about summary refs log tree commit diff
path: root/absl/base/internal/low_level_alloc.cc
blob: 5a8199e62ef1fb709b20817b06c137c9d7f2fd1c (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
// Copyright 2017 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// A low-level allocator that can be used by other low-level
// modules without introducing dependency cycles.
// This allocator is slow and wasteful of memory;
// it should not be used when performance is key.

#include "absl/base/internal/low_level_alloc.h"

#include <type_traits>

#include "absl/base/call_once.h"
#include "absl/base/config.h"
#include "absl/base/internal/direct_mmap.h"
#include "absl/base/internal/scheduling_mode.h"
#include "absl/base/macros.h"
#include "absl/base/thread_annotations.h"

// LowLevelAlloc requires that the platform support low-level
// allocation of virtual memory. Platforms lacking this cannot use
// LowLevelAlloc.
#ifndef ABSL_LOW_LEVEL_ALLOC_MISSING

#ifndef _WIN32
#include <pthread.h>
#include <signal.h>
#include <sys/mman.h>
#include <unistd.h>
#else
#include <windows.h>
#endif

#include <string.h>
#include <algorithm>
#include <atomic>
#include <cerrno>
#include <cstddef>
#include <new>                   // for placement-new

#include "absl/base/dynamic_annotations.h"
#include "absl/base/internal/raw_logging.h"
#include "absl/base/internal/spinlock.h"

// MAP_ANONYMOUS
#if defined(__APPLE__)
// For mmap, Linux defines both MAP_ANONYMOUS and MAP_ANON and says MAP_ANON is
// deprecated. In Darwin, MAP_ANON is all there is.
#if !defined MAP_ANONYMOUS
#define MAP_ANONYMOUS MAP_ANON
#endif  // !MAP_ANONYMOUS
#endif  // __APPLE__

namespace absl {
namespace base_internal {

// A first-fit allocator with amortized logarithmic free() time.

// ---------------------------------------------------------------------------
static const int kMaxLevel = 30;

namespace {
// This struct describes one allocated block, or one free block.
struct AllocList {
  struct Header {
    // Size of entire region, including this field. Must be
    // first. Valid in both allocated and unallocated blocks.
    uintptr_t size;

    // kMagicAllocated or kMagicUnallocated xor this.
    uintptr_t magic;

    // Pointer to parent arena.
    LowLevelAlloc::Arena *arena;

    // Aligns regions to 0 mod 2*sizeof(void*).
    void *dummy_for_alignment;
  } header;

  // Next two fields: in unallocated blocks: freelist skiplist data
  //                  in allocated blocks: overlaps with client data

  // Levels in skiplist used.
  int levels;

  // Actually has levels elements. The AllocList node may not have room
  // for all kMaxLevel entries. See max_fit in LLA_SkiplistLevels().
  AllocList *next[kMaxLevel];
};
}  // namespace

// ---------------------------------------------------------------------------
// A trivial skiplist implementation.  This is used to keep the freelist
// in address order while taking only logarithmic time per insert and delete.

// An integer approximation of log2(size/base)
// Requires size >= base.
static int IntLog2(size_t size, size_t base) {
  int result = 0;
  for (size_t i = size; i > base; i >>= 1) {  // i == floor(size/2**result)
    result++;
  }
  //    floor(size / 2**result) <= base < floor(size / 2**(result-1))
  // =>     log2(size/(base+1)) <= result < 1+log2(size/base)
  // => result ~= log2(size/base)
  return result;
}

// Return a random integer n:  p(n)=1/(2**n) if 1 <= n; p(n)=0 if n < 1.
static int Random(uint32_t *state) {
  uint32_t r = *state;
  int result = 1;
  while ((((r = r*1103515245 + 12345) >> 30) & 1) == 0) {
    result++;
  }
  *state = r;
  return result;
}

