runtime/mem_map.cc - platform/art - Gitiles

 /*
  * Copyright (C) 2008 The Android Open Source Project
  *
  * 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
  *
  *      http://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.
  */

 #include "mem_map.h"
 #include "thread-inl.h"

 #include <inttypes.h>
 #include <backtrace/BacktraceMap.h>
 #include <memory>

 // See CreateStartPos below.
 #ifdef __BIONIC__
 #include <sys/auxv.h>
 #endif

 #include "base/stringprintf.h"
 #include "ScopedFd.h"
 #include "utils.h"

 #define USE_ASHMEM 1

 #ifdef USE_ASHMEM
 #include <cutils/ashmem.h>
 #endif

 namespace art {

 static std::ostream& operator<<(
     std::ostream& os,
     std::pair<BacktraceMap::const_iterator, BacktraceMap::const_iterator> iters) {
   for (BacktraceMap::const_iterator it = iters.first; it != iters.second; ++it) {
     os << StringPrintf("0x%08x-0x%08x %c%c%c %s\n",
                        static_cast<uint32_t>(it->start),
                        static_cast<uint32_t>(it->end),
                        (it->flags & PROT_READ) ? 'r' : '-',
                        (it->flags & PROT_WRITE) ? 'w' : '-',
                        (it->flags & PROT_EXEC) ? 'x' : '-', it->name.c_str());
   }
   return os;
 }

 std::ostream& operator<<(std::ostream& os, const std::multimap<void*, MemMap*>& mem_maps) {
   os << "MemMap:" << std::endl;
   for (auto it = mem_maps.begin(); it != mem_maps.end(); ++it) {
     void* base = it->first;
     MemMap* map = it->second;
     CHECK_EQ(base, map->BaseBegin());
     os << *map << std::endl;
   }
   return os;
 }

 std::multimap<void*, MemMap*> MemMap::maps_;

 #if defined(__LP64__) && !defined(__x86_64__)
 // Handling mem_map in 32b address range for 64b architectures that do not support MAP_32BIT.

 // The regular start of memory allocations. The first 64KB is protected by SELinux.
 static constexpr uintptr_t LOW_MEM_START = 64 * KB;

 // Generate random starting position.
 // To not interfere with image position, take the image's address and only place it below. Current
 // formula (sketch):
 //
 // ART_BASE_ADDR      = 0001XXXXXXXXXXXXXXX
 // ----------------------------------------
 //                    = 0000111111111111111
 // & ~(kPageSize - 1) =~0000000000000001111
 // ----------------------------------------
 // mask               = 0000111111111110000
 // & random data      = YYYYYYYYYYYYYYYYYYY
 // -----------------------------------
 // tmp                = 0000YYYYYYYYYYY0000
 // + LOW_MEM_START    = 0000000000001000000
 // --------------------------------------
 // start
 //
 // getauxval as an entropy source is exposed in Bionic, but not in glibc before 2.16. When we
 // do not have Bionic, simply start with LOW_MEM_START.

 // Function is standalone so it can be tested somewhat in mem_map_test.cc.
 #ifdef __BIONIC__
 uintptr_t CreateStartPos(uint64_t input) {
   CHECK_NE(0, ART_BASE_ADDRESS);

   // Start with all bits below highest bit in ART_BASE_ADDRESS.
   constexpr size_t leading_zeros = CLZ(static_cast<uint32_t>(ART_BASE_ADDRESS));
   constexpr uintptr_t mask_ones = (1 << (31 - leading_zeros)) - 1;

   // Lowest (usually 12) bits are not used, as aligned by page size.
   constexpr uintptr_t mask = mask_ones & ~(kPageSize - 1);

   // Mask input data.
   return (input & mask) + LOW_MEM_START;
 }
 #endif

 static uintptr_t GenerateNextMemPos() {
 #ifdef __BIONIC__
   uint8_t* random_data = reinterpret_cast<uint8_t*>(getauxval(AT_RANDOM));
   // The lower 8B are taken for the stack guard. Use the upper 8B (with mask).
   return CreateStartPos(*reinterpret_cast<uintptr_t*>(random_data + 8));
 #else
   // No auxv on host, see above.
   return LOW_MEM_START;
 #endif
 }

 // Initialize linear scan to random position.
 uintptr_t MemMap::next_mem_pos_ = GenerateNextMemPos();
 #endif

 static bool CheckMapRequest(byte* expected_ptr, void* actual_ptr, size_t byte_count,
                             std::ostringstream* error_msg) {
   // Handled first by caller for more specific error messages.
   CHECK(actual_ptr != MAP_FAILED);

   if (expected_ptr == nullptr) {
     return true;
   }

   if (expected_ptr == actual_ptr) {
     return true;
   }

   // We asked for an address but didn't get what we wanted, all paths below here should fail.
   int result = munmap(actual_ptr, byte_count);
   if (result == -1) {
     PLOG(WARNING) << StringPrintf("munmap(%p, %zd) failed", actual_ptr, byte_count);
   }

   uintptr_t actual = reinterpret_cast<uintptr_t>(actual_ptr);
   uintptr_t expected = reinterpret_cast<uintptr_t>(expected_ptr);
   uintptr_t limit = expected + byte_count;

   std::unique_ptr<BacktraceMap> map(BacktraceMap::Create(getpid()));
   if (!map->Build()) {
     *error_msg << StringPrintf("Failed to build process map to determine why mmap returned "
                                "0x%08" PRIxPTR " instead of 0x%08" PRIxPTR, actual, expected);

