third_party/tcmalloc/chromium/src/system-alloc.cc

// Copyright (c) 2005, Google Inc.
// All rights reserved.
// 
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
// 
//     * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
//     * Neither the name of Google Inc. nor the names of its
// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
// 
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

// ---
// Author: Sanjay Ghemawat

#include <config.h>
#include <errno.h>                      // for EAGAIN, errno
#include <fcntl.h>                      // for open, O_RDWR
#include <stddef.h>                     // for size_t, NULL, ptrdiff_t
#if defined HAVE_STDINT_H
#include <stdint.h>                     // for uintptr_t, intptr_t
#elif defined HAVE_INTTYPES_H
#include <inttypes.h>
#else
#include <sys/types.h>
#endif
#ifdef HAVE_MMAP
#include <sys/mman.h>                   // for munmap, mmap, MADV_DONTNEED, etc
#endif
#ifdef HAVE_UNISTD_H
#include <unistd.h>                     // for sbrk, getpagesize, off_t
#endif
#include <new>                          // for operator new
#include <gperftools/malloc_extension.h>
#include "base/basictypes.h"
#include "base/commandlineflags.h"
#include "base/spinlock.h"              // for SpinLockHolder, SpinLock, etc
#include "common.h"
#include "internal_logging.h"

// On systems (like freebsd) that don't define MAP_ANONYMOUS, use the old
// form of the name instead.
#ifndef MAP_ANONYMOUS
# define MAP_ANONYMOUS MAP_ANON
#endif

// MADV_FREE is specifically designed for use by malloc(), but only
// FreeBSD supports it; in linux we fall back to the somewhat inferior
// MADV_DONTNEED.
#if !defined(MADV_FREE) && defined(MADV_DONTNEED)
# define MADV_FREE  MADV_DONTNEED
#endif

// Solaris has a bug where it doesn't declare madvise() for C++.
//    http://www.opensolaris.org/jive/thread.jspa?threadID=21035&tstart=0
#if defined(__sun) && defined(__SVR4)
# include <sys/types.h>    // for caddr_t
  extern "C" { extern int madvise(caddr_t, size_t, int); }
#endif

// Set kDebugMode mode so that we can have use C++ conditionals
// instead of preprocessor conditionals.
#ifdef NDEBUG
static const bool kDebugMode = false;
#else
static const bool kDebugMode = true;
#endif

// TODO(sanjay): Move the code below into the tcmalloc namespace
using tcmalloc::kLog;
using tcmalloc::Log;

// Anonymous namespace to avoid name conflicts on "CheckAddressBits".
namespace {

// Check that no bit is set at position ADDRESS_BITS or higher.
template <int ADDRESS_BITS> bool CheckAddressBits(uintptr_t ptr) {
  return (ptr >> ADDRESS_BITS) == 0;
}

// Specialize for the bit width of a pointer to avoid undefined shift.
template <> bool CheckAddressBits<8 * sizeof(void*)>(uintptr_t ptr) {
  return true;
}

#if defined(OS_LINUX) && defined(__x86_64__)
#define ASLR_IS_SUPPORTED
#endif

#if defined(ASLR_IS_SUPPORTED)
// From libdieharder, public domain library by Bob Jenkins (rngav.c).
// Described at http://burtleburtle.net/bob/rand/smallprng.html.
// Not cryptographically secure, but good enough for what we need.
typedef uint32_t u4;
struct ranctx {
  u4 a;
  u4 b;
  u4 c;
  u4 d;
};

#define rot(x,k) (((x)<<(k))|((x)>>(32-(k))))

u4 ranval(ranctx* x) {
  /* xxx: the generator being tested */
  u4 e = x->a - rot(x->b, 27);
  x->a = x->b ^ rot(x->c, 17);
  x->b = x->c + x->d;
  x->c = x->d + e;
  x->d = e + x->a;
  return x->d;
}

void raninit(ranctx* x, u4 seed) {
  u4 i;
  x->a = 0xf1ea5eed;
  x->b = x->c = x->d = seed;
  for (i = 0; i < 20; ++i) {
    (void) ranval(x);
  }
}

