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- // 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_bucket.h"
- #include <algorithm>
- #include <cstdint>
- #include <tuple>
- #include "base/allocator/partition_allocator/address_pool_manager.h"
- #include "base/allocator/partition_allocator/oom.h"
- #include "base/allocator/partition_allocator/page_allocator.h"
- #include "base/allocator/partition_allocator/page_allocator_constants.h"
- #include "base/allocator/partition_allocator/partition_address_space.h"
- #include "base/allocator/partition_allocator/partition_alloc.h"
- #include "base/allocator/partition_allocator/partition_alloc_base/bits.h"
- #include "base/allocator/partition_allocator/partition_alloc_base/compiler_specific.h"
- #include "base/allocator/partition_allocator/partition_alloc_base/component_export.h"
- #include "base/allocator/partition_allocator/partition_alloc_base/debug/alias.h"
- #include "base/allocator/partition_allocator/partition_alloc_base/debug/debugging_buildflags.h"
- #include "base/allocator/partition_allocator/partition_alloc_base/immediate_crash.h"
- #include "base/allocator/partition_allocator/partition_alloc_base/thread_annotations.h"
- #include "base/allocator/partition_allocator/partition_alloc_buildflags.h"
- #include "base/allocator/partition_allocator/partition_alloc_check.h"
- #include "base/allocator/partition_allocator/partition_alloc_config.h"
- #include "base/allocator/partition_allocator/partition_alloc_constants.h"
- #include "base/allocator/partition_allocator/partition_alloc_forward.h"
- #include "base/allocator/partition_allocator/partition_direct_map_extent.h"
- #include "base/allocator/partition_allocator/partition_oom.h"
- #include "base/allocator/partition_allocator/partition_page.h"
- #include "base/allocator/partition_allocator/partition_tag.h"
- #include "base/allocator/partition_allocator/partition_tag_bitmap.h"
- #include "base/allocator/partition_allocator/reservation_offset_table.h"
- #include "base/allocator/partition_allocator/starscan/state_bitmap.h"
- #include "base/allocator/partition_allocator/tagging.h"
- #include "build/build_config.h"
- namespace partition_alloc::internal {
- namespace {
- template <bool thread_safe>
- [[noreturn]] PA_NOINLINE void PartitionOutOfMemoryMappingFailure(
- PartitionRoot<thread_safe>* root,
- size_t size) PA_LOCKS_EXCLUDED(root->lock_) {
- PA_NO_CODE_FOLDING();
- root->OutOfMemory(size);
- PA_IMMEDIATE_CRASH(); // Not required, kept as documentation.
- }
- template <bool thread_safe>
- [[noreturn]] PA_NOINLINE void PartitionOutOfMemoryCommitFailure(
- PartitionRoot<thread_safe>* root,
- size_t size) PA_LOCKS_EXCLUDED(root->lock_) {
- PA_NO_CODE_FOLDING();
- root->OutOfMemory(size);
- PA_IMMEDIATE_CRASH(); // Not required, kept as documentation.
- }
- #if !defined(PA_HAS_64_BITS_POINTERS) && BUILDFLAG(USE_BACKUP_REF_PTR)
- // |start| has to be aligned to kSuperPageSize, but |end| doesn't. This means
- // that a partial super page is allowed at the end. Since the block list uses
- // kSuperPageSize granularity, a partial super page is considered blocked if
- // there is a raw_ptr<T> pointing anywhere in that super page, even if doesn't
- // point to that partially allocated region.
- bool AreAllowedSuperPagesForBRPPool(uintptr_t start, uintptr_t end) {
- PA_DCHECK(!(start % kSuperPageSize));
- for (uintptr_t super_page = start; super_page < end;
- super_page += kSuperPageSize) {
- // If any blocked super page is found inside the given memory region,
- // the memory region is blocked.
- if (!AddressPoolManagerBitmap::IsAllowedSuperPageForBRPPool(super_page)) {
- AddressPoolManagerBitmap::IncrementBlocklistHitCount();
- return false;
- }
- }
- return true;
- }
- #endif // !defined(PA_HAS_64_BITS_POINTERS) && BUILDFLAG(USE_BACKUP_REF_PTR)
- // Reserves |requested_size| worth of super pages from the specified pool of the
- // GigaCage. If BRP pool is requested this function will honor BRP block list.