     return false;
   }
   for (BacktraceMap::const_iterator it = map->begin(); it != map->end(); ++it) {
     if ((expected >= it->start && expected < it->end)  // start of new within old
         || (limit > it->start && limit < it->end)      // end of new within old
         || (expected <= it->start && limit > it->end)) {  // start/end of new includes all of old
       *error_msg
           << StringPrintf("Requested region 0x%08" PRIxPTR "-0x%08" PRIxPTR " overlaps with "
                           "existing map 0x%08" PRIxPTR "-0x%08" PRIxPTR " (%s)\n",
                           expected, limit,
                           static_cast<uintptr_t>(it->start), static_cast<uintptr_t>(it->end),
                           it->name.c_str())
           << std::make_pair(it, map->end());
       return false;
     }
   }
   *error_msg << StringPrintf("Failed to mmap at expected address, mapped at "
                              "0x%08" PRIxPTR " instead of 0x%08" PRIxPTR, actual, expected);
   return false;
 }

 MemMap* MemMap::MapAnonymous(const char* name, byte* expected, size_t byte_count, int prot,
                              bool low_4gb, std::string* error_msg) {
   if (byte_count == 0) {
     return new MemMap(name, nullptr, 0, nullptr, 0, prot);
   }
   size_t page_aligned_byte_count = RoundUp(byte_count, kPageSize);

 #ifdef USE_ASHMEM
   // android_os_Debug.cpp read_mapinfo assumes all ashmem regions associated with the VM are
   // prefixed "dalvik-".
   std::string debug_friendly_name("dalvik-");
   debug_friendly_name += name;
   ScopedFd fd(ashmem_create_region(debug_friendly_name.c_str(), page_aligned_byte_count));
   if (fd.get() == -1) {
     *error_msg = StringPrintf("ashmem_create_region failed for '%s': %s", name, strerror(errno));
     return nullptr;
   }
   int flags = MAP_PRIVATE;
 #else
   ScopedFd fd(-1);
   int flags = MAP_PRIVATE | MAP_ANONYMOUS;
 #endif

   // We need to store and potentially set an error number for pretty printing of errors
   int saved_errno = 0;

 #ifdef __LP64__
   // When requesting low_4g memory and having an expectation, the requested range should fit into
   // 4GB.
   if (low_4gb && (
       // Start out of bounds.
       (reinterpret_cast<uintptr_t>(expected) >> 32) != 0 ||
       // End out of bounds. For simplicity, this will fail for the last page of memory.
       (reinterpret_cast<uintptr_t>(expected + page_aligned_byte_count) >> 32) != 0)) {
     *error_msg = StringPrintf("The requested address space (%p, %p) cannot fit in low_4gb",
                               expected, expected + page_aligned_byte_count);
     return nullptr;
   }
 #endif

   // TODO:
   // A page allocator would be a useful abstraction here, as
   // 1) It is doubtful that MAP_32BIT on x86_64 is doing the right job for us
   // 2) The linear scheme, even with simple saving of the last known position, is very crude
 #if defined(__LP64__) && !defined(__x86_64__)
   // MAP_32BIT only available on x86_64.
   void* actual = MAP_FAILED;
   if (low_4gb && expected == nullptr) {
     bool first_run = true;

     for (uintptr_t ptr = next_mem_pos_; ptr < 4 * GB; ptr += kPageSize) {
       if (4U * GB - ptr < page_aligned_byte_count) {
         // Not enough memory until 4GB.
         if (first_run) {
           // Try another time from the bottom;
           ptr = LOW_MEM_START - kPageSize;
           first_run = false;
           continue;
         } else {
           // Second try failed.
           break;
         }
       }

       uintptr_t tail_ptr;

       // Check pages are free.
       bool safe = true;
       for (tail_ptr = ptr; tail_ptr < ptr + page_aligned_byte_count; tail_ptr += kPageSize) {
         if (msync(reinterpret_cast<void*>(tail_ptr), kPageSize, 0) == 0) {
           safe = false;
           break;
         } else {
           DCHECK_EQ(errno, ENOMEM);
         }
       }

       next_mem_pos_ = tail_ptr;  // update early, as we break out when we found and mapped a region

       if (safe == true) {
         actual = mmap(reinterpret_cast<void*>(ptr), page_aligned_byte_count, prot, flags, fd.get(),
                       0);
         if (actual != MAP_FAILED) {
           // Since we didn't use MAP_FIXED the kernel may have mapped it somewhere not in the low
           // 4GB. If this is the case, unmap and retry.
           if (reinterpret_cast<uintptr_t>(actual) + page_aligned_byte_count < 4 * GB) {
             break;
           } else {
             munmap(actual, page_aligned_byte_count);
             actual = MAP_FAILED;
           }
         }
       } else {
         // Skip over last page.
         ptr = tail_ptr;
       }
     }

     if (actual == MAP_FAILED) {
       LOG(ERROR) << "Could not find contiguous low-memory space.";
       saved_errno = ENOMEM;
     }
   } else {
     actual = mmap(expected, page_aligned_byte_count, prot, flags, fd.get(), 0);
     saved_errno = errno;
   }

 #else
 #ifdef __x86_64__
   if (low_4gb && expected == nullptr) {
     flags |= MAP_32BIT;
   }
 #endif

   void* actual = mmap(expected, page_aligned_byte_count, prot, flags, fd.get(), 0);
   saved_errno = errno;
 #endif

   if (actual == MAP_FAILED) {
     std::string maps;
     ReadFileToString("/proc/self/maps", &maps);