// If the kernel cannot honor the hint in arch_get_unmapped_area_topdown, it
// will simply ignore it. So we give a hint that has a good chance of
// working.
// The mmap top-down allocator will normally allocate below TASK_SIZE - gap,
// with a gap that depends on the max stack size. See x86/mm/mmap.c. We
// should make allocations that are below this area, which would be
// 0x7ffbf8000000.
// We use 0x3ffffffff000 as the mask so that we only "pollute" half of the
// address space. In the unlikely case where fragmentation would become an
// issue, the kernel will still have another half to use.
const uint64_t kRandomAddressMask = 0x3ffffffff000ULL;

#endif  // defined(ASLR_IS_SUPPORTED)

// Give a random "hint" that is suitable for use with mmap(). This cannot make
// mmap fail, as the kernel will simply not follow the hint if it can't.
// However, this will create address space fragmentation.  Currently, we only
// implement it on x86_64, where we have a 47 bits userland address space and
// fragmentation is not an issue.
void* GetRandomAddrHint() {
#if !defined(ASLR_IS_SUPPORTED)
  return NULL;
#else
  // Note: we are protected by the general TCMalloc_SystemAlloc spinlock. Given
  // the nature of what we're doing, it wouldn't be critical if we weren't for
  // ctx, but it is for the "initialized" variable.
  // It's nice to share the state between threads, because scheduling will add
  // some randomness to the succession of ranval() calls.
  static ranctx ctx;
  static bool initialized = false;
  if (!initialized) {
    initialized = true;
    // We really want this to be a stack variable and don't want any compiler
    // optimization. We're using its address as a poor-man source of
    // randomness.
    volatile char c;
    // Pre-initialize our seed with a "random" address in case /dev/urandom is
    // not available.
    uint32_t seed = (reinterpret_cast<uint64_t>(&c) >> 32) ^
                    reinterpret_cast<uint64_t>(&c);
    int urandom_fd = open("/dev/urandom", O_RDONLY);
    if (urandom_fd >= 0) {
      ssize_t len;
      len = read(urandom_fd, &seed, sizeof(seed));
      ASSERT(len == sizeof(seed));
      int ret = close(urandom_fd);
      ASSERT(ret == 0);
    }
    raninit(&ctx, seed);
  }
  uint64_t random_address = (static_cast<uint64_t>(ranval(&ctx)) << 32) |
                            ranval(&ctx);
  // A a bit-wise "and" won't bias our random distribution.
  random_address &= kRandomAddressMask;
  return reinterpret_cast<void*>(random_address);
#endif  // ASLR_IS_SUPPORTED
}

// Allocate |length| bytes of memory using mmap(). The memory will be
// readable and writeable, but not executable.
// Like mmap(), we will return MAP_FAILED on failure.
// |is_aslr_enabled| controls address space layout randomization. When true, we
// will put the first mapping at a random address and will then try to grow it.
// If it's not possible to grow an existing mapping, a new one will be created.
void* AllocWithMmap(size_t length, bool is_aslr_enabled) {
  // Note: we are protected by the general TCMalloc_SystemAlloc spinlock.
  static void* address_hint = NULL;
#if defined(ASLR_IS_SUPPORTED)
  if (is_aslr_enabled &&
      (!address_hint ||
       reinterpret_cast<uint64_t>(address_hint) & ~kRandomAddressMask)) {
    address_hint = GetRandomAddrHint();
  }
#endif  // ASLR_IS_SUPPORTED

  // address_hint is likely to make us grow an existing mapping.
  void* result = mmap(address_hint, length, PROT_READ|PROT_WRITE,
                      MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
#if defined(ASLR_IS_SUPPORTED)
  if (result == address_hint) {
    // If mmap() succeeded at a address_hint, our next mmap() will try to grow
    // the current mapping as long as it's compatible with our ASLR mask.
    // This has been done for performance reasons, see crbug.com/173371.
    // It should be possible to strike a better balance between performance
    // and security but will be done at a later date.
    // If this overflows, it could only set address_hint to NULL, which is
    // what we want (and can't happen on the currently supported architecture).
    address_hint = static_cast<char*>(result) + length;
  } else {
    // mmap failed or a collision prevented the kernel from honoring the hint,
    // reset the hint.
    address_hint = NULL;
  }
#endif  // ASLR_IS_SUPPORTED
  return result;
}

}  // Anonymous namespace to avoid name conflicts on "CheckAddressBits".