- //
- // The returned address will be aligned to kSuperPageSize, and so
- // |requested_address| should be. |requested_size| doesn't have to be, however.
- //
- // |requested_address| is merely a hint, which will be attempted, but easily
- // given up on if doesn't work the first time.
- //
- // The function doesn't need to hold root->lock_ or any other locks, because:
- // - It (1) reserves memory, (2) then consults AreAllowedSuperPagesForBRPPool
- // for that memory, and (3) returns the memory if
- // allowed, or unreserves and decommits if not allowed. So no other
- // overlapping region can be allocated while executing
- // AreAllowedSuperPagesForBRPPool.
- // - IsAllowedSuperPageForBRPPool (used by AreAllowedSuperPagesForBRPPool) is
- // designed to not need locking.
- uintptr_t ReserveMemoryFromGigaCage(pool_handle pool,
- uintptr_t requested_address,
- size_t requested_size) {
- PA_DCHECK(!(requested_address % kSuperPageSize));
- uintptr_t reserved_address = AddressPoolManager::GetInstance().Reserve(
- pool, requested_address, requested_size);
- // In 32-bit mode, when allocating from BRP pool, verify that the requested
- // allocation honors the block list. Find a better address otherwise.
- #if !defined(PA_HAS_64_BITS_POINTERS) && BUILDFLAG(USE_BACKUP_REF_PTR)
- if (pool == GetBRPPool()) {
- constexpr int kMaxRandomAddressTries = 10;
- for (int i = 0; i < kMaxRandomAddressTries; ++i) {
- if (!reserved_address ||
- AreAllowedSuperPagesForBRPPool(reserved_address,
- reserved_address + requested_size))
- break;
- AddressPoolManager::GetInstance().UnreserveAndDecommit(
- pool, reserved_address, requested_size);
- // No longer try to honor |requested_address|, because it didn't work for
- // us last time.
- reserved_address =
- AddressPoolManager::GetInstance().Reserve(pool, 0, requested_size);
- }
- // If the allocation attempt succeeds, we will break out of the following
- // loop immediately.
- //
- // Last resort: sequentially scan the whole 32-bit address space. The number
- // of blocked super-pages should be very small, so we expect to practically
- // never need to run the following code. Note that it may fail to find an
- // available super page, e.g., when it becomes available after the scan
- // passes through it, but we accept the risk.
- for (uintptr_t address_to_try = kSuperPageSize; address_to_try != 0;
- address_to_try += kSuperPageSize) {
- if (!reserved_address ||
- AreAllowedSuperPagesForBRPPool(reserved_address,
- reserved_address + requested_size))
- break;
- AddressPoolManager::GetInstance().UnreserveAndDecommit(
- pool, reserved_address, requested_size);
- // Reserve() can return a different pointer than attempted.
- reserved_address = AddressPoolManager::GetInstance().Reserve(
- pool, address_to_try, requested_size);
- }
- // If the loop ends naturally, the last allocated region hasn't been
- // verified. Do it now.
- if (reserved_address &&
- !AreAllowedSuperPagesForBRPPool(reserved_address,
- reserved_address + requested_size)) {
- AddressPoolManager::GetInstance().UnreserveAndDecommit(
- pool, reserved_address, requested_size);
- reserved_address = 0;
- }
- }
- #endif // !defined(PA_HAS_64_BITS_POINTERS) && BUILDFLAG(USE_BACKUP_REF_PTR)
- #if !defined(PA_HAS_64_BITS_POINTERS)
- // Only mark the region as belonging to the pool after it has passed the
- // blocklist check in order to avoid a potential race with destructing a
- // raw_ptr<T> object that points to non-PA memory in another thread.
- // If `MarkUsed` was called earlier, the other thread could incorrectly
- // determine that the allocation had come form PartitionAlloc.
- if (reserved_address)
- AddressPoolManager::GetInstance().MarkUsed(pool, reserved_address,
- requested_size);
- #endif
- PA_DCHECK(!(reserved_address % kSuperPageSize));
- return reserved_address;
- }
- template <bool thread_safe>
- SlotSpanMetadata<thread_safe>* PartitionDirectMap(
- PartitionRoot<thread_safe>* root,
- unsigned int flags,
- size_t raw_size,
- size_t slot_span_alignment) {
- PA_DCHECK((slot_span_alignment >= PartitionPageSize()) &&
- base::bits::IsPowerOfTwo(slot_span_alignment));
- // No static EXCLUSIVE_LOCKS_REQUIRED(), as the checker doesn't understand
- // scoped unlocking.