     *error_msg = StringPrintf("Failed anonymous mmap(%p, %zd, 0x%x, 0x%x, %d, 0): %s\n%s",
                               expected, page_aligned_byte_count, prot, flags, fd.get(),
                               strerror(saved_errno), maps.c_str());
     return nullptr;
   }
   std::ostringstream check_map_request_error_msg;
   if (!CheckMapRequest(expected, actual, page_aligned_byte_count, &check_map_request_error_msg)) {
     *error_msg = check_map_request_error_msg.str();
     return nullptr;
   }
   return new MemMap(name, reinterpret_cast<byte*>(actual), byte_count, actual,
                     page_aligned_byte_count, prot);
 }

 MemMap* MemMap::MapFileAtAddress(byte* expected, size_t byte_count, int prot, int flags, int fd,
                                  off_t start, bool reuse, const char* filename,
                                  std::string* error_msg) {
   CHECK_NE(0, prot);
   CHECK_NE(0, flags & (MAP_SHARED | MAP_PRIVATE));
   if (reuse) {
     // reuse means it is okay that it overlaps an existing page mapping.
     // Only use this if you actually made the page reservation yourself.
     CHECK(expected != nullptr);
     flags |= MAP_FIXED;
   } else {
     CHECK_EQ(0, flags & MAP_FIXED);
   }

   if (byte_count == 0) {
     return new MemMap(filename, nullptr, 0, nullptr, 0, prot);
   }
   // Adjust 'offset' to be page-aligned as required by mmap.
   int page_offset = start % kPageSize;
   off_t page_aligned_offset = start - page_offset;
   // Adjust 'byte_count' to be page-aligned as we will map this anyway.
   size_t page_aligned_byte_count = RoundUp(byte_count + page_offset, kPageSize);
   // The 'expected' is modified (if specified, ie non-null) to be page aligned to the file but not
   // necessarily to virtual memory. mmap will page align 'expected' for us.
   byte* page_aligned_expected = (expected == nullptr) ? nullptr : (expected - page_offset);

   byte* actual = reinterpret_cast<byte*>(mmap(page_aligned_expected,
                                               page_aligned_byte_count,
                                               prot,
                                               flags,
                                               fd,
                                               page_aligned_offset));
   if (actual == MAP_FAILED) {
     auto saved_errno = errno;

     std::string maps;
     ReadFileToString("/proc/self/maps", &maps);

     *error_msg = StringPrintf("mmap(%p, %zd, 0x%x, 0x%x, %d, %" PRId64
                               ") of file '%s' failed: %s\n%s",
                               page_aligned_expected, page_aligned_byte_count, prot, flags, fd,
                               static_cast<int64_t>(page_aligned_offset), filename,
                               strerror(saved_errno), maps.c_str());
     return nullptr;
   }
   std::ostringstream check_map_request_error_msg;
   if (!CheckMapRequest(expected, actual, page_aligned_byte_count, &check_map_request_error_msg)) {
     *error_msg = check_map_request_error_msg.str();
     return nullptr;
   }
   return new MemMap(filename, actual + page_offset, byte_count, actual, page_aligned_byte_count,
                     prot);
 }

 MemMap::~MemMap() {
   if (base_begin_ == nullptr && base_size_ == 0) {
     return;
   }
   int result = munmap(base_begin_, base_size_);
   if (result == -1) {
     PLOG(FATAL) << "munmap failed";
   }

   // Remove it from maps_.
   MutexLock mu(Thread::Current(), *Locks::mem_maps_lock_);
   bool found = false;
   for (auto it = maps_.lower_bound(base_begin_), end = maps_.end();
        it != end && it->first == base_begin_; ++it) {
     if (it->second == this) {
       found = true;
       maps_.erase(it);
       break;
     }
   }
   CHECK(found) << "MemMap not found";
 }

 MemMap::MemMap(const std::string& name, byte* begin, size_t size, void* base_begin,
                size_t base_size, int prot)
     : name_(name), begin_(begin), size_(size), base_begin_(base_begin), base_size_(base_size),
       prot_(prot) {
   if (size_ == 0) {
     CHECK(begin_ == nullptr);
     CHECK(base_begin_ == nullptr);
     CHECK_EQ(base_size_, 0U);
   } else {
     CHECK(begin_ != nullptr);
     CHECK(base_begin_ != nullptr);
     CHECK_NE(base_size_, 0U);

     // Add it to maps_.
     MutexLock mu(Thread::Current(), *Locks::mem_maps_lock_);
     maps_.insert(std::pair<void*, MemMap*>(base_begin_, this));
   }
 };

 MemMap* MemMap::RemapAtEnd(byte* new_end, const char* tail_name, int tail_prot,
                            std::string* error_msg) {
   DCHECK_GE(new_end, Begin());
   DCHECK_LE(new_end, End());
   DCHECK_LE(begin_ + size_, reinterpret_cast<byte*>(base_begin_) + base_size_);
   DCHECK(IsAligned<kPageSize>(begin_));
   DCHECK(IsAligned<kPageSize>(base_begin_));
   DCHECK(IsAligned<kPageSize>(reinterpret_cast<byte*>(base_begin_) + base_size_));
   DCHECK(IsAligned<kPageSize>(new_end));
   byte* old_end = begin_ + size_;
   byte* old_base_end = reinterpret_cast<byte*>(base_begin_) + base_size_;
   byte* new_base_end = new_end;
   DCHECK_LE(new_base_end, old_base_end);
   if (new_base_end == old_base_end) {
     return new MemMap(tail_name, nullptr, 0, nullptr, 0, tail_prot);
   }
   size_ = new_end - reinterpret_cast<byte*>(begin_);
   base_size_ = new_base_end - reinterpret_cast<byte*>(base_begin_);
   DCHECK_LE(begin_ + size_, reinterpret_cast<byte*>(base_begin_) + base_size_);
   size_t tail_size = old_end - new_end;
   byte* tail_base_begin = new_base_end;
   size_t tail_base_size = old_base_end - new_base_end;
   DCHECK_EQ(tail_base_begin + tail_base_size, old_base_end);
   DCHECK(IsAligned<kPageSize>(tail_base_size));