// Return a number of skiplist levels for a node of size bytes, where
// base is the minimum node size.  Compute level=log2(size / base)+n
// where n is 1 if random is false and otherwise a random number generated with
// the standard distribution for a skiplist:  See Random() above.
// Bigger nodes tend to have more skiplist levels due to the log2(size / base)
// term, so first-fit searches touch fewer nodes.  "level" is clipped so
// level<kMaxLevel and next[level-1] will fit in the node.
// 0 < LLA_SkiplistLevels(x,y,false) <= LLA_SkiplistLevels(x,y,true) < kMaxLevel
static int LLA_SkiplistLevels(size_t size, size_t base, uint32_t *random) {
  // max_fit is the maximum number of levels that will fit in a node for the
  // given size.   We can't return more than max_fit, no matter what the
  // random number generator says.
  size_t max_fit = (size - offsetof(AllocList, next)) / sizeof(AllocList *);
  int level = IntLog2(size, base) + (random != nullptr ? Random(random) : 1);
  if (static_cast<size_t>(level) > max_fit) level = static_cast<int>(max_fit);
  if (level > kMaxLevel-1) level = kMaxLevel - 1;
  ABSL_RAW_CHECK(level >= 1, "block not big enough for even one level");
  return level;
}

// Return "atleast", the first element of AllocList *head s.t. *atleast >= *e.
// For 0 <= i < head->levels, set prev[i] to "no_greater", where no_greater
// points to the last element at level i in the AllocList less than *e, or is
// head if no such element exists.
static AllocList *LLA_SkiplistSearch(AllocList *head,
                                     AllocList *e, AllocList **prev) {
  AllocList *p = head;
  for (int level = head->levels - 1; level >= 0; level--) {
    for (AllocList *n; (n = p->next[level]) != nullptr && n < e; p = n) {
    }
    prev[level] = p;
  }
  return (head->levels == 0) ? nullptr : prev[0]->next[0];
}

// Insert element *e into AllocList *head.  Set prev[] as LLA_SkiplistSearch.
// Requires that e->levels be previously set by the caller (using
// LLA_SkiplistLevels())
static void LLA_SkiplistInsert(AllocList *head, AllocList *e,
                               AllocList **prev) {
  LLA_SkiplistSearch(head, e, prev);
  for (; head->levels < e->levels; head->levels++) {  // extend prev pointers
    prev[head->levels] = head;                        // to all *e's levels
  }
  for (int i = 0; i != e->levels; i++) {  // add element to list
    e->next[i] = prev[i]->next[i];
    prev[i]->next[i] = e;
  }
}

// Remove element *e from AllocList *head.  Set prev[] as LLA_SkiplistSearch().
// Requires that e->levels be previous set by the caller (using
// LLA_SkiplistLevels())
static void LLA_SkiplistDelete(AllocList *head, AllocList *e,
                               AllocList **prev) {
  AllocList *found = LLA_SkiplistSearch(head, e, prev);
  ABSL_RAW_CHECK(e == found, "element not in freelist");
  for (int i = 0; i != e->levels && prev[i]->next[i] == e; i++) {
    prev[i]->next[i] = e->next[i];
  }
  while (head->levels > 0 && head->next[head->levels - 1] == nullptr) {
    head->levels--;   // reduce head->levels if level unused
  }
}

// ---------------------------------------------------------------------------
// Arena implementation

// Metadata for an LowLevelAlloc arena instance.
struct LowLevelAlloc::Arena {
  // Constructs an arena with the given LowLevelAlloc flags.
  explicit Arena(uint32_t flags_value);

  base_internal::SpinLock mu;
  // Head of free list, sorted by address
  AllocList freelist GUARDED_BY(mu);
  // Count of allocated blocks
  int32_t allocation_count GUARDED_BY(mu);
  // flags passed to NewArena
  const uint32_t flags;
  // Result of sysconf(_SC_PAGESIZE)
  const size_t pagesize;
  // Lowest power of two >= max(16, sizeof(AllocList))
  const size_t roundup;
  // Smallest allocation block size
  const size_t min_size;
  // PRNG state
  uint32_t random GUARDED_BY(mu);
};

namespace {
using ArenaStorage = std::aligned_storage<sizeof(LowLevelAlloc::Arena),
                                          alignof(LowLevelAlloc::Arena)>::type;