- root->lock_.AssertAcquired();
- const bool return_null = flags & AllocFlags::kReturnNull;
- if (PA_UNLIKELY(raw_size > MaxDirectMapped())) {
- if (return_null)
- return nullptr;
- // The lock is here to protect PA from:
- // 1. Concurrent calls
- // 2. Reentrant calls
- //
- // This is fine here however, as:
- // 1. Concurrency: |PartitionRoot::OutOfMemory()| never returns, so the lock
- // will not be re-acquired, which would lead to acting on inconsistent
- // data that could have been modified in-between releasing and acquiring
- // it.
- // 2. Reentrancy: This is why we release the lock. On some platforms,
- // terminating the process may free() memory, or even possibly try to
- // allocate some. Calling free() is fine, but will deadlock since
- // |PartitionRoot::lock_| is not recursive.
- //
- // Supporting reentrant calls properly is hard, and not a requirement for
- // PA. However up to that point, we've only *read* data, not *written* to
- // any state. Reentrant calls are then fine, especially as we don't continue
- // on this path. The only downside is possibly endless recursion if the OOM
- // handler allocates and fails to use UncheckedMalloc() or equivalent, but
- // that's violating the contract of base::TerminateBecauseOutOfMemory().
- ScopedUnlockGuard unlock{root->lock_};
- PartitionExcessiveAllocationSize(raw_size);
- }
- PartitionDirectMapExtent<thread_safe>* map_extent = nullptr;
- PartitionPage<thread_safe>* page = nullptr;
- #if defined(PA_USE_MTE_CHECKED_PTR_WITH_64_BITS_POINTERS)
- const PartitionTag tag = root->GetNewPartitionTag();
- #endif // defined(PA_USE_MTE_CHECKED_PTR_WITH_64_BITS_POINTERS)
- {
- // Getting memory for direct-mapped allocations doesn't interact with the
- // rest of the allocator, but takes a long time, as it involves several
- // system calls. With GigaCage, no mmap() (or equivalent) call is made on 64
- // bit systems, but page permissions are changed with mprotect(), which is a
- // syscall.
- //
- // These calls are almost always slow (at least a couple us per syscall on a
- // desktop Linux machine), and they also have a very long latency tail,
- // possibly from getting descheduled. As a consequence, we should not hold
- // the lock when performing a syscall. This is not the only problematic
- // location, but since this one doesn't interact with the rest of the
- // allocator, we can safely drop and then re-acquire the lock.
- //
- // Note that this only affects allocations that are not served out of the
- // thread cache, but as a simple example the buffer partition in blink is
- // frequently used for large allocations (e.g. ArrayBuffer), and frequent,
- // small ones (e.g. WTF::String), and does not have a thread cache.
- ScopedUnlockGuard scoped_unlock{root->lock_};
- const size_t slot_size =
- PartitionRoot<thread_safe>::GetDirectMapSlotSize(raw_size);
- // The super page starts with a partition page worth of metadata and guard
- // pages, hence alignment requests ==PartitionPageSize() will be
- // automatically satisfied. Padding is needed for higher-order alignment
- // requests. Note, |slot_span_alignment| is at least 1 partition page.
- const size_t padding_for_alignment =
- slot_span_alignment - PartitionPageSize();
- const size_t reservation_size =
- PartitionRoot<thread_safe>::GetDirectMapReservationSize(
- raw_size + padding_for_alignment);
- #if BUILDFLAG(PA_DCHECK_IS_ON)
- const size_t available_reservation_size =
- reservation_size - padding_for_alignment -
- PartitionRoot<thread_safe>::GetDirectMapMetadataAndGuardPagesSize();
- PA_DCHECK(slot_size <= available_reservation_size);
- #endif
- // Allocate from GigaCage. Route to the appropriate GigaCage pool based on
- // BackupRefPtr support.
- pool_handle pool = root->ChoosePool();
- uintptr_t reservation_start;
- {
- // Reserving memory from the GigaCage is actually not a syscall on 64 bit
- // platforms.
