) or recursively call into
 488          * stack depot again (such as call_rcu()).
 489          *
 490          * Instead, get an RCU cookie, so that we can ensure this entry isn't
 491          * moved onto another list until the next grace period, and concurrent
 492          * RCU list traversal remains safe.
 493          */
 494         stack->rcu_state = get_state_synchronize_rcu();
 495
 496         /*
 497          * Add the entry to the freelist tail, so that older entries are
 498          * considered first - their RCU cookie is more likely to no longer be
 499          * associated with the current grace period.
 500          */
 501         list_add_tail(&stack->free_list, &free_stacks);
 502
 503         counters[DEPOT_COUNTER_FREELIST_SIZE]++;
 504         counters[DEPOT_COUNTER_REFD_FREES]++;
 505         counters[DEPOT_COUNTER_REFD_INUSE]--;
 506
 507         printk_deferred_exit();
 508         raw_spin_unlock_irqrestore(&pool_lock, flags);
 509 }
 510
 511 /* Calculates the hash for a stack. */
 512 static inline u32 hash_stack(unsigned long *entries, unsigned int size)
 513 {
 514         return jhash2((u32 *)entries,
 515                       array_size(size,  sizeof(*entries)) / sizeof(u32),
 516                       STACK_HASH_SEED);
 517 }
 518
 519 /*
 520  * Non-instrumented version of memcmp().
 521  * Does not check the lexicographical order, only the equality.
 522  */
 523 static inline
 524 int stackdepot_memcmp(const unsigned long *u1, const unsigned long *u2,
 525                         unsigned int n)
 526 {
 527         for ( ; n-- ; u1++, u2++) {
 528                 if (*u1 != *u2)
 529                         return 1;
 530         }
 531         return 0;
 532 }
 533
 534 /* Finds a stack in a bucket of the hash table. */
 535 static inline struct stack_record *find_stack(struct list_head *bucket,
 536                                               unsigned long *entries, int size,
 537                                               u32 hash, depot_flags_t flags)
 538 {
 539         struct stack_record *stack, *ret = NULL;
 540
 541         /*
 542          * Stack depot may be used from instrumentation that instruments RCU or
 543          * tracing itself; use variant that does not call into RCU and cannot be
 544          * traced.
 545          *
 546          * Note: Such use cases must take care when using refcounting to evict
 547          * unused entries, because the stack record free-then-reuse code paths
 548          * do call into RCU.
 549          */
 550         rcu_read_lock_sched_notrace();
 551
 552         list_for_each_entry_rcu(stack, bucket, hash_list) {
 553                 if (stack->hash != hash || stack->size != size)
 554                         continue;
 555
 556                 /*
 557                  * This may race with depot_free_stack() accessing the freelist
 558                  * management state unioned with @entries. The refcount is zero
 559                  * in that case and the below refcount_inc_not_zero() will fail.
 560                  */
 561                 if (data_race(stackdepot_memcmp(entries, stack->entries, size)))
 562                         continue;
 563
 564                 /*
 565                  * Try to increment refcount. If this succeeds, the stack record
 566                  * is valid and has not yet been freed.
 567                  *
 568                  * If STACK_DEPOT_FLAG_GET is not used, it is undefined behavior
 569                  * to then call stack_depot_put() later, and we can assume that
 570                  * a stack record is never placed back on the freelist.
 571                  */
 572                 if ((flags & STACK_DEPOT_FLAG_GET) && !refcount_inc_not_zero(&stack->count))
 573                         continue;
 574
 575                 ret = stack;
 576                 break;
 577         }
 578
 579         rcu_read_unlock_sched_notrace();
 580
 581         return ret;
 582 }
 583
 584 depot_stack_handle_t stack_depot_save_flags(unsigned long *entries,
 585                                             unsigned int nr_entries,
 586                                             gfp_t alloc_flags,
 587                                             depot_flags_t depot_flags)
 588 {
 589         struct list_head *bucket;
 590         struct stack_record *found = NULL;
 591         depot_stack_handle_t handle = 0;
 592         struct page *page = NULL;
 593         void *prealloc = NULL;
 594         bool can_alloc = depot_flags & STACK_DEPOT_FLAG_CAN_ALLOC;
 595         unsigned long flags;
 596         u32 hash;
 597
 598         if (WARN_ON(depot_flags & ~STACK_DEPOT_FLAGS_MASK))
 599                 return 0;
 600
 601         /*
 602          * If this stack trace is from an interrupt, including anything before
 603          * interrupt entry usually leads to unbounded stack depot growth.