// Static storage space for the lazily-constructed, default global arena
// instances.  We require this space because the whole point of LowLevelAlloc
// is to avoid relying on malloc/new.
ArenaStorage default_arena_storage;
ArenaStorage unhooked_arena_storage;
#ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
ArenaStorage unhooked_async_sig_safe_arena_storage;
#endif

// We must use LowLevelCallOnce here to construct the global arenas, rather than
// using function-level statics, to avoid recursively invoking the scheduler.
absl::once_flag create_globals_once;

void CreateGlobalArenas() {
  new (&default_arena_storage)
      LowLevelAlloc::Arena(LowLevelAlloc::kCallMallocHook);
  new (&unhooked_arena_storage) LowLevelAlloc::Arena(0);
#ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
  new (&unhooked_async_sig_safe_arena_storage)
      LowLevelAlloc::Arena(LowLevelAlloc::kAsyncSignalSafe);
#endif
}

// Returns a global arena that does not call into hooks.  Used by NewArena()
// when kCallMallocHook is not set.
LowLevelAlloc::Arena* UnhookedArena() {
  base_internal::LowLevelCallOnce(&create_globals_once, CreateGlobalArenas);
  return reinterpret_cast<LowLevelAlloc::Arena*>(&unhooked_arena_storage);
}

#ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
// Returns a global arena that is async-signal safe.  Used by NewArena() when
// kAsyncSignalSafe is set.
LowLevelAlloc::Arena *UnhookedAsyncSigSafeArena() {
  base_internal::LowLevelCallOnce(&create_globals_once, CreateGlobalArenas);
  return reinterpret_cast<LowLevelAlloc::Arena *>(
      &unhooked_async_sig_safe_arena_storage);
}
#endif

}  // namespace

// Returns the default arena, as used by LowLevelAlloc::Alloc() and friends.
LowLevelAlloc::Arena *LowLevelAlloc::DefaultArena() {
  base_internal::LowLevelCallOnce(&create_globals_once, CreateGlobalArenas);
  return reinterpret_cast<LowLevelAlloc::Arena*>(&default_arena_storage);
}

// magic numbers to identify allocated and unallocated blocks
static const uintptr_t kMagicAllocated = 0x4c833e95U;
static const uintptr_t kMagicUnallocated = ~kMagicAllocated;

namespace {
class SCOPED_LOCKABLE ArenaLock {
 public:
  explicit ArenaLock(LowLevelAlloc::Arena *arena)
      EXCLUSIVE_LOCK_FUNCTION(arena->mu)
      : arena_(arena) {
#ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
    if ((arena->flags & LowLevelAlloc::kAsyncSignalSafe) != 0) {
      sigset_t all;
      sigfillset(&all);
      mask_valid_ = pthread_sigmask(SIG_BLOCK, &all, &mask_) == 0;
    }
#endif
    arena_->mu.Lock();
  }
  ~ArenaLock() { ABSL_RAW_CHECK(left_, "haven't left Arena region"); }
  void Leave() UNLOCK_FUNCTION() {
    arena_->mu.Unlock();
#ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
    if (mask_valid_) {
      pthread_sigmask(SIG_SETMASK, &mask_, nullptr);
    }
#endif
    left_ = true;
  }

 private:
  bool left_ = false;  // whether left region
#ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
  bool mask_valid_ = false;
  sigset_t mask_;  // old mask of blocked signals
#endif
  LowLevelAlloc::Arena *arena_;
  ArenaLock(const ArenaLock &) = delete;
  ArenaLock &operator=(const ArenaLock &) = delete;
};
}  // namespace