 #ifdef USE_ASHMEM
   // android_os_Debug.cpp read_mapinfo assumes all ashmem regions associated with the VM are
   // prefixed "dalvik-".
   std::string debug_friendly_name("dalvik-");
   debug_friendly_name += tail_name;
   ScopedFd fd(ashmem_create_region(debug_friendly_name.c_str(), tail_base_size));
   int flags = MAP_PRIVATE | MAP_FIXED;
   if (fd.get() == -1) {
     *error_msg = StringPrintf("ashmem_create_region failed for '%s': %s",
                               tail_name, strerror(errno));
     return nullptr;
   }
 #else
   ScopedFd fd(-1);
   int flags = MAP_PRIVATE | MAP_ANONYMOUS;
 #endif

   // Unmap/map the tail region.
   int result = munmap(tail_base_begin, tail_base_size);
   if (result == -1) {
     std::string maps;
     ReadFileToString("/proc/self/maps", &maps);
     *error_msg = StringPrintf("munmap(%p, %zd) failed for '%s'\n%s",
                               tail_base_begin, tail_base_size, name_.c_str(),
                               maps.c_str());
     return nullptr;
   }
   // Don't cause memory allocation between the munmap and the mmap
   // calls. Otherwise, libc (or something else) might take this memory
   // region. Note this isn't perfect as there's no way to prevent
   // other threads to try to take this memory region here.
   byte* actual = reinterpret_cast<byte*>(mmap(tail_base_begin, tail_base_size, tail_prot,
                                               flags, fd.get(), 0));
   if (actual == MAP_FAILED) {
     std::string maps;
     ReadFileToString("/proc/self/maps", &maps);
     *error_msg = StringPrintf("anonymous mmap(%p, %zd, 0x%x, 0x%x, %d, 0) failed\n%s",
                               tail_base_begin, tail_base_size, tail_prot, flags, fd.get(),
                               maps.c_str());
     return nullptr;
   }
   return new MemMap(tail_name, actual, tail_size, actual, tail_base_size, tail_prot);
 }

 void MemMap::MadviseDontNeedAndZero() {
   if (base_begin_ != nullptr || base_size_ != 0) {
     if (!kMadviseZeroes) {
       memset(base_begin_, 0, base_size_);
     }
     int result = madvise(base_begin_, base_size_, MADV_DONTNEED);
     if (result == -1) {
       PLOG(WARNING) << "madvise failed";
     }
   }
 }

 bool MemMap::Protect(int prot) {
   if (base_begin_ == nullptr && base_size_ == 0) {
     prot_ = prot;
     return true;
   }

   if (mprotect(base_begin_, base_size_, prot) == 0) {
     prot_ = prot;
     return true;
   }

   PLOG(ERROR) << "mprotect(" << reinterpret_cast<void*>(base_begin_) << ", " << base_size_ << ", "
               << prot << ") failed";
   return false;
 }

 bool MemMap::CheckNoGaps(MemMap* begin_map, MemMap* end_map) {
   MutexLock mu(Thread::Current(), *Locks::mem_maps_lock_);
   CHECK(begin_map != nullptr);
   CHECK(end_map != nullptr);
   CHECK(HasMemMap(begin_map));
   CHECK(HasMemMap(end_map));
   CHECK_LE(begin_map->BaseBegin(), end_map->BaseBegin());
   MemMap* map = begin_map;
   while (map->BaseBegin() != end_map->BaseBegin()) {
     MemMap* next_map = GetLargestMemMapAt(map->BaseEnd());
     if (next_map == nullptr) {
       // Found a gap.
       return false;
     }
     map = next_map;
   }
   return true;
 }

 void MemMap::DumpMaps(std::ostream& os) {
   DumpMaps(os, maps_);
 }

 void MemMap::DumpMaps(std::ostream& os, const std::multimap<void*, MemMap*>& mem_maps) {
   MutexLock mu(Thread::Current(), *Locks::mem_maps_lock_);
   DumpMapsLocked(os, mem_maps);
 }

 void MemMap::DumpMapsLocked(std::ostream& os, const std::multimap<void*, MemMap*>& mem_maps) {
   os << mem_maps;
 }

 bool MemMap::HasMemMap(MemMap* map) {
   void* base_begin = map->BaseBegin();
   for (auto it = maps_.lower_bound(base_begin), end = maps_.end();
        it != end && it->first == base_begin; ++it) {
     if (it->second == map) {
       return true;
     }
   }
   return false;
 }

 MemMap* MemMap::GetLargestMemMapAt(void* address) {
   size_t largest_size = 0;
   MemMap* largest_map = nullptr;
   for (auto it = maps_.lower_bound(address), end = maps_.end();
        it != end && it->first == address; ++it) {
     MemMap* map = it->second;
     CHECK(map != nullptr);
     if (largest_size < map->BaseSize()) {
       largest_size = map->BaseSize();
       largest_map = map;
     }
   }
   return largest_map;
 }

 std::ostream& operator<<(std::ostream& os, const MemMap& mem_map) {
   os << StringPrintf("[MemMap: %p-%p prot=0x%x %s]",
                      mem_map.BaseBegin(), mem_map.BaseEnd(), mem_map.GetProtect(),
                      mem_map.GetName().c_str());
   return os;
 }