COMPILE_ASSERT(kAddressBits <= 8 * sizeof(void*),
               address_bits_larger_than_pointer_size);

// Structure for discovering alignment
union MemoryAligner {
  void*  p;
  double d;
  size_t s;
} CACHELINE_ALIGNED;

static SpinLock spinlock(SpinLock::LINKER_INITIALIZED);

#if defined(HAVE_MMAP) || defined(MADV_FREE)
#ifdef HAVE_GETPAGESIZE
static size_t pagesize = 0;
#endif
#endif

// The current system allocator
SysAllocator* sys_alloc = NULL;

// Configuration parameters.
DEFINE_int32(malloc_devmem_start,
             EnvToInt("TCMALLOC_DEVMEM_START", 0),
             "Physical memory starting location in MB for /dev/mem allocation."
             "  Setting this to 0 disables /dev/mem allocation");
DEFINE_int32(malloc_devmem_limit,
             EnvToInt("TCMALLOC_DEVMEM_LIMIT", 0),
             "Physical memory limit location in MB for /dev/mem allocation."
             "  Setting this to 0 means no limit.");
DEFINE_bool(malloc_skip_sbrk,
            EnvToBool("TCMALLOC_SKIP_SBRK", false),
            "Whether sbrk can be used to obtain memory.");
DEFINE_bool(malloc_skip_mmap,
            EnvToBool("TCMALLOC_SKIP_MMAP", false),
            "Whether mmap can be used to obtain memory.");

DEFINE_bool(malloc_random_allocator,
#if defined(ASLR_IS_SUPPORTED)
            EnvToBool("TCMALLOC_ASLR", true),
#else
            EnvToBool("TCMALLOC_ASLR", false),
#endif
            "Whether to randomize the address space via mmap().");

// static allocators
class SbrkSysAllocator : public SysAllocator {
public:
  SbrkSysAllocator() : SysAllocator() {
  }
  void* Alloc(size_t size, size_t *actual_size, size_t alignment);
};
static char sbrk_space[sizeof(SbrkSysAllocator)];

class MmapSysAllocator : public SysAllocator {
public:
  MmapSysAllocator() : SysAllocator() {
  }
  void* Alloc(size_t size, size_t *actual_size, size_t alignment);
};
static char mmap_space[sizeof(MmapSysAllocator)];

class DevMemSysAllocator : public SysAllocator {
public:
  DevMemSysAllocator() : SysAllocator() {
  }
  void* Alloc(size_t size, size_t *actual_size, size_t alignment);
};

class DefaultSysAllocator : public SysAllocator {
 public:
  DefaultSysAllocator() : SysAllocator() {
    for (int i = 0; i < kMaxAllocators; i++) {
      failed_[i] = true;
      allocs_[i] = NULL;
      names_[i] = NULL;
    }
  }
  void SetChildAllocator(SysAllocator* alloc, unsigned int index,
                         const char* name) {
    if (index < kMaxAllocators && alloc != NULL) {
      allocs_[index] = alloc;
      failed_[index] = false;
      names_[index] = name;
    }
  }
  void* Alloc(size_t size, size_t *actual_size, size_t alignment);

 private:
  static const int kMaxAllocators = 2;
  bool failed_[kMaxAllocators];
  SysAllocator* allocs_[kMaxAllocators];
  const char* names_[kMaxAllocators];
};
static char default_space[sizeof(DefaultSysAllocator)];
static const char sbrk_name[] = "SbrkSysAllocator";
static const char mmap_name[] = "MmapSysAllocator";


void* SbrkSysAllocator::Alloc(size_t size, size_t *actual_size,
                              size_t alignment) {
#ifndef HAVE_SBRK
  return NULL;
#else
  // Check if we should use sbrk allocation.
  // FLAGS_malloc_skip_sbrk starts out as false (its uninitialized
  // state) and eventually gets initialized to the specified value.  Note
  // that this code runs for a while before the flags are initialized.
  // That means that even if this flag is set to true, some (initial)
  // memory will be allocated with sbrk before the flag takes effect.
  if (FLAGS_malloc_skip_sbrk) {
    return NULL;
  }