// create an appropriate magic number for an object at "ptr"
// "magic" should be kMagicAllocated or kMagicUnallocated
inline static uintptr_t Magic(uintptr_t magic, AllocList::Header *ptr) {
  return magic ^ reinterpret_cast<uintptr_t>(ptr);
}

namespace {
size_t GetPageSize() {
#ifdef _WIN32
  SYSTEM_INFO system_info;
  GetSystemInfo(&system_info);
  return std::max(system_info.dwPageSize, system_info.dwAllocationGranularity);
#elif defined(__wasm__) || defined(__asmjs__)
  return getpagesize();
#else
  return sysconf(_SC_PAGESIZE);
#endif
}

size_t RoundedUpBlockSize() {
  // Round up block sizes to a power of two close to the header size.
  size_t roundup = 16;
  while (roundup < sizeof(AllocList::Header)) {
    roundup += roundup;
  }
  return roundup;
}

}  // namespace

LowLevelAlloc::Arena::Arena(uint32_t flags_value)
    : mu(base_internal::SCHEDULE_KERNEL_ONLY),
      allocation_count(0),
      flags(flags_value),
      pagesize(GetPageSize()),
      roundup(RoundedUpBlockSize()),
      min_size(2 * roundup),
      random(0) {
  freelist.header.size = 0;
  freelist.header.magic =
      Magic(kMagicUnallocated, &freelist.header);
  freelist.header.arena = this;
  freelist.levels = 0;
  memset(freelist.next, 0, sizeof(freelist.next));
}

// L < meta_data_arena->mu
LowLevelAlloc::Arena *LowLevelAlloc::NewArena(int32_t flags) {
  Arena *meta_data_arena = DefaultArena();
#ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
  if ((flags & LowLevelAlloc::kAsyncSignalSafe) != 0) {
    meta_data_arena = UnhookedAsyncSigSafeArena();
  } else  // NOLINT(readability/braces)
#endif
      if ((flags & LowLevelAlloc::kCallMallocHook) == 0) {
    meta_data_arena = UnhookedArena();
  }
  Arena *result =
    new (AllocWithArena(sizeof (*result), meta_data_arena)) Arena(flags);
  return result;
}

// L < arena->mu, L < arena->arena->mu
bool LowLevelAlloc::DeleteArena(Arena *arena) {
  ABSL_RAW_CHECK(
      arena != nullptr && arena != DefaultArena() && arena != UnhookedArena(),
      "may not delete default arena");
  ArenaLock section(arena);
  if (arena->allocation_count != 0) {
    section.Leave();
    return false;
  }
  while (arena->freelist.next[0] != nullptr) {
    AllocList *region = arena->freelist.next[0];
    size_t size = region->header.size;
    arena->freelist.next[0] = region->next[0];
    ABSL_RAW_CHECK(
        region->header.magic == Magic(kMagicUnallocated, &region->header),
        "bad magic number in DeleteArena()");
    ABSL_RAW_CHECK(region->header.arena == arena,
                   "bad arena pointer in DeleteArena()");
    ABSL_RAW_CHECK(size % arena->pagesize == 0,
                   "empty arena has non-page-aligned block size");
    ABSL_RAW_CHECK(reinterpret_cast<uintptr_t>(region) % arena->pagesize == 0,
                   "empty arena has non-page-aligned block");
    int munmap_result;
#ifdef _WIN32
    munmap_result = VirtualFree(region, 0, MEM_RELEASE);
    ABSL_RAW_CHECK(munmap_result != 0,
                   "LowLevelAlloc::DeleteArena: VitualFree failed");
#else
#ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
    if ((arena->flags & LowLevelAlloc::kAsyncSignalSafe) == 0) {
      munmap_result = munmap(region, size);
    } else {
      munmap_result = base_internal::DirectMunmap(region, size);
    }
#else
    munmap_result = munmap(region, size);
#endif  // ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
    if (munmap_result != 0) {
      ABSL_RAW_LOG(FATAL, "LowLevelAlloc::DeleteArena: munmap failed: %d",
                   errno);
    }
#endif  // _WIN32
  }
  section.Leave();
  arena->~Arena();
  Free(arena);
  return true;
}

// ---------------------------------------------------------------------------

// Addition, checking for overflow.  The intent is to die if an external client
// manages to push through a request that would cause arithmetic to fail.
static inline uintptr_t CheckedAdd(uintptr_t a, uintptr_t b) {
  uintptr_t sum = a + b;
  ABSL_RAW_CHECK(sum >= a, "LowLevelAlloc arithmetic overflow");
  return sum;
}

