blob: 9205266754d286573168794a853f5ed1b7fb21a1 [file] [log] [blame]
// Copyright (c) 2015 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "base/metrics/persistent_memory_allocator.h"
#include <assert.h>
#include <algorithm>
#if defined(OS_WIN)
#include <windows.h>
#include "winbase.h"
#elif defined(OS_POSIX) || defined(OS_FUCHSIA)
#include <sys/mman.h>
#endif
#include "base/files/memory_mapped_file.h"
#include "base/logging.h"
#include "base/memory/shared_memory.h"
#include "base/metrics/histogram_functions.h"
#include "base/metrics/sparse_histogram.h"
#include "base/numerics/safe_conversions.h"
#include "base/sys_info.h"
#include "base/threading/thread_restrictions.h"
#include "build/build_config.h"
namespace {
// Limit of memory segment size. It has to fit in an unsigned 32-bit number
// and should be a power of 2 in order to accomodate almost any page size.
const uint32_t kSegmentMaxSize = 1 << 30; // 1 GiB
// A constant (random) value placed in the shared metadata to identify
// an already initialized memory segment.
const uint32_t kGlobalCookie = 0x408305DC;
// The current version of the metadata. If updates are made that change
// the metadata, the version number can be queried to operate in a backward-
// compatible manner until the memory segment is completely re-initalized.
const uint32_t kGlobalVersion = 2;
// Constant values placed in the block headers to indicate its state.
const uint32_t kBlockCookieFree = 0;
const uint32_t kBlockCookieQueue = 1;
const uint32_t kBlockCookieWasted = (uint32_t)-1;
const uint32_t kBlockCookieAllocated = 0xC8799269;
// TODO(bcwhite): When acceptable, consider moving flags to std::atomic<char>
// types rather than combined bitfield.
// Flags stored in the flags_ field of the SharedMetadata structure below.
enum : int {
kFlagCorrupt = 1 << 0,
kFlagFull = 1 << 1
};
// Errors that are logged in "errors" histogram.
enum AllocatorError : int {
kMemoryIsCorrupt = 1,
};
bool CheckFlag(const volatile std::atomic<uint32_t>* flags, int flag) {
uint32_t loaded_flags = flags->load(std::memory_order_relaxed);
return (loaded_flags & flag) != 0;
}
void SetFlag(volatile std::atomic<uint32_t>* flags, int flag) {
uint32_t loaded_flags = flags->load(std::memory_order_relaxed);
for (;;) {
uint32_t new_flags = (loaded_flags & ~flag) | flag;
// In the failue case, actual "flags" value stored in loaded_flags.
// These access are "relaxed" because they are completely independent
// of all other values.
if (flags->compare_exchange_weak(loaded_flags, new_flags,
std::memory_order_relaxed,
std::memory_order_relaxed)) {
break;
}
}
}
} // namespace
namespace base {
// All allocations and data-structures must be aligned to this byte boundary.
// Alignment as large as the physical bus between CPU and RAM is _required_
// for some architectures, is simply more efficient on other CPUs, and
// generally a Good Idea(tm) for all platforms as it reduces/eliminates the
// chance that a type will span cache lines. Alignment mustn't be less
// than 8 to ensure proper alignment for all types. The rest is a balance
// between reducing spans across multiple cache lines and wasted space spent
// padding out allocations. An alignment of 16 would ensure that the block
// header structure always sits in a single cache line. An average of about
// 1/2 this value will be wasted with every allocation.
const uint32_t PersistentMemoryAllocator::kAllocAlignment = 8;
// The block-header is placed at the top of every allocation within the
// segment to describe the data that follows it.
struct PersistentMemoryAllocator::BlockHeader {
uint32_t size; // Number of bytes in this block, including header.
uint32_t cookie; // Constant value indicating completed allocation.
std::atomic<uint32_t> type_id; // Arbitrary number indicating data type.
std::atomic<uint32_t> next; // Pointer to the next block when iterating.
};
// The shared metadata exists once at the top of the memory segment to
// describe the state of the allocator to all processes. The size of this
// structure must be a multiple of 64-bits to ensure compatibility between
// architectures.
struct PersistentMemoryAllocator::SharedMetadata {
uint32_t cookie; // Some value that indicates complete initialization.
uint32_t size; // Total size of memory segment.
uint32_t page_size; // Paging size within memory segment.
uint32_t version; // Version code so upgrades don't break.
uint64_t id; // Arbitrary ID number given by creator.
uint32_t name; // Reference to stored name string.
uint32_t padding1; // Pad-out read-only data to 64-bit alignment.
// Above is read-only after first construction. Below may be changed and
// so must be marked "volatile" to provide correct inter-process behavior.
// State of the memory, plus some padding to keep alignment.
volatile std::atomic<uint8_t> memory_state; // MemoryState enum values.
uint8_t padding2[3];
// Bitfield of information flags. Access to this should be done through
// the CheckFlag() and SetFlag() methods defined above.
volatile std::atomic<uint32_t> flags;
// Offset/reference to first free space in segment.
volatile std::atomic<uint32_t> freeptr;
// The "iterable" queue is an M&S Queue as described here, append-only:
// https://www.research.ibm.com/people/m/michael/podc-1996.pdf
// |queue| needs to be 64-bit aligned and is itself a multiple of 64 bits.
volatile std::atomic<uint32_t> tailptr; // Last block of iteration queue.
volatile BlockHeader queue; // Empty block for linked-list head/tail.
