memory-chunk.cc

// Copyright 2020 the V8 project 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 "src/heap/memory-chunk.h"

#include "src/heap/memory-chunk-inl.h"
#include "src/heap/spaces.h"

namespace v8 {
namespace internal {

void MemoryChunk::DiscardUnusedMemory(Address addr, size_t size) {
  base::AddressRegion memory_area =
      MemoryAllocator::ComputeDiscardMemoryArea(addr, size);
  if (memory_area.size() != 0) {
    MemoryAllocator* memory_allocator = heap_->memory_allocator();
    v8::PageAllocator* page_allocator =
        memory_allocator->page_allocator(executable());
    CHECK(page_allocator->DiscardSystemPages(
        reinterpret_cast<void*>(memory_area.begin()), memory_area.size()));
  }
}

size_t MemoryChunkLayout::CodePageGuardStartOffset() {
  // We are guarding code pages: the first OS page after the header
  // will be protected as non-writable.
  return ::RoundUp(Page::kHeaderSize, MemoryAllocator::GetCommitPageSize());
}

size_t MemoryChunkLayout::CodePageGuardSize() {
  return MemoryAllocator::GetCommitPageSize();
}

intptr_t MemoryChunkLayout::ObjectStartOffsetInCodePage() {
  // We are guarding code pages: the first OS page after the header
  // will be protected as non-writable.
  return CodePageGuardStartOffset() + CodePageGuardSize();
}

intptr_t MemoryChunkLayout::ObjectEndOffsetInCodePage() {
  // We are guarding code pages: the last OS page will be protected as
  // non-writable.
  return Page::kPageSize -
         static_cast<int>(MemoryAllocator::GetCommitPageSize());
}

size_t MemoryChunkLayout::AllocatableMemoryInCodePage() {
  size_t memory = ObjectEndOffsetInCodePage() - ObjectStartOffsetInCodePage();
  return memory;
}

intptr_t MemoryChunkLayout::ObjectStartOffsetInDataPage() {
  return RoundUp(MemoryChunk::kHeaderSize, kTaggedSize);
}

size_t MemoryChunkLayout::ObjectStartOffsetInMemoryChunk(
    AllocationSpace space) {
  if (space == CODE_SPACE) {
    return ObjectStartOffsetInCodePage();
  }
  return ObjectStartOffsetInDataPage();
}

size_t MemoryChunkLayout::AllocatableMemoryInDataPage() {
  size_t memory = MemoryChunk::kPageSize - ObjectStartOffsetInDataPage();
  DCHECK_LE(kMaxRegularHeapObjectSize, memory);
  return memory;
}

size_t MemoryChunkLayout::AllocatableMemoryInMemoryChunk(
    AllocationSpace space) {
  if (space == CODE_SPACE) {
    return AllocatableMemoryInCodePage();
  }
  return AllocatableMemoryInDataPage();
}

int MemoryChunkLayout::MaxRegularCodeObjectSize() {
  int size = static_cast<int>(AllocatableMemoryInCodePage() / 2);
  DCHECK_LE(size, kMaxRegularHeapObjectSize);
  return size;
}

#ifdef THREAD_SANITIZER
void MemoryChunk::SynchronizedHeapLoad() {
  CHECK(reinterpret_cast<Heap*>(base::Acquire_Load(
            reinterpret_cast<base::AtomicWord*>(&heap_))) != nullptr ||
        InReadOnlySpace());
}
#endif

void MemoryChunk::InitializationMemoryFence() {
  base::SeqCst_MemoryFence();
#ifdef THREAD_SANITIZER
  // Since TSAN does not process memory fences, we use the following annotation
  // to tell TSAN that there is no data race when emitting a
  // InitializationMemoryFence. Note that the other thread still needs to
  // perform MemoryChunk::synchronized_heap().
  base::Release_Store(reinterpret_cast<base::AtomicWord*>(&heap_),
                      reinterpret_cast<base::AtomicWord>(heap_));
#endif
}

void MemoryChunk::DecrementWriteUnprotectCounterAndMaybeSetPermissions(
    PageAllocator::Permission permission) {
  DCHECK(permission == PageAllocator::kRead ||
         permission == PageAllocator::kReadExecute);
  DCHECK(IsFlagSet(MemoryChunk::IS_EXECUTABLE));
  DCHECK(owner_identity() == CODE_SPACE || owner_identity() == CODE_LO_SPACE);
  // Decrementing the write_unprotect_counter_ and changing the page
  // protection mode has to be atomic.
  base::MutexGuard guard(page_protection_change_mutex_);
  if (write_unprotect_counter_ == 0) {
    // This is a corner case that may happen when we have a
    // CodeSpaceMemoryModificationScope open and this page was newly
    // added.
    return;
  }
  write_unprotect_counter_--;
  DCHECK_LT(write_unprotect_counter_, kMaxWriteUnprotectCounter);
  if (write_unprotect_counter_ == 0) {
    Address protect_start =
        address() + MemoryChunkLayout::ObjectStartOffsetInCodePage();
    size_t page_size = MemoryAllocator::GetCommitPageSize();
    DCHECK(IsAligned(protect_start, page_size));
    size_t protect_size = RoundUp(area_size(), page_size);
    CHECK(reservation_.SetPermissions(protect_start, protect_size, permission));
  }
}

void MemoryChunk::SetReadable() {
  DecrementWriteUnprotectCounterAndMaybeSetPermissions(PageAllocator::kRead);
}

void MemoryChunk::SetReadAndExecutable() {
  DCHECK(!FLAG_jitless);
  DecrementWriteUnprotectCounterAndMaybeSetPermissions(
      PageAllocator::kReadExecute);
}

void MemoryChunk::SetReadAndWritable() {
  DCHECK(IsFlagSet(MemoryChunk::IS_EXECUTABLE));
  DCHECK(owner_identity() == CODE_SPACE || owner_identity() == CODE_LO_SPACE);
  // Incrementing the write_unprotect_counter_ and changing the page
  // protection mode has to be atomic.
  base::MutexGuard guard(page_protection_change_mutex_);
  write_unprotect_counter_++;
  DCHECK_LE(write_unprotect_counter_, kMaxWriteUnprotectCounter);
  if (write_unprotect_counter_ == 1) {
    Address unprotect_start =
        address() + MemoryChunkLayout::ObjectStartOffsetInCodePage();
    size_t page_size = MemoryAllocator::GetCommitPageSize();
    DCHECK(IsAligned(unprotect_start, page_size));
    size_t unprotect_size = RoundUp(area_size(), page_size);
    CHECK(reservation_.SetPermissions(unprotect_start, unprotect_size,
                                      PageAllocator::kReadWrite));
  }
}

}  // namespace internal
}  // namespace v8