 }  // namespace art
	/*
	* Copyright (C) 2008 The Android Open Source Project
	*
	* 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
	*
	* http://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.
	*/

	#include "mem_map.h"
	#include "thread-inl.h"

	#include <inttypes.h>
	#include <backtrace/BacktraceMap.h>
	#include <memory>

	// See CreateStartPos below.
	#ifdef __BIONIC__
	#include <sys/auxv.h>
	#endif

	#include "base/stringprintf.h"
	#include "ScopedFd.h"
	#include "utils.h"

	#define USE_ASHMEM 1

	#ifdef USE_ASHMEM
	#include <cutils/ashmem.h>
	#endif

	namespace art {

	static std::ostream& operator<<(
	std::ostream& os,
	std::pair<BacktraceMap::const_iterator, BacktraceMap::const_iterator> iters) {
	for (BacktraceMap::const_iterator it = iters.first; it != iters.second; ++it) {
	os << StringPrintf("0x%08x-0x%08x %c%c%c %s\n",
	static_cast<uint32_t>(it->start),
	static_cast<uint32_t>(it->end),
	(it->flags & PROT_READ) ? 'r' : '-',
	(it->flags & PROT_WRITE) ? 'w' : '-',
	(it->flags & PROT_EXEC) ? 'x' : '-', it->name.c_str());
	}
	return os;
	}

	std::ostream& operator<<(std::ostream& os, const std::multimap<void, MemMap>& mem_maps) {
	os << "MemMap:" << std::endl;
	for (auto it = mem_maps.begin(); it != mem_maps.end(); ++it) {
	void* base = it->first;
	MemMap* map = it->second;
	CHECK_EQ(base, map->BaseBegin());
	os << *map << std::endl;
	}
	return os;
	}

	std::multimap<void, MemMap> MemMap::maps_;

	#if defined(__LP64__) && !defined(__x86_64__)
	// Handling mem_map in 32b address range for 64b architectures that do not support MAP_32BIT.

	// The regular start of memory allocations. The first 64KB is protected by SELinux.
	static constexpr uintptr_t LOW_MEM_START = 64 * KB;

	// Generate random starting position.
	// To not interfere with image position, take the image's address and only place it below. Current
	// formula (sketch):
	//
	// ART_BASE_ADDR = 0001XXXXXXXXXXXXXXX
	// ----------------------------------------
	// = 0000111111111111111
	// & ~(kPageSize - 1) =~0000000000000001111
	// ----------------------------------------
	// mask = 0000111111111110000
	// & random data = YYYYYYYYYYYYYYYYYYY
	// -----------------------------------
	// tmp = 0000YYYYYYYYYYY0000
	// + LOW_MEM_START = 0000000000001000000
	// --------------------------------------
	// start
	//
	// getauxval as an entropy source is exposed in Bionic, but not in glibc before 2.16. When we
	// do not have Bionic, simply start with LOW_MEM_START.

	// Function is standalone so it can be tested somewhat in mem_map_test.cc.
	#ifdef __BIONIC__
	uintptr_t CreateStartPos(uint64_t input) {
	CHECK_NE(0, ART_BASE_ADDRESS);

	// Start with all bits below highest bit in ART_BASE_ADDRESS.
	constexpr size_t leading_zeros = CLZ(static_cast<uint32_t>(ART_BASE_ADDRESS));
	constexpr uintptr_t mask_ones = (1 << (31 - leading_zeros)) - 1;

	// Lowest (usually 12) bits are not used, as aligned by page size.
	constexpr uintptr_t mask = mask_ones & ~(kPageSize - 1);

	// Mask input data.
	return (input & mask) + LOW_MEM_START;
	}
	#endif

	static uintptr_t GenerateNextMemPos() {
	#ifdef __BIONIC__
	uint8_t* random_data = reinterpret_cast<uint8_t*>(getauxval(AT_RANDOM));
	// The lower 8B are taken for the stack guard. Use the upper 8B (with mask).
	return CreateStartPos(reinterpret_cast<uintptr_t>(random_data + 8));
	#else
	// No auxv on host, see above.
	return LOW_MEM_START;
	#endif
	}

	// Initialize linear scan to random position.
	uintptr_t MemMap::next_mem_pos_ = GenerateNextMemPos();
	#endif

	static bool CheckMapRequest(byte* expected_ptr, void* actual_ptr, size_t byte_count,
	std::ostringstream* error_msg) {
	// Handled first by caller for more specific error messages.
	CHECK(actual_ptr != MAP_FAILED);

	if (expected_ptr == nullptr) {
	return true;
	}

	if (expected_ptr == actual_ptr) {
	return true;
	}

	// We asked for an address but didn't get what we wanted, all paths below here should fail.
	int result = munmap(actual_ptr, byte_count);
	if (result == -1) {
	PLOG(WARNING) << StringPrintf("munmap(%p, %zd) failed", actual_ptr, byte_count);
	}

	uintptr_t actual = reinterpret_cast<uintptr_t>(actual_ptr);
	uintptr_t expected = reinterpret_cast<uintptr_t>(expected_ptr);
	uintptr_t limit = expected + byte_count;

	std::unique_ptr<BacktraceMap> map(BacktraceMap::Create(getpid()));
	if (!map->Build()) {
	*error_msg << StringPrintf("Failed to build process map to determine why mmap returned "
	"0x%08" PRIxPTR " instead of 0x%08" PRIxPTR, actual, expected);

