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* 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
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* See the License for the specific language governing permissions and
* limitations under the License.
#include "base/mutex.h"
#include <string>
#include <map>
#include <stddef.h>
#include <sys/mman.h> // For the PROT_* and MAP_* constants.
#include <sys/types.h>
#include "base/allocator.h"
#include "globals.h"
namespace art {
#if defined(__LP64__) && (!defined(__x86_64__) || defined(__APPLE__))
#ifdef __linux__
static constexpr bool kMadviseZeroes = true;
static constexpr bool kMadviseZeroes = false;
// Used to keep track of mmap segments.
// On 64b systems not supporting MAP_32BIT, the implementation of MemMap will do a linear scan
// for free pages. For security, the start of this scan should be randomized. This requires a
// dynamic initializer.
// For this to work, it is paramount that there are no other static initializers that access MemMap.
// Otherwise, calls might see uninitialized values.
class MemMap {
// Request an anonymous region of length 'byte_count' and a requested base address.
// Use null as the requested base address if you don't care.
// "reuse" allows re-mapping an address range from an existing mapping.
// The word "anonymous" in this context means "not backed by a file". The supplied
// 'ashmem_name' will be used -- on systems that support it -- to give the mapping
// a name.
// On success, returns returns a MemMap instance. On failure, returns null.
static MemMap* MapAnonymous(const char* ashmem_name, uint8_t* addr, size_t byte_count, int prot,
bool low_4gb, bool reuse, std::string* error_msg);
// Map part of a file, taking care of non-page aligned offsets. The
// "start" offset is absolute, not relative.
// On success, returns returns a MemMap instance. On failure, returns null.
static MemMap* MapFile(size_t byte_count, int prot, int flags, int fd, off_t start,
const char* filename, std::string* error_msg) {
return MapFileAtAddress(
nullptr, byte_count, prot, flags, fd, start, false, filename, error_msg);
// Map part of a file, taking care of non-page aligned offsets. The
// "start" offset is absolute, not relative. This version allows
// requesting a specific address for the base of the
// mapping. "reuse" allows us to create a view into an existing
// mapping where we do not take ownership of the memory.
// On success, returns returns a MemMap instance. On failure, returns null.
static MemMap* MapFileAtAddress(uint8_t* addr, size_t byte_count, int prot, int flags, int fd,
off_t start, bool reuse, const char* filename,
std::string* error_msg);
// Releases the memory mapping.
~MemMap() LOCKS_EXCLUDED(Locks::mem_maps_lock_);
const std::string& GetName() const {
return name_;
bool Protect(int prot);
void MadviseDontNeedAndZero();
int GetProtect() const {
return prot_;
uint8_t* Begin() const {
return begin_;
size_t Size() const {
return size_;
// Resize the mem-map by unmapping pages at the end. Currently only supports shrinking.
void SetSize(size_t new_size);
uint8_t* End() const {
return Begin() + Size();
void* BaseBegin() const {
return base_begin_;
size_t BaseSize() const {
return base_size_;
void* BaseEnd() const {
return reinterpret_cast<uint8_t*>(BaseBegin()) + BaseSize();
bool HasAddress(const void* addr) const {
return Begin() <= addr && addr < End();
// Unmap the pages at end and remap them to create another memory map.
MemMap* RemapAtEnd(uint8_t* new_end, const char* tail_name, int tail_prot,
std::string* error_msg);
static bool CheckNoGaps(MemMap* begin_map, MemMap* end_map)
static void DumpMaps(std::ostream& os)
typedef AllocationTrackingMultiMap<void*, MemMap*, kAllocatorTagMaps> Maps;
static void Init() LOCKS_EXCLUDED(Locks::mem_maps_lock_);
static void Shutdown() LOCKS_EXCLUDED(Locks::mem_maps_lock_);
MemMap(const std::string& name, uint8_t* begin, size_t size, void* base_begin, size_t base_size,
int prot, bool reuse) LOCKS_EXCLUDED(Locks::mem_maps_lock_);
static void DumpMapsLocked(std::ostream& os)
static bool HasMemMap(MemMap* map)
static MemMap* GetLargestMemMapAt(void* address)
const std::string name_;
uint8_t* const begin_; // Start of data.
size_t size_; // Length of data.
void* const base_begin_; // Page-aligned base address.
size_t base_size_; // Length of mapping. May be changed by RemapAtEnd (ie Zygote).
int prot_; // Protection of the map.
// When reuse_ is true, this is just a view of an existing mapping
// and we do not take ownership and are not responsible for
// unmapping.
const bool reuse_;
static uintptr_t next_mem_pos_; // Next memory location to check for low_4g extent.
// All the non-empty MemMaps. Use a multimap as we do a reserve-and-divide (eg ElfMap::Load()).
static Maps* maps_ GUARDED_BY(Locks::mem_maps_lock_);
friend class MemMapTest; // To allow access to base_begin_ and base_size_.
std::ostream& operator<<(std::ostream& os, const MemMap& mem_map);
std::ostream& operator<<(std::ostream& os, const MemMap::Maps& mem_maps);
} // namespace art