- #if !defined(PA_HAS_64_BITS_POINTERS)
- ScopedSyscallTimer timer{root};
- #endif
- reservation_start = ReserveMemoryFromGigaCage(pool, 0, reservation_size);
- }
- if (PA_UNLIKELY(!reservation_start)) {
- if (return_null)
- return nullptr;
- PartitionOutOfMemoryMappingFailure(root, reservation_size);
- }
- root->total_size_of_direct_mapped_pages.fetch_add(
- reservation_size, std::memory_order_relaxed);
- // Shift by 1 partition page (metadata + guard pages) and alignment padding.
- const uintptr_t slot_start =
- reservation_start + PartitionPageSize() + padding_for_alignment;
- {
- ScopedSyscallTimer timer{root};
- RecommitSystemPages(
- reservation_start + SystemPageSize(),
- #if BUILDFLAG(PUT_REF_COUNT_IN_PREVIOUS_SLOT)
- // If PUT_REF_COUNT_IN_PREVIOUS_SLOT is on, and if the BRP pool is
- // used, allocate 2 SystemPages, one for SuperPage metadata and the
- // other for RefCount "bitmap" (only one of its elements will be
- // used).
- (pool == GetBRPPool()) ? SystemPageSize() * 2 : SystemPageSize(),
- #else
- SystemPageSize(),
- #endif
- PageAccessibilityConfiguration::kReadWrite,
- PageAccessibilityDisposition::kRequireUpdate);
- }
- // No need to hold root->lock_. Now that memory is reserved, no other
- // overlapping region can be allocated (because of how GigaCage works),
- // so no other thread can update the same offset table entries at the
- // same time. Furthermore, nobody will be ready these offsets until this
- // function returns.
- uintptr_t address_start = reservation_start;
- uintptr_t address_end = address_start + reservation_size;
- auto* offset_ptr = ReservationOffsetPointer(address_start);
- uint16_t offset = 0;
- while (address_start < address_end) {
- PA_DCHECK(offset_ptr < GetReservationOffsetTableEnd(address_start));
- PA_DCHECK(offset < kOffsetTagNormalBuckets);
- *offset_ptr++ = offset++;
- address_start += kSuperPageSize;
- }
- auto* super_page_extent =
- PartitionSuperPageToExtent<thread_safe>(reservation_start);
- super_page_extent->root = root;
- // The new structures are all located inside a fresh system page so they
- // will all be zeroed out. These DCHECKs are for documentation and to assert
- // our expectations of the kernel.
- PA_DCHECK(!super_page_extent->number_of_consecutive_super_pages);
- PA_DCHECK(!super_page_extent->next);
- PartitionPage<thread_safe>* first_page =
- reinterpret_cast<PartitionPage<thread_safe>*>(super_page_extent) + 1;
- page = PartitionPage<thread_safe>::FromAddr(slot_start);
- // |first_page| and |page| may be equal, if there is no alignment padding.
- if (page != first_page) {
- PA_DCHECK(page > first_page);
- PA_DCHECK(page - first_page <=
- PartitionPage<thread_safe>::kMaxSlotSpanMetadataOffset);
- PA_CHECK(!first_page->is_valid);
- first_page->has_valid_span_after_this = true;
- first_page->slot_span_metadata_offset = page - first_page;
- }
- auto* metadata =
- reinterpret_cast<PartitionDirectMapMetadata<thread_safe>*>(page);
- // Since direct map metadata is larger than PartitionPage, make sure the
- // first and the last bytes are on the same system page, i.e. within the
- // super page metadata region.
- PA_DCHECK(base::bits::AlignDown(reinterpret_cast<uintptr_t>(metadata),
- SystemPageSize()) ==
- base::bits::AlignDown(
- reinterpret_cast<uintptr_t>(metadata) +
- sizeof(PartitionDirectMapMetadata<thread_safe>) - 1,
- SystemPageSize()));
- PA_DCHECK(page == &metadata->page);
- page->is_valid = true;
- PA_DCHECK(!page->has_valid_span_after_this);
- PA_DCHECK(!page->slot_span_metadata_offset);
- PA_DCHECK(!page->slot_span_metadata.next_slot_span);
- PA_DCHECK(!page->slot_span_metadata.marked_full);
- PA_DCHECK(!page->slot_span_metadata.num_allocated_slots);
- PA_DCHECK(!page->slot_span_metadata.num_unprovisioned_slots);
- PA_DCHECK(!page->slot_span_metadata.in_empty_cache());
- PA_DCHECK(!metadata->subsequent_page.subsequent_page_metadata.raw_size);
- // Raw size is set later, by the caller.