};
// The "queue" block header is used to detect "last node" so that zero/null
// can be used to indicate that it hasn't been added at all. It is part of
// the SharedMetadata structure which itself is always located at offset zero.
const PersistentMemoryAllocator::Reference
PersistentMemoryAllocator::kReferenceQueue =
offsetof(SharedMetadata, queue);
const base::FilePath::CharType PersistentMemoryAllocator::kFileExtension[] =
FILE_PATH_LITERAL(".pma");
PersistentMemoryAllocator::Iterator::Iterator(
const PersistentMemoryAllocator* allocator)
: allocator_(allocator), last_record_(kReferenceQueue), record_count_(0) {}
PersistentMemoryAllocator::Iterator::Iterator(
const PersistentMemoryAllocator* allocator,
Reference starting_after)
: allocator_(allocator), last_record_(0), record_count_(0) {
Reset(starting_after);
}
void PersistentMemoryAllocator::Iterator::Reset() {
last_record_.store(kReferenceQueue, std::memory_order_relaxed);
record_count_.store(0, std::memory_order_relaxed);
}
void PersistentMemoryAllocator::Iterator::Reset(Reference starting_after) {
if (starting_after == 0) {
Reset();
return;
}
last_record_.store(starting_after, std::memory_order_relaxed);
record_count_.store(0, std::memory_order_relaxed);
// Ensure that the starting point is a valid, iterable block (meaning it can
// be read and has a non-zero "next" pointer).
const volatile BlockHeader* block =
allocator_->GetBlock(starting_after, 0, 0, false, false);
if (!block || block->next.load(std::memory_order_relaxed) == 0) {
NOTREACHED();
last_record_.store(kReferenceQueue, std::memory_order_release);
}
}
PersistentMemoryAllocator::Reference
PersistentMemoryAllocator::Iterator::GetLast() {
Reference last = last_record_.load(std::memory_order_relaxed);
if (last == kReferenceQueue)
return kReferenceNull;
return last;
}
PersistentMemoryAllocator::Reference
PersistentMemoryAllocator::Iterator::GetNext(uint32_t* type_return) {
// Make a copy of the existing count of found-records, acquiring all changes
// made to the allocator, notably "freeptr" (see comment in loop for why
// the load of that value cannot be moved above here) that occurred during
// any previous runs of this method, including those by parallel threads
// that interrupted it. It pairs with the Release at the end of this method.
//
// Otherwise, if the compiler were to arrange the two loads such that
// "count" was fetched _after_ "freeptr" then it would be possible for
// this thread to be interrupted between them and other threads perform
// multiple allocations, make-iterables, and iterations (with the included
// increment of |record_count_|) culminating in the check at the bottom
// mistakenly determining that a loop exists. Isn't this stuff fun?
uint32_t count = record_count_.load(std::memory_order_acquire);
Reference last = last_record_.load(std::memory_order_acquire);
Reference next;
while (true) {
const volatile BlockHeader* block =
allocator_->GetBlock(last, 0, 0, true, false);
if (!block) // Invalid iterator state.
return kReferenceNull;
// The compiler and CPU can freely reorder all memory accesses on which
// there are no dependencies. It could, for example, move the load of
// "freeptr" to above this point because there are no explicit dependencies
// between it and "next". If it did, however, then another block could
// be queued after that but before the following load meaning there is
// one more queued block than the future "detect loop by having more
// blocks that could fit before freeptr" will allow.
//
// By "acquiring" the "next" value here, it's synchronized to the enqueue
// of the node which in turn is synchronized to the allocation (which sets
// freeptr). Thus, the scenario above cannot happen.
next = block->next.load(std::memory_order_acquire);
if (next == kReferenceQueue) // No next allocation in queue.
return kReferenceNull;
block = allocator_->GetBlock(next, 0, 0, false, false);
if (!block) { // Memory is corrupt.
allocator_->SetCorrupt();
return kReferenceNull;
}
// Update the "last_record" pointer to be the reference being returned.
// If it fails then another thread has already iterated past it so loop
// again. Failing will also load the existing value into "last" so there
// is no need to do another such load when the while-loop restarts. A
// "strong" compare-exchange is used because failing unnecessarily would
// mean repeating some fairly costly validations above.
if (last_record_.compare_exchange_strong(
last, next, std::memory_order_acq_rel, std::memory_order_acquire)) {
*type_return = block->type_id.load(std::memory_order_relaxed);
break;
}
}
// Memory corruption could cause a loop in the list. Such must be detected
// so as to not cause an infinite loop in the caller. This is done by simply
// making sure it doesn't iterate more times than the absolute maximum
// number of allocations that could have been made. Callers are likely
// to loop multiple times before it is detected but at least it stops.
const uint32_t freeptr = std::min(
allocator_->shared_meta()->freeptr.load(std::memory_order_relaxed),
allocator_->mem_size_);
const uint32_t max_records =
freeptr / (sizeof(BlockHeader) + kAllocAlignment);
if (count > max_records) {
allocator_->SetCorrupt();
return kReferenceNull;
}
// Increment the count and release the changes made above. It pairs with
// the Acquire at the top of this method. Note that this operation is not
// strictly synchonized with fetching of the object to return, which would
// have to be done inside the loop and is somewhat complicated to achieve.