  // sbrk will release memory if passed a negative number, so we do
  // a strict check here
  if (static_cast<ptrdiff_t>(size + alignment) < 0) return NULL;

  // This doesn't overflow because TCMalloc_SystemAlloc has already
  // tested for overflow at the alignment boundary.
  size = ((size + alignment - 1) / alignment) * alignment;

  // "actual_size" indicates that the bytes from the returned pointer
  // p up to and including (p + actual_size - 1) have been allocated.
  if (actual_size) {
    *actual_size = size;
  }

  // Check that we we're not asking for so much more memory that we'd
  // wrap around the end of the virtual address space.  (This seems
  // like something sbrk() should check for us, and indeed opensolaris
  // does, but glibc does not:
  //    http://src.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/lib/libc/port/sys/sbrk.c?a=true
  //    http://sourceware.org/cgi-bin/cvsweb.cgi/~checkout~/libc/misc/sbrk.c?rev=1.1.2.1&content-type=text/plain&cvsroot=glibc
  // Without this check, sbrk may succeed when it ought to fail.)
  if (reinterpret_cast<intptr_t>(sbrk(0)) + size < size) {
    return NULL;
  }

  void* result = sbrk(size);
  if (result == reinterpret_cast<void*>(-1)) {
    return NULL;
  }

  // Is it aligned?
  uintptr_t ptr = reinterpret_cast<uintptr_t>(result);
  if ((ptr & (alignment-1)) == 0)  return result;

  // Try to get more memory for alignment
  size_t extra = alignment - (ptr & (alignment-1));
  void* r2 = sbrk(extra);
  if (reinterpret_cast<uintptr_t>(r2) == (ptr + size)) {
    // Contiguous with previous result
    return reinterpret_cast<void*>(ptr + extra);
  }

  // Give up and ask for "size + alignment - 1" bytes so
  // that we can find an aligned region within it.
  result = sbrk(size + alignment - 1);
  if (result == reinterpret_cast<void*>(-1)) {
    return NULL;
  }
  ptr = reinterpret_cast<uintptr_t>(result);
  if ((ptr & (alignment-1)) != 0) {
    ptr += alignment - (ptr & (alignment-1));
  }
  return reinterpret_cast<void*>(ptr);
#endif  // HAVE_SBRK
}

void* MmapSysAllocator::Alloc(size_t size, size_t *actual_size,
                              size_t alignment) {
#ifndef HAVE_MMAP
  return NULL;
#else
  // Check if we should use mmap allocation.
  // FLAGS_malloc_skip_mmap starts out as false (its uninitialized
  // state) and eventually gets initialized to the specified value.  Note
  // that this code runs for a while before the flags are initialized.
  // Chances are we never get here before the flags are initialized since
  // sbrk is used until the heap is exhausted (before mmap is used).
  if (FLAGS_malloc_skip_mmap) {
    return NULL;
  }

  // Enforce page alignment
  if (pagesize == 0) pagesize = getpagesize();
  if (alignment < pagesize) alignment = pagesize;
  size_t aligned_size = ((size + alignment - 1) / alignment) * alignment;
  if (aligned_size < size) {
    return NULL;
  }
  size = aligned_size;

  // "actual_size" indicates that the bytes from the returned pointer
  // p up to and including (p + actual_size - 1) have been allocated.
  if (actual_size) {
    *actual_size = size;
  }

  // Ask for extra memory if alignment > pagesize
  size_t extra = 0;
  if (alignment > pagesize) {
    extra = alignment - pagesize;
  }

  // Note: size + extra does not overflow since:
  //            size + alignment < (1<<NBITS).
  // and        extra <= alignment
  // therefore  size + extra < (1<<NBITS)
  void* result = AllocWithMmap(size + extra, FLAGS_malloc_random_allocator);
  if (result == reinterpret_cast<void*>(MAP_FAILED)) {
    return NULL;
  }

  // Adjust the return memory so it is aligned
  uintptr_t ptr = reinterpret_cast<uintptr_t>(result);
  size_t adjust = 0;
  if ((ptr & (alignment - 1)) != 0) {
    adjust = alignment - (ptr & (alignment - 1));
  }