- metadata->subsequent_page.slot_span_metadata_offset = 1;
- PA_DCHECK(!metadata->bucket.active_slot_spans_head);
- PA_DCHECK(!metadata->bucket.empty_slot_spans_head);
- PA_DCHECK(!metadata->bucket.decommitted_slot_spans_head);
- PA_DCHECK(!metadata->bucket.num_system_pages_per_slot_span);
- PA_DCHECK(!metadata->bucket.num_full_slot_spans);
- metadata->bucket.slot_size = slot_size;
- new (&page->slot_span_metadata)
- SlotSpanMetadata<thread_safe>(&metadata->bucket);
- // It is typically possible to map a large range of inaccessible pages, and
- // this is leveraged in multiple places, including the GigaCage. However,
- // this doesn't mean that we can commit all this memory. For the vast
- // majority of allocations, this just means that we crash in a slightly
- // different place, but for callers ready to handle failures, we have to
- // return nullptr. See crbug.com/1187404.
- //
- // Note that we didn't check above, because if we cannot even commit a
- // single page, then this is likely hopeless anyway, and we will crash very
- // soon.
- const bool ok = root->TryRecommitSystemPagesForData(
- slot_start, slot_size, PageAccessibilityDisposition::kRequireUpdate);
- if (!ok) {
- if (!return_null) {
- PartitionOutOfMemoryCommitFailure(root, slot_size);
- }
- {
- ScopedSyscallTimer timer{root};
- #if !defined(PA_HAS_64_BITS_POINTERS)
- AddressPoolManager::GetInstance().MarkUnused(pool, reservation_start,
- reservation_size);
- #endif
- AddressPoolManager::GetInstance().UnreserveAndDecommit(
- pool, reservation_start, reservation_size);
- }
- root->total_size_of_direct_mapped_pages.fetch_sub(
- reservation_size, std::memory_order_relaxed);
- return nullptr;
- }
- auto* next_entry = PartitionFreelistEntry::EmplaceAndInitNull(slot_start);
- page->slot_span_metadata.SetFreelistHead(next_entry);
- map_extent = &metadata->direct_map_extent;
- map_extent->reservation_size = reservation_size;
- map_extent->padding_for_alignment = padding_for_alignment;
- map_extent->bucket = &metadata->bucket;
- #if defined(PA_USE_MTE_CHECKED_PTR_WITH_64_BITS_POINTERS)
- DirectMapPartitionTagSetValue(slot_start, tag);
- #endif // defined(PA_USE_MTE_CHECKED_PTR_WITH_64_BITS_POINTERS)
- }
- root->lock_.AssertAcquired();
- // Maintain the doubly-linked list of all direct mappings.
- map_extent->next_extent = root->direct_map_list;
- if (map_extent->next_extent)
- map_extent->next_extent->prev_extent = map_extent;
- map_extent->prev_extent = nullptr;
- root->direct_map_list = map_extent;
- return &page->slot_span_metadata;
- }
- uint8_t ComputeSystemPagesPerSlotSpanPreferSmall(size_t slot_size) {
- if (slot_size > MaxRegularSlotSpanSize()) {
- // This is technically not needed, as for now all the larger slot sizes are
- // multiples of the system page size.
- return base::bits::AlignUp(slot_size, SystemPageSize()) / SystemPageSize();
- }
- // Smaller slot spans waste less address space, as well as potentially lower
- // fragmentation:
- // - Address space: This comes from fuller SuperPages (since the tail end of a
- // SuperPage is more likely to be used when the slot span is smaller. Also,
- // if a slot span is partially used, a smaller slot span will use less
- // address space.
- // - In-slot fragmentation: Slot span management code will prioritize
- // almost-full slot spans, as well as trying to keep empty slot spans
- // empty. The more granular this logic can work, the better.
- //
- // Since metadata space overhead is constant per-PartitionPage, keeping
- // smaller slot spans makes sense.
- //
- // Underlying memory allocation is done per-PartitionPage, but memory commit
- // is done per system page. This means that we prefer to fill the entirety of
- // a PartitionPage with a slot span, but we can tolerate some system pages
- // being empty at the end, as these will not cost committed or dirty memory.
- //
- // The choice below is, for multi-slot slot spans:
- // - If a full PartitionPage slot span is possible with less than 2% of a
- // *single* system page wasted, use it. The smallest possible size wins.