	return false;
	}
	for (BacktraceMap::const_iterator it = map->begin(); it != map->end(); ++it) {
	if ((expected >= it->start && expected < it->end) // start of new within old
	\|\| (limit > it->start && limit < it->end) // end of new within old
	\|\| (expected <= it->start && limit > it->end)) { // start/end of new includes all of old
	*error_msg
	<< StringPrintf("Requested region 0x%08" PRIxPTR "-0x%08" PRIxPTR " overlaps with "
	"existing map 0x%08" PRIxPTR "-0x%08" PRIxPTR " (%s)\n",
	expected, limit,
	static_cast<uintptr_t>(it->start), static_cast<uintptr_t>(it->end),
	it->name.c_str())
	<< std::make_pair(it, map->end());
	return false;
	}
	}
	*error_msg << StringPrintf("Failed to mmap at expected address, mapped at "
	"0x%08" PRIxPTR " instead of 0x%08" PRIxPTR, actual, expected);
	return false;
	}

	MemMap* MemMap::MapAnonymous(const char* name, byte* expected, size_t byte_count, int prot,
	bool low_4gb, std::string* error_msg) {
	if (byte_count == 0) {
	return new MemMap(name, nullptr, 0, nullptr, 0, prot);
	}
	size_t page_aligned_byte_count = RoundUp(byte_count, kPageSize);

	#ifdef USE_ASHMEM
	// android_os_Debug.cpp read_mapinfo assumes all ashmem regions associated with the VM are
	// prefixed "dalvik-".
	std::string debug_friendly_name("dalvik-");
	debug_friendly_name += name;
	ScopedFd fd(ashmem_create_region(debug_friendly_name.c_str(), page_aligned_byte_count));
	if (fd.get() == -1) {
	*error_msg = StringPrintf("ashmem_create_region failed for '%s': %s", name, strerror(errno));
	return nullptr;
	}
	int flags = MAP_PRIVATE;
	#else
	ScopedFd fd(-1);
	int flags = MAP_PRIVATE \| MAP_ANONYMOUS;
	#endif

	// We need to store and potentially set an error number for pretty printing of errors
	int saved_errno = 0;

	#ifdef __LP64__
	// When requesting low_4g memory and having an expectation, the requested range should fit into
	// 4GB.
	if (low_4gb && (
	// Start out of bounds.
	(reinterpret_cast<uintptr_t>(expected) >> 32) != 0 \|\|
	// End out of bounds. For simplicity, this will fail for the last page of memory.
	(reinterpret_cast<uintptr_t>(expected + page_aligned_byte_count) >> 32) != 0)) {
	*error_msg = StringPrintf("The requested address space (%p, %p) cannot fit in low_4gb",
	expected, expected + page_aligned_byte_count);
	return nullptr;
	}
	#endif

	// TODO:
	// A page allocator would be a useful abstraction here, as
	// 1) It is doubtful that MAP_32BIT on x86_64 is doing the right job for us
	// 2) The linear scheme, even with simple saving of the last known position, is very crude
	#if defined(__LP64__) && !defined(__x86_64__)
	// MAP_32BIT only available on x86_64.
	void* actual = MAP_FAILED;
	if (low_4gb && expected == nullptr) {
	bool first_run = true;

	for (uintptr_t ptr = next_mem_pos_; ptr < 4 * GB; ptr += kPageSize) {
	if (4U * GB - ptr < page_aligned_byte_count) {
	// Not enough memory until 4GB.
	if (first_run) {
	// Try another time from the bottom;
	ptr = LOW_MEM_START - kPageSize;
	first_run = false;
	continue;
	} else {
	// Second try failed.
	break;
	}
	}

	uintptr_t tail_ptr;

	// Check pages are free.
	bool safe = true;
	for (tail_ptr = ptr; tail_ptr < ptr + page_aligned_byte_count; tail_ptr += kPageSize) {
	if (msync(reinterpret_cast<void*>(tail_ptr), kPageSize, 0) == 0) {
	safe = false;
	break;
	} else {
	DCHECK_EQ(errno, ENOMEM);
	}
	}

	next_mem_pos_ = tail_ptr; // update early, as we break out when we found and mapped a region

	if (safe == true) {
	actual = mmap(reinterpret_cast<void*>(ptr), page_aligned_byte_count, prot, flags, fd.get(),
	0);
	if (actual != MAP_FAILED) {
	// Since we didn't use MAP_FIXED the kernel may have mapped it somewhere not in the low
	// 4GB. If this is the case, unmap and retry.
	if (reinterpret_cast<uintptr_t>(actual) + page_aligned_byte_count < 4 * GB) {
	break;
	} else {
	munmap(actual, page_aligned_byte_count);
	actual = MAP_FAILED;
	}
	}
	} else {
	// Skip over last page.
	ptr = tail_ptr;
	}
	}

	if (actual == MAP_FAILED) {
	LOG(ERROR) << "Could not find contiguous low-memory space.";
	saved_errno = ENOMEM;
	}
	} else {
	actual = mmap(expected, page_aligned_byte_count, prot, flags, fd.get(), 0);
	saved_errno = errno;
	}

	#else
	#ifdef __x86_64__
	if (low_4gb && expected == nullptr) {
	flags \|= MAP_32BIT;
	}
	#endif

	void* actual = mmap(expected, page_aligned_byte_count, prot, flags, fd.get(), 0);
	saved_errno = errno;
	#endif

	if (actual == MAP_FAILED) {
	std::string maps;
	ReadFileToString("/proc/self/maps", &maps);