  // Return the unused memory to the system
  if (adjust > 0) {
    munmap(reinterpret_cast<void*>(ptr), adjust);
  }
  if (adjust < extra) {
    munmap(reinterpret_cast<void*>(ptr + adjust + size), extra - adjust);
  }

  ptr += adjust;
  return reinterpret_cast<void*>(ptr);
#endif  // HAVE_MMAP
}

void* DevMemSysAllocator::Alloc(size_t size, size_t *actual_size,
                                size_t alignment) {
#ifndef HAVE_MMAP
  return NULL;
#else
  static bool initialized = false;
  static off_t physmem_base;  // next physical memory address to allocate
  static off_t physmem_limit; // maximum physical address allowed
  static int physmem_fd;      // file descriptor for /dev/mem

  // Check if we should use /dev/mem allocation.  Note that it may take
  // a while to get this flag initialized, so meanwhile we fall back to
  // the next allocator.  (It looks like 7MB gets allocated before
  // this flag gets initialized -khr.)
  if (FLAGS_malloc_devmem_start == 0) {
    // NOTE: not a devmem_failure - we'd like TCMalloc_SystemAlloc to
    // try us again next time.
    return NULL;
  }

  if (!initialized) {
    physmem_fd = open("/dev/mem", O_RDWR);
    if (physmem_fd < 0) {
      return NULL;
    }
    physmem_base = FLAGS_malloc_devmem_start*1024LL*1024LL;
    physmem_limit = FLAGS_malloc_devmem_limit*1024LL*1024LL;
    initialized = true;
  }

  // Enforce page alignment
  if (pagesize == 0) pagesize = getpagesize();
  if (alignment < pagesize) alignment = pagesize;
  size_t aligned_size = ((size + alignment - 1) / alignment) * alignment;
  if (aligned_size < size) {
    return NULL;
  }
  size = aligned_size;

  // "actual_size" indicates that the bytes from the returned pointer
  // p up to and including (p + actual_size - 1) have been allocated.
  if (actual_size) {
    *actual_size = size;
  }

  // Ask for extra memory if alignment > pagesize
  size_t extra = 0;
  if (alignment > pagesize) {
    extra = alignment - pagesize;
  }

  // check to see if we have any memory left
  if (physmem_limit != 0 &&
      ((size + extra) > (physmem_limit - physmem_base))) {
    return NULL;
  }

  // Note: size + extra does not overflow since:
  //            size + alignment < (1<<NBITS).
  // and        extra <= alignment
  // therefore  size + extra < (1<<NBITS)
  void *result = mmap(0, size + extra, PROT_WRITE|PROT_READ,
                      MAP_SHARED, physmem_fd, physmem_base);
  if (result == reinterpret_cast<void*>(MAP_FAILED)) {
    return NULL;
  }
  uintptr_t ptr = reinterpret_cast<uintptr_t>(result);

  // Adjust the return memory so it is aligned
  size_t adjust = 0;
  if ((ptr & (alignment - 1)) != 0) {
    adjust = alignment - (ptr & (alignment - 1));
  }

  // Return the unused virtual memory to the system
  if (adjust > 0) {
    munmap(reinterpret_cast<void*>(ptr), adjust);
  }
  if (adjust < extra) {
    munmap(reinterpret_cast<void*>(ptr + adjust + size), extra - adjust);
  }

  ptr += adjust;
  physmem_base += adjust + size;

  return reinterpret_cast<void*>(ptr);
#endif  // HAVE_MMAP
}

void* DefaultSysAllocator::Alloc(size_t size, size_t *actual_size,
                                 size_t alignment) {
  for (int i = 0; i < kMaxAllocators; i++) {
    if (!failed_[i] && allocs_[i] != NULL) {
      void* result = allocs_[i]->Alloc(size, actual_size, alignment);
      if (result != NULL) {
        return result;
      }
      failed_[i] = true;
    }
  }
  // After both failed, reset "failed_" to false so that a single failed
  // allocation won't make the allocator never work again.
  for (int i = 0; i < kMaxAllocators; i++) {
    failed_[i] = false;
  }
  return NULL;
}

static bool system_alloc_inited = false;
void InitSystemAllocators(void) {
  MmapSysAllocator *mmap = new (mmap_space) MmapSysAllocator();
  SbrkSysAllocator *sbrk = new (sbrk_space) SbrkSysAllocator();