// Return value rounded up to next multiple of align.
// align must be a power of two.
static inline uintptr_t RoundUp(uintptr_t addr, uintptr_t align) {
  return CheckedAdd(addr, align - 1) & ~(align - 1);
}

// Equivalent to "return prev->next[i]" but with sanity checking
// that the freelist is in the correct order, that it
// consists of regions marked "unallocated", and that no two regions
// are adjacent in memory (they should have been coalesced).
// L < arena->mu
static AllocList *Next(int i, AllocList *prev, LowLevelAlloc::Arena *arena) {
  ABSL_RAW_CHECK(i < prev->levels, "too few levels in Next()");
  AllocList *next = prev->next[i];
  if (next != nullptr) {
    ABSL_RAW_CHECK(
        next->header.magic == Magic(kMagicUnallocated, &next->header),
        "bad magic number in Next()");
    ABSL_RAW_CHECK(next->header.arena == arena, "bad arena pointer in Next()");
    if (prev != &arena->freelist) {
      ABSL_RAW_CHECK(prev < next, "unordered freelist");
      ABSL_RAW_CHECK(reinterpret_cast<char *>(prev) + prev->header.size <
                         reinterpret_cast<char *>(next),
                     "malformed freelist");
    }
  }
  return next;
}

// Coalesce list item "a" with its successor if they are adjacent.
static void Coalesce(AllocList *a) {
  AllocList *n = a->next[0];
  if (n != nullptr && reinterpret_cast<char *>(a) + a->header.size ==
                          reinterpret_cast<char *>(n)) {
    LowLevelAlloc::Arena *arena = a->header.arena;
    a->header.size += n->header.size;
    n->header.magic = 0;
    n->header.arena = nullptr;
    AllocList *prev[kMaxLevel];
    LLA_SkiplistDelete(&arena->freelist, n, prev);
    LLA_SkiplistDelete(&arena->freelist, a, prev);
    a->levels = LLA_SkiplistLevels(a->header.size, arena->min_size,
                                   &arena->random);
    LLA_SkiplistInsert(&arena->freelist, a, prev);
  }
}

// Adds block at location "v" to the free list
// L >= arena->mu
static void AddToFreelist(void *v, LowLevelAlloc::Arena *arena) {
  AllocList *f = reinterpret_cast<AllocList *>(
                        reinterpret_cast<char *>(v) - sizeof (f->header));
  ABSL_RAW_CHECK(f->header.magic == Magic(kMagicAllocated, &f->header),
                 "bad magic number in AddToFreelist()");
  ABSL_RAW_CHECK(f->header.arena == arena,
                 "bad arena pointer in AddToFreelist()");
  f->levels = LLA_SkiplistLevels(f->header.size, arena->min_size,
                                 &arena->random);
  AllocList *prev[kMaxLevel];
  LLA_SkiplistInsert(&arena->freelist, f, prev);
  f->header.magic = Magic(kMagicUnallocated, &f->header);
  Coalesce(f);                  // maybe coalesce with successor
  Coalesce(prev[0]);            // maybe coalesce with predecessor
}

// Frees storage allocated by LowLevelAlloc::Alloc().
// L < arena->mu
void LowLevelAlloc::Free(void *v) {
  if (v != nullptr) {
    AllocList *f = reinterpret_cast<AllocList *>(
                        reinterpret_cast<char *>(v) - sizeof (f->header));
    ABSL_RAW_CHECK(f->header.magic == Magic(kMagicAllocated, &f->header),
                   "bad magic number in Free()");
    LowLevelAlloc::Arena *arena = f->header.arena;
    ArenaLock section(arena);
    AddToFreelist(v, arena);
    ABSL_RAW_CHECK(arena->allocation_count > 0, "nothing in arena to free");
    arena->allocation_count--;
    section.Leave();
  }
}

