src/base/allocator/partition_allocator/partition_page.cc

// Copyright (c) 2018 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/allocator/partition_allocator/partition_page.h"

#include "base/allocator/partition_allocator/partition_direct_map_extent.h"
#include "base/allocator/partition_allocator/partition_root_base.h"

namespace base {
namespace internal {

namespace {

ALWAYS_INLINE void PartitionDirectUnmap(PartitionPage* page) {
  PartitionRootBase* root = PartitionRootBase::FromPage(page);
  const PartitionDirectMapExtent* extent =
      PartitionDirectMapExtent::FromPage(page);
  size_t unmap_size = extent->map_size;

  // Maintain the doubly-linked list of all direct mappings.
  if (extent->prev_extent) {
    DCHECK(extent->prev_extent->next_extent == extent);
    extent->prev_extent->next_extent = extent->next_extent;
  } else {
    root->direct_map_list = extent->next_extent;
  }
  if (extent->next_extent) {
    DCHECK(extent->next_extent->prev_extent == extent);
    extent->next_extent->prev_extent = extent->prev_extent;
  }

  // Add on the size of the trailing guard page and preceeding partition
  // page.
  unmap_size += kPartitionPageSize + kSystemPageSize;

  size_t uncommitted_page_size = page->bucket->slot_size + kSystemPageSize;
  root->DecreaseCommittedPages(uncommitted_page_size);
  DCHECK(root->total_size_of_direct_mapped_pages >= uncommitted_page_size);
  root->total_size_of_direct_mapped_pages -= uncommitted_page_size;

  DCHECK(!(unmap_size & kPageAllocationGranularityOffsetMask));

  char* ptr = reinterpret_cast<char*>(PartitionPage::ToPointer(page));
  // Account for the mapping starting a partition page before the actual
  // allocation address.
  ptr -= kPartitionPageSize;

  FreePages(ptr, unmap_size);
}

ALWAYS_INLINE void PartitionRegisterEmptyPage(PartitionPage* page) {
  DCHECK(page->is_empty());
  PartitionRootBase* root = PartitionRootBase::FromPage(page);

  // If the page is already registered as empty, give it another life.
  if (page->empty_cache_index != -1) {
    DCHECK(page->empty_cache_index >= 0);
    DCHECK(static_cast<unsigned>(page->empty_cache_index) < kMaxFreeableSpans);
    DCHECK(root->global_empty_page_ring[page->empty_cache_index] == page);
    root->global_empty_page_ring[page->empty_cache_index] = nullptr;
  }

  int16_t current_index = root->global_empty_page_ring_index;
  PartitionPage* page_to_decommit = root->global_empty_page_ring[current_index];
  // The page might well have been re-activated, filled up, etc. before we get
  // around to looking at it here.
  if (page_to_decommit)
    page_to_decommit->DecommitIfPossible(root);

  // We put the empty slot span on our global list of "pages that were once
  // empty". thus providing it a bit of breathing room to get re-used before
  // we really free it. This improves performance, particularly on Mac OS X
  // which has subpar memory management performance.
  root->global_empty_page_ring[current_index] = page;
  page->empty_cache_index = current_index;
  ++current_index;
  if (current_index == kMaxFreeableSpans)
    current_index = 0;
  root->global_empty_page_ring_index = current_index;
}

}  // namespace

// static
PartitionPage PartitionPage::sentinel_page_;

PartitionPage* PartitionPage::get_sentinel_page() {
  return &sentinel_page_;
}

void PartitionPage::FreeSlowPath() {
  DCHECK(this != get_sentinel_page());
  if (LIKELY(this->num_allocated_slots == 0)) {
    // Page became fully unused.
    if (UNLIKELY(bucket->is_direct_mapped())) {
      PartitionDirectUnmap(this);
      return;
    }
    // If it's the current active page, change it. We bounce the page to
    // the empty list as a force towards defragmentation.
    if (LIKELY(this == bucket->active_pages_head))
      bucket->SetNewActivePage();
    DCHECK(bucket->active_pages_head != this);

    set_raw_size(0);
    DCHECK(!get_raw_size());

    PartitionRegisterEmptyPage(this);
  } else {
    DCHECK(!bucket->is_direct_mapped());
    // Ensure that the page is full. That's the only valid case if we
    // arrive here.
    DCHECK(this->num_allocated_slots < 0);
    // A transition of num_allocated_slots from 0 to -1 is not legal, and
    // likely indicates a double-free.
    CHECK(this->num_allocated_slots != -1);
    this->num_allocated_slots = -this->num_allocated_slots - 2;
    DCHECK(this->num_allocated_slots == bucket->get_slots_per_span() - 1);
    // Fully used page became partially used. It must be put back on the
    // non-full page list. Also make it the current page to increase the
    // chances of it being filled up again. The old current page will be
    // the next page.
    DCHECK(!this->next_page);
    if (LIKELY(bucket->active_pages_head != get_sentinel_page()))
      this->next_page = bucket->active_pages_head;
    bucket->active_pages_head = this;
    --bucket->num_full_pages;
    // Special case: for a partition page with just a single slot, it may
    // now be empty and we want to run it through the empty logic.
    if (UNLIKELY(this->num_allocated_slots == 0))
      FreeSlowPath();
  }
}

void PartitionPage::Decommit(PartitionRootBase* root) {
  DCHECK(is_empty());
  DCHECK(!bucket->is_direct_mapped());
  void* addr = PartitionPage::ToPointer(this);
  root->DecommitSystemPages(addr, bucket->get_bytes_per_span());

  // We actually leave the decommitted page in the active list. We'll sweep
  // it on to the decommitted page list when we next walk the active page
  // list.
  // Pulling this trick enables us to use a singly-linked page list for all
  // cases, which is critical in keeping the page metadata structure down to
  // 32 bytes in size.
  freelist_head = nullptr;
  num_unprovisioned_slots = 0;
  DCHECK(is_decommitted());
}

void PartitionPage::DecommitIfPossible(PartitionRootBase* root) {
  DCHECK(empty_cache_index >= 0);
  DCHECK(static_cast<unsigned>(empty_cache_index) < kMaxFreeableSpans);
  DCHECK(this == root->global_empty_page_ring[empty_cache_index]);
  empty_cache_index = -1;
  if (is_empty())
    Decommit(root);
}

}  // namespace internal
}  // namespace base