// It does not matter if it falls behind temporarily so long as it never
// gets ahead.
record_count_.fetch_add(1, std::memory_order_release);
return next;
}
PersistentMemoryAllocator::Reference
PersistentMemoryAllocator::Iterator::GetNextOfType(uint32_t type_match) {
Reference ref;
uint32_t type_found;
while ((ref = GetNext(&type_found)) != 0) {
if (type_found == type_match)
return ref;
}
return kReferenceNull;
}
// static
bool PersistentMemoryAllocator::IsMemoryAcceptable(const void* base,
size_t size,
size_t page_size,
bool readonly) {
return ((base && reinterpret_cast<uintptr_t>(base) % kAllocAlignment == 0) &&
(size >= sizeof(SharedMetadata) && size <= kSegmentMaxSize) &&
(size % kAllocAlignment == 0 || readonly) &&
(page_size == 0 || size % page_size == 0 || readonly));
}
PersistentMemoryAllocator::PersistentMemoryAllocator(void* base,
size_t size,
size_t page_size,
uint64_t id,
base::StringPiece name,
bool readonly)
: PersistentMemoryAllocator(Memory(base, MEM_EXTERNAL),
size,
page_size,
id,
name,
readonly) {}
PersistentMemoryAllocator::PersistentMemoryAllocator(Memory memory,
size_t size,
size_t page_size,
uint64_t id,
base::StringPiece name,
bool readonly)
: mem_base_(static_cast<char*>(memory.base)),
mem_type_(memory.type),
mem_size_(static_cast<uint32_t>(size)),
mem_page_(static_cast<uint32_t>((page_size ? page_size : size))),
#if defined(OS_NACL)
vm_page_size_(4096U), // SysInfo is not built for NACL.
#else
vm_page_size_(SysInfo::VMAllocationGranularity()),
#endif
readonly_(readonly),
corrupt_(0),
allocs_histogram_(nullptr),
used_histogram_(nullptr),
errors_histogram_(nullptr) {
// These asserts ensure that the structures are 32/64-bit agnostic and meet
// all the requirements of use within the allocator. They access private
// definitions and so cannot be moved to the global scope.
static_assert(sizeof(PersistentMemoryAllocator::BlockHeader) == 16,
"struct is not portable across different natural word widths");
static_assert(sizeof(PersistentMemoryAllocator::SharedMetadata) == 64,
"struct is not portable across different natural word widths");
static_assert(sizeof(BlockHeader) % kAllocAlignment == 0,
"BlockHeader is not a multiple of kAllocAlignment");
static_assert(sizeof(SharedMetadata) % kAllocAlignment == 0,
"SharedMetadata is not a multiple of kAllocAlignment");
static_assert(kReferenceQueue % kAllocAlignment == 0,
"\"queue\" is not aligned properly; must be at end of struct");
// Ensure that memory segment is of acceptable size.
CHECK(IsMemoryAcceptable(memory.base, size, page_size, readonly));
// These atomics operate inter-process and so must be lock-free. The local
// casts are to make sure it can be evaluated at compile time to a constant.
CHECK(((SharedMetadata*)nullptr)->freeptr.is_lock_free());
CHECK(((SharedMetadata*)nullptr)->flags.is_lock_free());
CHECK(((BlockHeader*)nullptr)->next.is_lock_free());
CHECK(corrupt_.is_lock_free());
if (shared_meta()->cookie != kGlobalCookie) {
if (readonly) {
SetCorrupt();
return;
}
// This block is only executed when a completely new memory segment is
// being initialized. It's unshared and single-threaded...
volatile BlockHeader* const first_block =
reinterpret_cast<volatile BlockHeader*>(mem_base_ +
sizeof(SharedMetadata));
if (shared_meta()->cookie != 0 ||
shared_meta()->size != 0 ||
shared_meta()->version != 0 ||
shared_meta()->freeptr.load(std::memory_order_relaxed) != 0 ||
shared_meta()->flags.load(std::memory_order_relaxed) != 0 ||
shared_meta()->id != 0 ||
shared_meta()->name != 0 ||
shared_meta()->tailptr != 0 ||
shared_meta()->queue.cookie != 0 ||
shared_meta()->queue.next.load(std::memory_order_relaxed) != 0 ||
first_block->size != 0 ||
first_block->cookie != 0 ||
first_block->type_id.load(std::memory_order_relaxed) != 0 ||
first_block->next != 0) {
// ...or something malicious has been playing with the metadata.
SetCorrupt();
}
// This is still safe to do even if corruption has been detected.
shared_meta()->cookie = kGlobalCookie;
shared_meta()->size = mem_size_;
shared_meta()->page_size = mem_page_;
shared_meta()->version = kGlobalVersion;
shared_meta()->id = id;
shared_meta()->freeptr.store(sizeof(SharedMetadata),
std::memory_order_release);
// Set up the queue of iterable allocations.
shared_meta()->queue.size = sizeof(BlockHeader);
shared_meta()->queue.cookie = kBlockCookieQueue;
shared_meta()->queue.next.store(kReferenceQueue, std::memory_order_release);
shared_meta()->tailptr.store(kReferenceQueue, std::memory_order_release);
// Allocate space for the name so other processes can learn it.
if (!name.empty()) {
const size_t name_length = name.length() + 1;
shared_meta()->name = Allocate(name_length, 0);
char* name_cstr = GetAsArray<char>(shared_meta()->name, 0, name_length);
if (name_cstr)
memcpy(name_cstr, name.data(), name.length());
}
shared_meta()->memory_state.store(MEMORY_INITIALIZED,
std::memory_order_release);
} else {
if (shared_meta()->size == 0 || shared_meta()->version != kGlobalVersion ||
shared_meta()->freeptr.load(std::memory_order_relaxed) == 0 ||
shared_meta()->tailptr == 0 || shared_meta()->queue.cookie == 0 ||
shared_meta()->queue.next.load(std::memory_order_relaxed) == 0) {
SetCorrupt();
}
if (!readonly) {
// The allocator is attaching to a previously initialized segment of
// memory. If the initialization parameters differ, make the best of it
// by reducing the local construction parameters to match those of
// the actual memory area. This ensures that the local object never
// tries to write outside of the original bounds.