	*error_msg = StringPrintf("Failed anonymous mmap(%p, %zd, 0x%x, 0x%x, %d, 0): %s\n%s",
	expected, page_aligned_byte_count, prot, flags, fd.get(),
	strerror(saved_errno), maps.c_str());
	return nullptr;
	}
	std::ostringstream check_map_request_error_msg;
	if (!CheckMapRequest(expected, actual, page_aligned_byte_count, &check_map_request_error_msg)) {
	*error_msg = check_map_request_error_msg.str();
	return nullptr;
	}
	return new MemMap(name, reinterpret_cast<byte*>(actual), byte_count, actual,
	page_aligned_byte_count, prot);
	}

	MemMap* MemMap::MapFileAtAddress(byte* expected, size_t byte_count, int prot, int flags, int fd,
	off_t start, bool reuse, const char* filename,
	std::string* error_msg) {
	CHECK_NE(0, prot);
	CHECK_NE(0, flags & (MAP_SHARED \| MAP_PRIVATE));
	if (reuse) {
	// reuse means it is okay that it overlaps an existing page mapping.
	// Only use this if you actually made the page reservation yourself.
	CHECK(expected != nullptr);
	flags \|= MAP_FIXED;
	} else {
	CHECK_EQ(0, flags & MAP_FIXED);
	}

	if (byte_count == 0) {
	return new MemMap(filename, nullptr, 0, nullptr, 0, prot);
	}
	// Adjust 'offset' to be page-aligned as required by mmap.
	int page_offset = start % kPageSize;
	off_t page_aligned_offset = start - page_offset;
	// Adjust 'byte_count' to be page-aligned as we will map this anyway.
	size_t page_aligned_byte_count = RoundUp(byte_count + page_offset, kPageSize);
	// The 'expected' is modified (if specified, ie non-null) to be page aligned to the file but not
	// necessarily to virtual memory. mmap will page align 'expected' for us.
	byte* page_aligned_expected = (expected == nullptr) ? nullptr : (expected - page_offset);

	byte* actual = reinterpret_cast<byte*>(mmap(page_aligned_expected,
	page_aligned_byte_count,
	prot,
	flags,
	fd,
	page_aligned_offset));
	if (actual == MAP_FAILED) {
	auto saved_errno = errno;

	std::string maps;
	ReadFileToString("/proc/self/maps", &maps);

	*error_msg = StringPrintf("mmap(%p, %zd, 0x%x, 0x%x, %d, %" PRId64
	") of file '%s' failed: %s\n%s",
	page_aligned_expected, page_aligned_byte_count, prot, flags, fd,
	static_cast<int64_t>(page_aligned_offset), filename,
	strerror(saved_errno), maps.c_str());
	return nullptr;
	}
	std::ostringstream check_map_request_error_msg;
	if (!CheckMapRequest(expected, actual, page_aligned_byte_count, &check_map_request_error_msg)) {
	*error_msg = check_map_request_error_msg.str();
	return nullptr;
	}
	return new MemMap(filename, actual + page_offset, byte_count, actual, page_aligned_byte_count,
	prot);
	}

	MemMap::~MemMap() {
	if (base_begin_ == nullptr && base_size_ == 0) {
	return;
	}
	int result = munmap(base_begin_, base_size_);
	if (result == -1) {
	PLOG(FATAL) << "munmap failed";
	}

	// Remove it from maps_.
	MutexLock mu(Thread::Current(), *Locks::mem_maps_lock_);
	bool found = false;
	for (auto it = maps_.lower_bound(base_begin_), end = maps_.end();
	it != end && it->first == base_begin_; ++it) {
	if (it->second == this) {
	found = true;
	maps_.erase(it);
	break;
	}
	}
	CHECK(found) << "MemMap not found";
	}

	MemMap::MemMap(const std::string& name, byte* begin, size_t size, void* base_begin,
	size_t base_size, int prot)
	: name_(name), begin_(begin), size_(size), base_begin_(base_begin), base_size_(base_size),
	prot_(prot) {
	if (size_ == 0) {
	CHECK(begin_ == nullptr);
	CHECK(base_begin_ == nullptr);
	CHECK_EQ(base_size_, 0U);
	} else {
	CHECK(begin_ != nullptr);
	CHECK(base_begin_ != nullptr);
	CHECK_NE(base_size_, 0U);

	// Add it to maps_.
	MutexLock mu(Thread::Current(), *Locks::mem_maps_lock_);
	maps_.insert(std::pair<void, MemMap>(base_begin_, this));
	}
	};

	MemMap* MemMap::RemapAtEnd(byte* new_end, const char* tail_name, int tail_prot,
	std::string* error_msg) {
	DCHECK_GE(new_end, Begin());
	DCHECK_LE(new_end, End());
	DCHECK_LE(begin_ + size_, reinterpret_cast<byte*>(base_begin_) + base_size_);
	DCHECK(IsAligned<kPageSize>(begin_));
	DCHECK(IsAligned<kPageSize>(base_begin_));
	DCHECK(IsAligned<kPageSize>(reinterpret_cast<byte*>(base_begin_) + base_size_));
	DCHECK(IsAligned<kPageSize>(new_end));
	byte* old_end = begin_ + size_;
	byte* old_base_end = reinterpret_cast<byte*>(base_begin_) + base_size_;
	byte* new_base_end = new_end;
	DCHECK_LE(new_base_end, old_base_end);
	if (new_base_end == old_base_end) {
	return new MemMap(tail_name, nullptr, 0, nullptr, 0, tail_prot);
	}
	size_ = new_end - reinterpret_cast<byte*>(begin_);
	base_size_ = new_base_end - reinterpret_cast<byte*>(base_begin_);
	DCHECK_LE(begin_ + size_, reinterpret_cast<byte*>(base_begin_) + base_size_);
	size_t tail_size = old_end - new_end;
	byte* tail_base_begin = new_base_end;
	size_t tail_base_size = old_base_end - new_base_end;
	DCHECK_EQ(tail_base_begin + tail_base_size, old_base_end);
	DCHECK(IsAligned<kPageSize>(tail_base_size));