  // In 64-bit debug mode, place the mmap allocator first since it
  // allocates pointers that do not fit in 32 bits and therefore gives
  // us better testing of code's 64-bit correctness.  It also leads to
  // less false negatives in heap-checking code.  (Numbers are less
  // likely to look like pointers and therefore the conservative gc in
  // the heap-checker is less likely to misinterpret a number as a
  // pointer).
  DefaultSysAllocator *sdef = new (default_space) DefaultSysAllocator();
  // Unfortunately, this code runs before flags are initialized. So
  // we can't use FLAGS_malloc_random_allocator.
#if defined(ASLR_IS_SUPPORTED)
  // Our only random allocator is mmap.
  sdef->SetChildAllocator(mmap, 0, mmap_name);
#else
  if (kDebugMode && sizeof(void*) > 4) {
    sdef->SetChildAllocator(mmap, 0, mmap_name);
    sdef->SetChildAllocator(sbrk, 1, sbrk_name);
  } else {
    sdef->SetChildAllocator(sbrk, 0, sbrk_name);
    sdef->SetChildAllocator(mmap, 1, mmap_name);
  }
#endif  // ASLR_IS_SUPPORTED
  sys_alloc = sdef;
}

void* TCMalloc_SystemAlloc(size_t size, size_t *actual_size,
                           size_t alignment) {
  // Discard requests that overflow
  if (size + alignment < size) return NULL;

  SpinLockHolder lock_holder(&spinlock);

  if (!system_alloc_inited) {
    InitSystemAllocators();
    system_alloc_inited = true;
  }

  // Enforce minimum alignment
  if (alignment < sizeof(MemoryAligner)) alignment = sizeof(MemoryAligner);

  void* result = sys_alloc->Alloc(size, actual_size, alignment);
  if (result != NULL) {
    if (actual_size) {
      CheckAddressBits<kAddressBits>(
          reinterpret_cast<uintptr_t>(result) + *actual_size - 1);
    } else {
      CheckAddressBits<kAddressBits>(
          reinterpret_cast<uintptr_t>(result) + size - 1);
    }
  }
  return result;
}

size_t TCMalloc_SystemAddGuard(void* start, size_t size) {
#ifdef HAVE_GETPAGESIZE
  if (pagesize == 0)
    pagesize = getpagesize();

  if (size < pagesize || (reinterpret_cast<size_t>(start) % pagesize) != 0)
    return 0;

  if (!mprotect(start, pagesize, PROT_NONE))
    return pagesize;
#endif

  return 0;
}

void TCMalloc_SystemRelease(void* start, size_t length) {
#ifdef MADV_FREE
  if (FLAGS_malloc_devmem_start) {
    // It's not safe to use MADV_FREE/MADV_DONTNEED if we've been
    // mapping /dev/mem for heap memory.
    return;
  }
  if (pagesize == 0) pagesize = getpagesize();
  const size_t pagemask = pagesize - 1;

  size_t new_start = reinterpret_cast<size_t>(start);
  size_t end = new_start + length;
  size_t new_end = end;

  // Round up the starting address and round down the ending address
  // to be page aligned:
  new_start = (new_start + pagesize - 1) & ~pagemask;
  new_end = new_end & ~pagemask;

  ASSERT((new_start & pagemask) == 0);
  ASSERT((new_end & pagemask) == 0);
  ASSERT(new_start >= reinterpret_cast<size_t>(start));
  ASSERT(new_end <= end);

  if (new_end > new_start) {
    // Note -- ignoring most return codes, because if this fails it
    // doesn't matter...
    while (madvise(reinterpret_cast<char*>(new_start), new_end - new_start,
                   MADV_FREE) == -1 &&
           errno == EAGAIN) {
      // NOP
    }
  }
#endif
}

void TCMalloc_SystemCommit(void* start, size_t length) {
  // Nothing to do here.  TCMalloc_SystemRelease does not alter pages
  // such that they need to be re-committed before they can be used by the
  // application.
}