// Because the fields are const to ensure that no code other than the
// constructor makes changes to them as well as to give optimization
// hints to the compiler, it's necessary to const-cast them for changes
// here.
if (shared_meta()->size < mem_size_)
*const_cast<uint32_t*>(&mem_size_) = shared_meta()->size;
if (shared_meta()->page_size < mem_page_)
*const_cast<uint32_t*>(&mem_page_) = shared_meta()->page_size;
// Ensure that settings are still valid after the above adjustments.
if (!IsMemoryAcceptable(memory.base, mem_size_, mem_page_, readonly))
SetCorrupt();
}
}
}
PersistentMemoryAllocator::~PersistentMemoryAllocator() {
// It's strictly forbidden to do any memory access here in case there is
// some issue with the underlying memory segment. The "Local" allocator
// makes use of this to allow deletion of the segment on the heap from
// within its destructor.
}
uint64_t PersistentMemoryAllocator::Id() const {
return shared_meta()->id;
}
const char* PersistentMemoryAllocator::Name() const {
Reference name_ref = shared_meta()->name;
const char* name_cstr =
GetAsArray<char>(name_ref, 0, PersistentMemoryAllocator::kSizeAny);
if (!name_cstr)
return "";
size_t name_length = GetAllocSize(name_ref);
if (name_cstr[name_length - 1] != '\0') {
NOTREACHED();
SetCorrupt();
return "";
}
return name_cstr;
}
void PersistentMemoryAllocator::CreateTrackingHistograms(
base::StringPiece name) {
if (name.empty() || readonly_)
return;
std::string name_string = name.as_string();
#if 0
// This histogram wasn't being used so has been disabled. It is left here
// in case development of a new use of the allocator could benefit from
// recording (temporarily and locally) the allocation sizes.
DCHECK(!allocs_histogram_);
allocs_histogram_ = Histogram::FactoryGet(
"UMA.PersistentAllocator." + name_string + ".Allocs", 1, 10000, 50,
HistogramBase::kUmaTargetedHistogramFlag);
#endif
DCHECK(!used_histogram_);
used_histogram_ = LinearHistogram::FactoryGet(
"UMA.PersistentAllocator." + name_string + ".UsedPct", 1, 101, 21,
HistogramBase::kUmaTargetedHistogramFlag);
DCHECK(!errors_histogram_);
errors_histogram_ = SparseHistogram::FactoryGet(
"UMA.PersistentAllocator." + name_string + ".Errors",
HistogramBase::kUmaTargetedHistogramFlag);
}
void PersistentMemoryAllocator::Flush(bool sync) {
FlushPartial(used(), sync);
}
void PersistentMemoryAllocator::SetMemoryState(uint8_t memory_state) {
shared_meta()->memory_state.store(memory_state, std::memory_order_relaxed);
FlushPartial(sizeof(SharedMetadata), false);
}
uint8_t PersistentMemoryAllocator::GetMemoryState() const {
return shared_meta()->memory_state.load(std::memory_order_relaxed);
}
size_t PersistentMemoryAllocator::used() const {
return std::min(shared_meta()->freeptr.load(std::memory_order_relaxed),
mem_size_);
}
PersistentMemoryAllocator::Reference PersistentMemoryAllocator::GetAsReference(
const void* memory,
uint32_t type_id) const {
uintptr_t address = reinterpret_cast<uintptr_t>(memory);
if (address < reinterpret_cast<uintptr_t>(mem_base_))
return kReferenceNull;
uintptr_t offset = address - reinterpret_cast<uintptr_t>(mem_base_);
if (offset >= mem_size_ || offset < sizeof(BlockHeader))
return kReferenceNull;
Reference ref = static_cast<Reference>(offset) - sizeof(BlockHeader);
if (!GetBlockData(ref, type_id, kSizeAny))
return kReferenceNull;
return ref;
}
size_t PersistentMemoryAllocator::GetAllocSize(Reference ref) const {
const volatile BlockHeader* const block = GetBlock(ref, 0, 0, false, false);
if (!block)
return 0;
uint32_t size = block->size;
// Header was verified by GetBlock() but a malicious actor could change
// the value between there and here. Check it again.
if (size <= sizeof(BlockHeader) || ref + size > mem_size_) {
SetCorrupt();
return 0;
}
return size - sizeof(BlockHeader);
}
uint32_t PersistentMemoryAllocator::GetType(Reference ref) const {
const volatile BlockHeader* const block = GetBlock(ref, 0, 0, false, false);
if (!block)
return 0;
return block->type_id.load(std::memory_order_relaxed);
}
bool PersistentMemoryAllocator::ChangeType(Reference ref,
uint32_t to_type_id,
uint32_t from_type_id,
bool clear) {
DCHECK(!readonly_);
volatile BlockHeader* const block = GetBlock(ref, 0, 0, false, false);
if (!block)
return false;
// "Strong" exchanges are used below because there is no loop that can retry
// in the wake of spurious failures possible with "weak" exchanges. It is,
// in aggregate, an "acquire-release" operation so no memory accesses can be
// reordered either before or after this method (since changes based on type
// could happen on either side).
if (clear) {
// If clearing the memory, first change it to the "transitioning" type so
// there can be no confusion by other threads. After the memory is cleared,
// it can be changed to its final type.
if (!block->type_id.compare_exchange_strong(
from_type_id, kTypeIdTransitioning, std::memory_order_acquire,
std::memory_order_acquire)) {
// Existing type wasn't what was expected: fail (with no changes)
return false;
}
// Clear the memory in an atomic manner. Using "release" stores force
// every write to be done after the ones before it. This is better than
// using memset because (a) it supports "volatile" and (b) it creates a
// reliable pattern upon which other threads may rely.