 604          *
 605          * Since use of filter_irq_stacks() is a requirement to ensure stack
 606          * depot can efficiently deduplicate interrupt stacks, always
 607          * filter_irq_stacks() to simplify all callers' use of stack depot.
 608          */
 609         nr_entries = filter_irq_stacks(entries, nr_entries);
 610
 611         if (unlikely(nr_entries == 0) || stack_depot_disabled)
 612                 return 0;
 613
 614         hash = hash_stack(entries, nr_entries);
 615         bucket = &stack_table[hash & stack_hash_mask];
 616
 617         /* Fast path: look the stack trace up without locking. */
 618         found = find_stack(bucket, entries, nr_entries, hash, depot_flags);
 619         if (found)
 620                 goto exit;
 621
 622         /*
 623          * Allocate memory for a new pool if required now:
 624          * we won't be able to do that under the lock.
 625          */
 626         if (unlikely(can_alloc && !READ_ONCE(new_pool))) {
 627                 /*
 628                  * Zero out zone modifiers, as we don't have specific zone
 629                  * requirements. Keep the flags related to allocation in atomic
 630                  * contexts and I/O.
 631                  */
 632                 alloc_flags &= ~GFP_ZONEMASK;
 633                 alloc_flags &= (GFP_ATOMIC | GFP_KERNEL);
 634                 alloc_flags |= __GFP_NOWARN;
 635                 page = alloc_pages(alloc_flags, DEPOT_POOL_ORDER);
 636                 if (page)
 637                         prealloc = page_address(page);
 638         }
 639
 640         raw_spin_lock_irqsave(&pool_lock, flags);
 641         printk_deferred_enter();
 642
 643         /* Try to find again, to avoid concurrently inserting duplicates. */
 644         found = find_stack(bucket, entries, nr_entries, hash, depot_flags);
 645         if (!found) {
 646                 struct stack_record *new =
 647                         depot_alloc_stack(entries, nr_entries, hash, depot_flags, &prealloc);
 648
 649                 if (new) {
 650                         /*
 651                          * This releases the stack record into the bucket and
 652                          * makes it visible to readers in find_stack().
 653                          */
 654                         list_add_rcu(&new->hash_list, bucket);
 655                         found = new;
 656                 }
 657         }
 658
 659         if (prealloc) {
 660                 /*
 661                  * Either stack depot already contains this stack trace, or
 662                  * depot_alloc_stack() did not consume the preallocated memory.
 663                  * Try to keep the preallocated memory for future.
 664                  */
 665                 depot_keep_new_pool(&prealloc);
 666         }
 667
 668         printk_deferred_exit();
 669         raw_spin_unlock_irqrestore(&pool_lock, flags);
 670 exit:
 671         if (prealloc) {
 672                 /* Stack depot didn't use this memory, free it. */
 673                 free_pages((unsigned long)prealloc, DEPOT_POOL_ORDER);
 674         }
 675         if (found)
 676                 handle = found->handle.handle;
 677         return handle;
 678 }
 679 EXPORT_SYMBOL_GPL(stack_depot_save_flags);
 680
 681 depot_stack_handle_t stack_depot_save(unsigned long *entries,
 682                                       unsigned int nr_entries,
 683                                       gfp_t alloc_flags)
 684 {
 685         return stack_depot_save_flags(entries, nr_entries, alloc_flags,
 686                                       STACK_DEPOT_FLAG_CAN_ALLOC);
 687 }
 688 EXPORT_SYMBOL_GPL(stack_depot_save);
 689
 690 struct stack_record *__stack_depot_get_stack_record(depot_stack_handle_t handle)
 691 {
 692         if (!handle)
 693                 return NULL;
 694
 695         return depot_fetch_stack(handle);
 696 }
 697
 698 unsigned int stack_depot_fetch(depot_stack_handle_t handle,
 699                                unsigned long **entries)
 700 {
 701         struct stack_record *stack;
 702
 703         *entries = NULL;
 704         /*
 705          * Let KMSAN know *entries is initialized. This shall prevent false
 706          * positive reports if instrumented code accesses it.