// allocates and returns a block of size bytes, to be freed with Free()
// L < arena->mu
static void *DoAllocWithArena(size_t request, LowLevelAlloc::Arena *arena) {
  void *result = nullptr;
  if (request != 0) {
    AllocList *s;       // will point to region that satisfies request
    ArenaLock section(arena);
    // round up with header
    size_t req_rnd = RoundUp(CheckedAdd(request, sizeof (s->header)),
                             arena->roundup);
    for (;;) {      // loop until we find a suitable region
      // find the minimum levels that a block of this size must have
      int i = LLA_SkiplistLevels(req_rnd, arena->min_size, nullptr) - 1;
      if (i < arena->freelist.levels) {   // potential blocks exist
        AllocList *before = &arena->freelist;  // predecessor of s
        while ((s = Next(i, before, arena)) != nullptr &&
               s->header.size < req_rnd) {
          before = s;
        }
        if (s != nullptr) {       // we found a region
          break;
        }
      }
      // we unlock before mmap() both because mmap() may call a callback hook,
      // and because it may be slow.
      arena->mu.Unlock();
      // mmap generous 64K chunks to decrease
      // the chances/impact of fragmentation:
      size_t new_pages_size = RoundUp(req_rnd, arena->pagesize * 16);
      void *new_pages;
#ifdef _WIN32
      new_pages = VirtualAlloc(0, new_pages_size,
                               MEM_RESERVE | MEM_COMMIT, PAGE_READWRITE);
      ABSL_RAW_CHECK(new_pages != nullptr, "VirtualAlloc failed");
#else
#ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
      if ((arena->flags & LowLevelAlloc::kAsyncSignalSafe) != 0) {
        new_pages = base_internal::DirectMmap(nullptr, new_pages_size,
            PROT_WRITE|PROT_READ, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
      } else {
        new_pages = mmap(nullptr, new_pages_size, PROT_WRITE | PROT_READ,
                         MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
      }
#else
      new_pages = mmap(nullptr, new_pages_size, PROT_WRITE | PROT_READ,
                       MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
#endif  // ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
      if (new_pages == MAP_FAILED) {
        ABSL_RAW_LOG(FATAL, "mmap error: %d", errno);
      }

#endif  // _WIN32
      arena->mu.Lock();
      s = reinterpret_cast<AllocList *>(new_pages);
      s->header.size = new_pages_size;
      // Pretend the block is allocated; call AddToFreelist() to free it.
      s->header.magic = Magic(kMagicAllocated, &s->header);
      s->header.arena = arena;
      AddToFreelist(&s->levels, arena);  // insert new region into free list
    }
    AllocList *prev[kMaxLevel];
    LLA_SkiplistDelete(&arena->freelist, s, prev);    // remove from free list
    // s points to the first free region that's big enough
    if (CheckedAdd(req_rnd, arena->min_size) <= s->header.size) {
      // big enough to split
      AllocList *n = reinterpret_cast<AllocList *>
                        (req_rnd + reinterpret_cast<char *>(s));
      n->header.size = s->header.size - req_rnd;
      n->header.magic = Magic(kMagicAllocated, &n->header);
      n->header.arena = arena;
      s->header.size = req_rnd;
      AddToFreelist(&n->levels, arena);
    }
    s->header.magic = Magic(kMagicAllocated, &s->header);
    ABSL_RAW_CHECK(s->header.arena == arena, "");
    arena->allocation_count++;
    section.Leave();
    result = &s->levels;
  }
  ANNOTATE_MEMORY_IS_UNINITIALIZED(result, request);
  return result;
}

void *LowLevelAlloc::Alloc(size_t request) {
  void *result = DoAllocWithArena(request, DefaultArena());
  return result;
}

void *LowLevelAlloc::AllocWithArena(size_t request, Arena *arena) {
  ABSL_RAW_CHECK(arena != nullptr, "must pass a valid arena");
  void *result = DoAllocWithArena(request, arena);
  return result;
}

}  // namespace base_internal
}  // namespace absl

#endif  // ABSL_LOW_LEVEL_ALLOC_MISSING