volatile std::atomic<int>* data =
reinterpret_cast<volatile std::atomic<int>*>(
reinterpret_cast<volatile char*>(block) + sizeof(BlockHeader));
const uint32_t words = (block->size - sizeof(BlockHeader)) / sizeof(int);
DCHECK_EQ(0U, (block->size - sizeof(BlockHeader)) % sizeof(int));
for (uint32_t i = 0; i < words; ++i) {
data->store(0, std::memory_order_release);
++data;
}
// If the destination type is "transitioning" then skip the final exchange.
if (to_type_id == kTypeIdTransitioning)
return true;
// Finish the change to the desired type.
from_type_id = kTypeIdTransitioning; // Exchange needs modifiable original.
bool success = block->type_id.compare_exchange_strong(
from_type_id, to_type_id, std::memory_order_release,
std::memory_order_relaxed);
DCHECK(success); // Should never fail.
return success;
}
// One step change to the new type. Will return false if the existing value
// doesn't match what is expected.
return block->type_id.compare_exchange_strong(from_type_id, to_type_id,
std::memory_order_acq_rel,
std::memory_order_acquire);
}
PersistentMemoryAllocator::Reference PersistentMemoryAllocator::Allocate(
size_t req_size,
uint32_t type_id) {
Reference ref = AllocateImpl(req_size, type_id);
if (ref) {
// Success: Record this allocation in usage stats (if active).
if (allocs_histogram_)
allocs_histogram_->Add(static_cast<HistogramBase::Sample>(req_size));
} else {
// Failure: Record an allocation of zero for tracking.
if (allocs_histogram_)
allocs_histogram_->Add(0);
}
return ref;
}
PersistentMemoryAllocator::Reference PersistentMemoryAllocator::AllocateImpl(
size_t req_size,
uint32_t type_id) {
DCHECK(!readonly_);
// Validate req_size to ensure it won't overflow when used as 32-bit value.
if (req_size > kSegmentMaxSize - sizeof(BlockHeader)) {
NOTREACHED();
return kReferenceNull;
}
// Round up the requested size, plus header, to the next allocation alignment.
uint32_t size = static_cast<uint32_t>(req_size + sizeof(BlockHeader));
size = (size + (kAllocAlignment - 1)) & ~(kAllocAlignment - 1);
if (size <= sizeof(BlockHeader) || size > mem_page_) {
NOTREACHED();
return kReferenceNull;
}
// Get the current start of unallocated memory. Other threads may
// update this at any time and cause us to retry these operations.
// This value should be treated as "const" to avoid confusion through
// the code below but recognize that any failed compare-exchange operation
// involving it will cause it to be loaded with a more recent value. The
// code should either exit or restart the loop in that case.
/* const */ uint32_t freeptr =
shared_meta()->freeptr.load(std::memory_order_acquire);
// Allocation is lockless so we do all our caculation and then, if saving
// indicates a change has occurred since we started, scrap everything and
// start over.
for (;;) {
if (IsCorrupt())
return kReferenceNull;
if (freeptr + size > mem_size_) {
SetFlag(&shared_meta()->flags, kFlagFull);
return kReferenceNull;
}
// Get pointer to the "free" block. If something has been allocated since
// the load of freeptr above, it is still safe as nothing will be written
// to that location until after the compare-exchange below.
volatile BlockHeader* const block = GetBlock(freeptr, 0, 0, false, true);
if (!block) {
SetCorrupt();
return kReferenceNull;
}
// An allocation cannot cross page boundaries. If it would, create a
// "wasted" block and begin again at the top of the next page. This
// area could just be left empty but we fill in the block header just
// for completeness sake.
const uint32_t page_free = mem_page_ - freeptr % mem_page_;
if (size > page_free) {
if (page_free <= sizeof(BlockHeader)) {
SetCorrupt();
return kReferenceNull;
}
const uint32_t new_freeptr = freeptr + page_free;
if (shared_meta()->freeptr.compare_exchange_strong(
freeptr, new_freeptr, std::memory_order_acq_rel,
std::memory_order_acquire)) {
block->size = page_free;
block->cookie = kBlockCookieWasted;
}
continue;
}
// Don't leave a slice at the end of a page too small for anything. This
// can result in an allocation up to two alignment-sizes greater than the
// minimum required by requested-size + header + alignment.
if (page_free - size < sizeof(BlockHeader) + kAllocAlignment)
size = page_free;
const uint32_t new_freeptr = freeptr + size;
if (new_freeptr > mem_size_) {
SetCorrupt();
return kReferenceNull;
}
// Save our work. Try again if another thread has completed an allocation
// while we were processing. A "weak" exchange would be permissable here
// because the code will just loop and try again but the above processing
// is significant so make the extra effort of a "strong" exchange.
if (!shared_meta()->freeptr.compare_exchange_strong(
freeptr, new_freeptr, std::memory_order_acq_rel,
std::memory_order_acquire)) {
continue;
}
// Given that all memory was zeroed before ever being given to an instance
// of this class and given that we only allocate in a monotomic fashion
// going forward, it must be that the newly allocated block is completely
// full of zeros. If we find anything in the block header that is NOT a
// zero then something must have previously run amuck through memory,
// writing beyond the allocated space and into unallocated space.
if (block->size != 0 ||
block->cookie != kBlockCookieFree ||
block->type_id.load(std::memory_order_relaxed) != 0 ||
block->next.load(std::memory_order_relaxed) != 0) {
SetCorrupt();
return kReferenceNull;
}
// Make sure the memory exists by writing to the first byte of every memory
// page it touches beyond the one containing the block header itself.