 707          */
 708         kmsan_unpoison_memory(entries, sizeof(*entries));
 709
 710         if (!handle || stack_depot_disabled)
 711                 return 0;
 712
 713         stack = depot_fetch_stack(handle);
 714         /*
 715          * Should never be NULL, otherwise this is a use-after-put (or just a
 716          * corrupt handle).
 717          */
 718         if (WARN(!stack, "corrupt handle or use after stack_depot_put()"))
 719                 return 0;
 720
 721         *entries = stack->entries;
 722         return stack->size;
 723 }
 724 EXPORT_SYMBOL_GPL(stack_depot_fetch);
 725
 726 void stack_depot_put(depot_stack_handle_t handle)
 727 {
 728         struct stack_record *stack;
 729
 730         if (!handle || stack_depot_disabled)
 731                 return;
 732
 733         stack = depot_fetch_stack(handle);
 734         /*
 735          * Should always be able to find the stack record, otherwise this is an
 736          * unbalanced put attempt (or corrupt handle).
 737          */
 738         if (WARN(!stack, "corrupt handle or unbalanced stack_depot_put()"))
 739                 return;
 740
 741         if (refcount_dec_and_test(&stack->count))
 742                 depot_free_stack(stack);
 743 }
 744 EXPORT_SYMBOL_GPL(stack_depot_put);
 745
 746 void stack_depot_print(depot_stack_handle_t stack)
 747 {
 748         unsigned long *entries;
 749         unsigned int nr_entries;
 750
 751         nr_entries = stack_depot_fetch(stack, &entries);
 752         if (nr_entries > 0)
 753                 stack_trace_print(entries, nr_entries, 0);
 754 }
 755 EXPORT_SYMBOL_GPL(stack_depot_print);
 756
 757 int stack_depot_snprint(depot_stack_handle_t handle, char *buf, size_t size,
 758                        int spaces)
 759 {
 760         unsigned long *entries;
 761         unsigned int nr_entries;
 762
 763         nr_entries = stack_depot_fetch(handle, &entries);
 764         return nr_entries ? stack_trace_snprint(buf, size, entries, nr_entries,
 765                                                 spaces) : 0;
 766 }
 767 EXPORT_SYMBOL_GPL(stack_depot_snprint);
 768
 769 depot_stack_handle_t __must_check stack_depot_set_extra_bits(
 770                         depot_stack_handle_t handle, unsigned int extra_bits)
 771 {
 772         union handle_parts parts = { .handle = handle };
 773
 774         /* Don't set extra bits on empty handles. */
 775         if (!handle)
 776                 return 0;
 777
 778         parts.extra = extra_bits;
 779         return parts.handle;
 780 }
 781 EXPORT_SYMBOL(stack_depot_set_extra_bits);
 782
 783 unsigned int stack_depot_get_extra_bits(depot_stack_handle_t handle)
 784 {
 785         union handle_parts parts = { .handle = handle };
 786
 787         return parts.extra;
 788 }
 789 EXPORT_SYMBOL(stack_depot_get_extra_bits);
 790
 791 static int stats_show(struct seq_file *seq, void *v)
 792 {
 793         /*
 794          * data race ok: These are just statistics counters, and approximate
 795          * statistics are ok for debugging.
 796          */
 797         seq_printf(seq, "pools: %d\n", data_race(pools_num));
 798         for (int i = 0; i < DEPOT_COUNTER_COUNT; i++)
 799                 seq_printf(seq, "%s: %ld\n", counter_names[i], data_race(counters[i]));
 800
 801         return 0;
 802 }
 803 DEFINE_SHOW_ATTRIBUTE(stats);
 804
 805 static int depot_debugfs_init(void)
 806 {
 807         struct dentry *dir;
 808
 809         if (stack_depot_disabled)
 810                 return 0;
 811
 812         dir = debugfs_create_dir("stackdepot", NULL);
 813         debugfs_create_file("stats", 0444, dir, NULL, &stats_fops);
 814         return 0;
 815 }
 816 late_initcall(depot_debugfs_init);