	#ifdef USE_ASHMEM
	// android_os_Debug.cpp read_mapinfo assumes all ashmem regions associated with the VM are
	// prefixed "dalvik-".
	std::string debug_friendly_name("dalvik-");
	debug_friendly_name += tail_name;
	ScopedFd fd(ashmem_create_region(debug_friendly_name.c_str(), tail_base_size));
	int flags = MAP_PRIVATE \| MAP_FIXED;
	if (fd.get() == -1) {
	*error_msg = StringPrintf("ashmem_create_region failed for '%s': %s",
	tail_name, strerror(errno));
	return nullptr;
	}
	#else
	ScopedFd fd(-1);
	int flags = MAP_PRIVATE \| MAP_ANONYMOUS;
	#endif

	// Unmap/map the tail region.
	int result = munmap(tail_base_begin, tail_base_size);
	if (result == -1) {
	std::string maps;
	ReadFileToString("/proc/self/maps", &maps);
	*error_msg = StringPrintf("munmap(%p, %zd) failed for '%s'\n%s",
	tail_base_begin, tail_base_size, name_.c_str(),
	maps.c_str());
	return nullptr;
	}
	// Don't cause memory allocation between the munmap and the mmap
	// calls. Otherwise, libc (or something else) might take this memory
	// region. Note this isn't perfect as there's no way to prevent
	// other threads to try to take this memory region here.
	byte* actual = reinterpret_cast<byte*>(mmap(tail_base_begin, tail_base_size, tail_prot,
	flags, fd.get(), 0));
	if (actual == MAP_FAILED) {
	std::string maps;
	ReadFileToString("/proc/self/maps", &maps);
	*error_msg = StringPrintf("anonymous mmap(%p, %zd, 0x%x, 0x%x, %d, 0) failed\n%s",
	tail_base_begin, tail_base_size, tail_prot, flags, fd.get(),
	maps.c_str());
	return nullptr;
	}
	return new MemMap(tail_name, actual, tail_size, actual, tail_base_size, tail_prot);
	}

	void MemMap::MadviseDontNeedAndZero() {
	if (base_begin_ != nullptr \|\| base_size_ != 0) {
	if (!kMadviseZeroes) {
	memset(base_begin_, 0, base_size_);
	}
	int result = madvise(base_begin_, base_size_, MADV_DONTNEED);
	if (result == -1) {
	PLOG(WARNING) << "madvise failed";
	}
	}
	}

	bool MemMap::Protect(int prot) {
	if (base_begin_ == nullptr && base_size_ == 0) {
	prot_ = prot;
	return true;
	}

	if (mprotect(base_begin_, base_size_, prot) == 0) {
	prot_ = prot;
	return true;
	}

	PLOG(ERROR) << "mprotect(" << reinterpret_cast<void*>(base_begin_) << ", " << base_size_ << ", "
	<< prot << ") failed";
	return false;
	}

	bool MemMap::CheckNoGaps(MemMap* begin_map, MemMap* end_map) {
	MutexLock mu(Thread::Current(), *Locks::mem_maps_lock_);
	CHECK(begin_map != nullptr);
	CHECK(end_map != nullptr);
	CHECK(HasMemMap(begin_map));
	CHECK(HasMemMap(end_map));
	CHECK_LE(begin_map->BaseBegin(), end_map->BaseBegin());
	MemMap* map = begin_map;
	while (map->BaseBegin() != end_map->BaseBegin()) {
	MemMap* next_map = GetLargestMemMapAt(map->BaseEnd());
	if (next_map == nullptr) {
	// Found a gap.
	return false;
	}
	map = next_map;
	}
	return true;
	}

	void MemMap::DumpMaps(std::ostream& os) {
	DumpMaps(os, maps_);
	}

	void MemMap::DumpMaps(std::ostream& os, const std::multimap<void, MemMap>& mem_maps) {
	MutexLock mu(Thread::Current(), *Locks::mem_maps_lock_);
	DumpMapsLocked(os, mem_maps);
	}

	void MemMap::DumpMapsLocked(std::ostream& os, const std::multimap<void, MemMap>& mem_maps) {
	os << mem_maps;
	}

	bool MemMap::HasMemMap(MemMap* map) {
	void* base_begin = map->BaseBegin();
	for (auto it = maps_.lower_bound(base_begin), end = maps_.end();
	it != end && it->first == base_begin; ++it) {
	if (it->second == map) {
	return true;
	}
	}
	return false;
	}

	MemMap* MemMap::GetLargestMemMapAt(void* address) {
	size_t largest_size = 0;
	MemMap* largest_map = nullptr;
	for (auto it = maps_.lower_bound(address), end = maps_.end();
	it != end && it->first == address; ++it) {
	MemMap* map = it->second;
	CHECK(map != nullptr);
	if (largest_size < map->BaseSize()) {
	largest_size = map->BaseSize();
	largest_map = map;
	}
	}
	return largest_map;
	}

	std::ostream& operator<<(std::ostream& os, const MemMap& mem_map) {
	os << StringPrintf("[MemMap: %p-%p prot=0x%x %s]",
	mem_map.BaseBegin(), mem_map.BaseEnd(), mem_map.GetProtect(),
	mem_map.GetName().c_str());
	return os;
	}

	} // namespace art