// As the underlying storage is often memory mapped from disk or shared
// space, sometimes things go wrong and those address don't actually exist
// leading to a SIGBUS (or Windows equivalent) at some arbitrary location
// in the code. This should concentrate all those failures into this
// location for easy tracking and, eventually, proper handling.
volatile char* mem_end = reinterpret_cast<volatile char*>(block) + size;
volatile char* mem_begin = reinterpret_cast<volatile char*>(
(reinterpret_cast<uintptr_t>(block) + sizeof(BlockHeader) +
(vm_page_size_ - 1)) &
~static_cast<uintptr_t>(vm_page_size_ - 1));
for (volatile char* memory = mem_begin; memory < mem_end;
memory += vm_page_size_) {
// It's required that a memory segment start as all zeros and thus the
// newly allocated block is all zeros at this point. Thus, writing a
// zero to it allows testing that the memory exists without actually
// changing its contents. The compiler doesn't know about the requirement
// and so cannot optimize-away these writes.
*memory = 0;
}
// Load information into the block header. There is no "release" of the
// data here because this memory can, currently, be seen only by the thread
// performing the allocation. When it comes time to share this, the thread
// will call MakeIterable() which does the release operation.
block->size = size;
block->cookie = kBlockCookieAllocated;
block->type_id.store(type_id, std::memory_order_relaxed);
return freeptr;
}
}
void PersistentMemoryAllocator::GetMemoryInfo(MemoryInfo* meminfo) const {
uint32_t remaining = std::max(
mem_size_ - shared_meta()->freeptr.load(std::memory_order_relaxed),
(uint32_t)sizeof(BlockHeader));
meminfo->total = mem_size_;
meminfo->free = remaining - sizeof(BlockHeader);
}
void PersistentMemoryAllocator::MakeIterable(Reference ref) {
DCHECK(!readonly_);
if (IsCorrupt())
return;
volatile BlockHeader* block = GetBlock(ref, 0, 0, false, false);
if (!block) // invalid reference
return;
if (block->next.load(std::memory_order_acquire) != 0) // Already iterable.
return;
block->next.store(kReferenceQueue, std::memory_order_release); // New tail.
// Try to add this block to the tail of the queue. May take multiple tries.
// If so, tail will be automatically updated with a more recent value during
// compare-exchange operations.
uint32_t tail = shared_meta()->tailptr.load(std::memory_order_acquire);
for (;;) {
// Acquire the current tail-pointer released by previous call to this
// method and validate it.
block = GetBlock(tail, 0, 0, true, false);
if (!block) {
SetCorrupt();
return;
}
// Try to insert the block at the tail of the queue. The tail node always
// has an existing value of kReferenceQueue; if that is somehow not the
// existing value then another thread has acted in the meantime. A "strong"
// exchange is necessary so the "else" block does not get executed when
// that is not actually the case (which can happen with a "weak" exchange).
uint32_t next = kReferenceQueue; // Will get replaced with existing value.
if (block->next.compare_exchange_strong(next, ref,
std::memory_order_acq_rel,
std::memory_order_acquire)) {
// Update the tail pointer to the new offset. If the "else" clause did
// not exist, then this could be a simple Release_Store to set the new
// value but because it does, it's possible that other threads could add
// one or more nodes at the tail before reaching this point. We don't
// have to check the return value because it either operates correctly
// or the exact same operation has already been done (by the "else"
// clause) on some other thread.
shared_meta()->tailptr.compare_exchange_strong(tail, ref,
std::memory_order_release,
std::memory_order_relaxed);
return;
}
// In the unlikely case that a thread crashed or was killed between the
// update of "next" and the update of "tailptr", it is necessary to
// perform the operation that would have been done. There's no explicit
// check for crash/kill which means that this operation may also happen
// even when the other thread is in perfect working order which is what
// necessitates the CompareAndSwap above.
shared_meta()->tailptr.compare_exchange_strong(
tail, next, std::memory_order_acq_rel, std::memory_order_acquire);
}
}
// The "corrupted" state is held both locally and globally (shared). The
// shared flag can't be trusted since a malicious actor could overwrite it.
// Because corruption can be detected during read-only operations such as
// iteration, this method may be called by other "const" methods. In this
// case, it's safe to discard the constness and modify the local flag and
// maybe even the shared flag if the underlying data isn't actually read-only.
void PersistentMemoryAllocator::SetCorrupt() const {
if (!corrupt_.load(std::memory_order_relaxed) &&
!CheckFlag(
const_cast<volatile std::atomic<uint32_t>*>(&shared_meta()->flags),
kFlagCorrupt)) {
LOG(ERROR) << "Corruption detected in shared-memory segment.";
RecordError(kMemoryIsCorrupt);
}
corrupt_.store(true, std::memory_order_relaxed);
if (!readonly_) {
SetFlag(const_cast<volatile std::atomic<uint32_t>*>(&shared_meta()->flags),
kFlagCorrupt);
}
}
bool PersistentMemoryAllocator::IsCorrupt() const {
if (corrupt_.load(std::memory_order_relaxed) ||
CheckFlag(&shared_meta()->flags, kFlagCorrupt)) {
SetCorrupt(); // Make sure all indicators are set.
return true;
}
return false;
}
bool PersistentMemoryAllocator::IsFull() const {
return CheckFlag(&shared_meta()->flags, kFlagFull);
}
// Dereference a block |ref| and ensure that it's valid for the desired
// |type_id| and |size|. |special| indicates that we may try to access block
// headers not available to callers but still accessed by this module. By
// having internal dereferences go through this same function, the allocator
// is hardened against corruption.
const volatile PersistentMemoryAllocator::BlockHeader*
PersistentMemoryAllocator::GetBlock(Reference ref, uint32_t type_id,
uint32_t size, bool queue_ok,
bool free_ok) const {
// Handle special cases.
if (ref == kReferenceQueue && queue_ok)
return reinterpret_cast<const volatile BlockHeader*>(mem_base_ + ref);
// Validation of parameters.
if (ref < sizeof(SharedMetadata))
return nullptr;
if (ref % kAllocAlignment != 0)
return nullptr;
size += sizeof(BlockHeader);
if (ref + size > mem_size_)
return nullptr;
// Validation of referenced block-header.
if (!free_ok) {
const volatile BlockHeader* const block =
reinterpret_cast<volatile BlockHeader*>(mem_base_ + ref);
if (block->cookie != kBlockCookieAllocated)
return nullptr;
if (block->size < size)
return nullptr;
if (ref + block->size > mem_size_)
return nullptr;
if (type_id != 0 &&
block->type_id.load(std::memory_order_relaxed) != type_id) {
return nullptr;
}
}
// Return pointer to block data.
return reinterpret_cast<const volatile BlockHeader*>(mem_base_ + ref);
}
void PersistentMemoryAllocator::FlushPartial(size_t length, bool sync) {
// Generally there is nothing to do as every write is done through volatile
// memory with atomic instructions to guarantee consistency. This (virtual)
// method exists so that derivced classes can do special things, such as
// tell the OS to write changes to disk now rather than when convenient.
}
void PersistentMemoryAllocator::RecordError(int error) const {
if (errors_histogram_)
errors_histogram_->Add(error);
}
const volatile void* PersistentMemoryAllocator::GetBlockData(
Reference ref,
uint32_t type_id,
uint32_t size) const {
DCHECK(size > 0);
const volatile BlockHeader* block =
GetBlock(ref, type_id, size, false, false);
if (!block)
return nullptr;
return reinterpret_cast<const volatile char*>(block) + sizeof(BlockHeader);
}
void PersistentMemoryAllocator::UpdateTrackingHistograms() {
DCHECK(!readonly_);
if (used_histogram_) {
MemoryInfo meminfo;
GetMemoryInfo(&meminfo);
HistogramBase::Sample used_percent = static_cast<HistogramBase::Sample>(
((meminfo.total - meminfo.free) * 100ULL / meminfo.total));
used_histogram_->Add(used_percent);
}
}
//----- LocalPersistentMemoryAllocator -----------------------------------------
LocalPersistentMemoryAllocator::LocalPersistentMemoryAllocator(
size_t size,
uint64_t id,
base::StringPiece name)
: PersistentMemoryAllocator(AllocateLocalMemory(size),
size, 0, id, name, false) {}
LocalPersistentMemoryAllocator::~LocalPersistentMemoryAllocator() {
DeallocateLocalMemory(const_cast<char*>(mem_base_), mem_size_, mem_type_);
}
// static
PersistentMemoryAllocator::Memory
LocalPersistentMemoryAllocator::AllocateLocalMemory(size_t size) {
void* address;
#if defined(OS_WIN)
address =
::VirtualAlloc(nullptr, size, MEM_RESERVE | MEM_COMMIT, PAGE_READWRITE);
if (address)
return Memory(address, MEM_VIRTUAL);
UmaHistogramSparse("UMA.LocalPersistentMemoryAllocator.Failures.Win",
::GetLastError());
#elif defined(OS_POSIX) || defined(OS_FUCHSIA)
// MAP_ANON is deprecated on Linux but MAP_ANONYMOUS is not universal on Mac.
// MAP_SHARED is not available on Linux <2.4 but required on Mac.
address = ::mmap(nullptr, size, PROT_READ | PROT_WRITE,
MAP_ANON | MAP_SHARED, -1, 0);
if (address != MAP_FAILED)
return Memory(address, MEM_VIRTUAL);
UmaHistogramSparse("UMA.LocalPersistentMemoryAllocator.Failures.Posix",
errno);
#else
#error This architecture is not (yet) supported.
#endif
// As a last resort, just allocate the memory from the heap. This will
// achieve the same basic result but the acquired memory has to be
// explicitly zeroed and thus realized immediately (i.e. all pages are
// added to the process now istead of only when first accessed).
address = malloc(size);
DPCHECK(address);
memset(address, 0, size);
return Memory(address, MEM_MALLOC);
}
// static
void LocalPersistentMemoryAllocator::DeallocateLocalMemory(void* memory,
size_t size,
MemoryType type) {
if (type == MEM_MALLOC) {
free(memory);
return;
}
DCHECK_EQ(MEM_VIRTUAL, type);
#if defined(OS_WIN)
BOOL success = ::VirtualFree(memory, 0, MEM_DECOMMIT);
DCHECK(success);
#elif defined(OS_POSIX) || defined(OS_FUCHSIA)
int result = ::munmap(memory, size);
DCHECK_EQ(0, result);
#else
#error This architecture is not (yet) supported.
#endif
}
//----- SharedPersistentMemoryAllocator ----------------------------------------
SharedPersistentMemoryAllocator::SharedPersistentMemoryAllocator(
std::unique_ptr<SharedMemory> memory,
uint64_t id,
base::StringPiece name,
bool read_only)
: PersistentMemoryAllocator(
Memory(static_cast<uint8_t*>(memory->memory()), MEM_SHARED),
memory->mapped_size(),
0,
id,
name,
read_only),
shared_memory_(std::move(memory)) {}
SharedPersistentMemoryAllocator::~SharedPersistentMemoryAllocator() = default;
// static
bool SharedPersistentMemoryAllocator::IsSharedMemoryAcceptable(
const SharedMemory& memory) {
return IsMemoryAcceptable(memory.memory(), memory.mapped_size(), 0, false);
}
#if !defined(OS_NACL)
//----- FilePersistentMemoryAllocator ------------------------------------------
FilePersistentMemoryAllocator::FilePersistentMemoryAllocator(
std::unique_ptr<MemoryMappedFile> file,
size_t max_size,
uint64_t id,
base::StringPiece name,
bool read_only)
: PersistentMemoryAllocator(
Memory(const_cast<uint8_t*>(file->data()), MEM_FILE),
max_size != 0 ? max_size : file->length(),
0,
id,
name,
read_only),
mapped_file_(std::move(file)) {}
FilePersistentMemoryAllocator::~FilePersistentMemoryAllocator() = default;
// static
bool FilePersistentMemoryAllocator::IsFileAcceptable(
const MemoryMappedFile& file,
bool read_only) {
return IsMemoryAcceptable(file.data(), file.length(), 0, read_only);
}
void FilePersistentMemoryAllocator::FlushPartial(size_t length, bool sync) {
if (sync)
AssertBlockingAllowed();
if (IsReadonly())
return;
#if defined(OS_WIN)
// Windows doesn't support asynchronous flush.
AssertBlockingAllowed();
BOOL success = ::FlushViewOfFile(data(), length);
DPCHECK(success);
#elif defined(OS_MACOSX)
// On OSX, "invalidate" removes all cached pages, forcing a re-read from
// disk. That's not applicable to "flush" so omit it.
int result =
::msync(const_cast<void*>(data()), length, sync ? MS_SYNC : MS_ASYNC);
DCHECK_NE(EINVAL, result);
#elif defined(OS_POSIX) || defined(OS_FUCHSIA)
// On POSIX, "invalidate" forces _other_ processes to recognize what has
// been written to disk and so is applicable to "flush".
int result = ::msync(const_cast<void*>(data()), length,
MS_INVALIDATE | (sync ? MS_SYNC : MS_ASYNC));
DCHECK_NE(EINVAL, result);
#else
#error Unsupported OS.
#endif
}
#endif // !defined(OS_NACL)
//----- DelayedPersistentAllocation --------------------------------------------
// Forwarding constructors.
DelayedPersistentAllocation::DelayedPersistentAllocation(
PersistentMemoryAllocator* allocator,
subtle::Atomic32* ref,
uint32_t type,
size_t size,
bool make_iterable)
: DelayedPersistentAllocation(
allocator,
reinterpret_cast<std::atomic<Reference>*>(ref),
type,
size,
0,
make_iterable) {}
DelayedPersistentAllocation::DelayedPersistentAllocation(
PersistentMemoryAllocator* allocator,
subtle::Atomic32* ref,
uint32_t type,
size_t size,
size_t offset,
bool make_iterable)
: DelayedPersistentAllocation(
allocator,
reinterpret_cast<std::atomic<Reference>*>(ref),
type,
size,
offset,
make_iterable) {}
DelayedPersistentAllocation::DelayedPersistentAllocation(
PersistentMemoryAllocator* allocator,
std::atomic<Reference>* ref,
uint32_t type,
size_t size,
bool make_iterable)
: DelayedPersistentAllocation(allocator,
ref,
type,
size,
0,
make_iterable) {}
// Real constructor.
DelayedPersistentAllocation::DelayedPersistentAllocation(
PersistentMemoryAllocator* allocator,
std::atomic<Reference>* ref,
uint32_t type,
size_t size,
size_t offset,
bool make_iterable)
: allocator_(allocator),
type_(type),
size_(checked_cast<uint32_t>(size)),
offset_(checked_cast<uint32_t>(offset)),
make_iterable_(make_iterable),
reference_(ref) {
DCHECK(allocator_);
DCHECK_NE(0U, type_);
DCHECK_LT(0U, size_);
DCHECK(reference_);
}
DelayedPersistentAllocation::~DelayedPersistentAllocation() = default;
void* DelayedPersistentAllocation::Get() const {
// Relaxed operations are acceptable here because it's not protecting the
// contents of the allocation in any way.
Reference ref = reference_->load(std::memory_order_acquire);
if (!ref) {
ref = allocator_->Allocate(size_, type_);
if (!ref)
return nullptr;
// Store the new reference in its proper location using compare-and-swap.
// Use a "strong" exchange to ensure no false-negatives since the operation
// cannot be retried.
Reference existing = 0; // Must be mutable; receives actual value.
if (reference_->compare_exchange_strong(existing, ref,
std::memory_order_release,
std::memory_order_relaxed)) {
if (make_iterable_)
allocator_->MakeIterable(ref);
} else {
// Failure indicates that something else has raced ahead, performed the
// allocation, and stored its reference. Purge the allocation that was
// just done and use the other one instead.
DCHECK_EQ(type_, allocator_->GetType(existing));
DCHECK_LE(size_, allocator_->GetAllocSize(existing));
allocator_->ChangeType(ref, 0, type_, /*clear=*/false);
ref = existing;
}
}
char* mem = allocator_->GetAsArray<char>(ref, type_, size_);
if (!mem) {
// This should never happen but be tolerant if it does as corruption from
// the outside is something to guard against.
NOTREACHED();
return nullptr;
}
return mem + offset_;
}
} // namespace base