- // - Otherwise, select the size with the smallest virtual address space
- // loss. Allow a SlotSpan to leave some slack in its PartitionPage, up to
- // 1/4 of the total.
- for (size_t partition_page_count = 1;
- partition_page_count <= kMaxPartitionPagesPerRegularSlotSpan;
- partition_page_count++) {
- size_t candidate_size = partition_page_count * PartitionPageSize();
- size_t waste = candidate_size % slot_size;
- if (waste <= .02 * SystemPageSize())
- return partition_page_count * NumSystemPagesPerPartitionPage();
- }
- size_t best_count = 0;
- size_t best_waste = std::numeric_limits<size_t>::max();
- for (size_t partition_page_count = 1;
- partition_page_count <= kMaxPartitionPagesPerRegularSlotSpan;
- partition_page_count++) {
- // Prefer no slack.
- for (size_t slack = 0; slack < partition_page_count; slack++) {
- size_t system_page_count =
- partition_page_count * NumSystemPagesPerPartitionPage() - slack;
- size_t candidate_size = system_page_count * SystemPageSize();
- size_t waste = candidate_size % slot_size;
- if (waste < best_waste) {
- best_waste = waste;
- best_count = system_page_count;
- }
- }
- }
- return best_count;
- }
- uint8_t ComputeSystemPagesPerSlotSpanInternal(size_t slot_size) {
- // This works out reasonably for the current bucket sizes of the generic
- // allocator, and the current values of partition page size and constants.
- // Specifically, we have enough room to always pack the slots perfectly into
- // some number of system pages. The only waste is the waste associated with
- // unfaulted pages (i.e. wasted address space).
- // TODO: we end up using a lot of system pages for very small sizes. For
- // example, we'll use 12 system pages for slot size 24. The slot size is so
- // small that the waste would be tiny with just 4, or 1, system pages. Later,
- // we can investigate whether there are anti-fragmentation benefits to using
- // fewer system pages.
- double best_waste_ratio = 1.0f;
- uint16_t best_pages = 0;
- if (slot_size > MaxRegularSlotSpanSize()) {
- // TODO(ajwong): Why is there a DCHECK here for this?
- // http://crbug.com/776537
- PA_DCHECK(!(slot_size % SystemPageSize()));
- best_pages = static_cast<uint16_t>(slot_size >> SystemPageShift());
- PA_CHECK(best_pages <= std::numeric_limits<uint8_t>::max());
- return static_cast<uint8_t>(best_pages);
- }
- PA_DCHECK(slot_size <= MaxRegularSlotSpanSize());
- for (uint16_t i = NumSystemPagesPerPartitionPage() - 1;
- i <= MaxSystemPagesPerRegularSlotSpan(); ++i) {
- size_t page_size = i << SystemPageShift();
- size_t num_slots = page_size / slot_size;
- size_t waste = page_size - (num_slots * slot_size);
- // Leaving a page unfaulted is not free; the page will occupy an empty page
- // table entry. Make a simple attempt to account for that.
- //
- // TODO(ajwong): This looks wrong. PTEs are allocated for all pages
- // regardless of whether or not they are wasted. Should it just
- // be waste += i * sizeof(void*)?
- // http://crbug.com/776537
- size_t num_remainder_pages = i & (NumSystemPagesPerPartitionPage() - 1);
- size_t num_unfaulted_pages =
- num_remainder_pages
- ? (NumSystemPagesPerPartitionPage() - num_remainder_pages)
- : 0;
- waste += sizeof(void*) * num_unfaulted_pages;
- double waste_ratio =
- static_cast<double>(waste) / static_cast<double>(page_size);
- if (waste_ratio < best_waste_ratio) {
- best_waste_ratio = waste_ratio;
- best_pages = i;
- }
- }
- PA_DCHECK(best_pages > 0);
- PA_CHECK(best_pages <= MaxSystemPagesPerRegularSlotSpan());
- return static_cast<uint8_t>(best_pages);
- }
- } // namespace
- uint8_t ComputeSystemPagesPerSlotSpan(size_t slot_size,
- bool prefer_smaller_slot_spans) {
- if (prefer_smaller_slot_spans)
- return ComputeSystemPagesPerSlotSpanPreferSmall(slot_size);
- else
- return ComputeSystemPagesPerSlotSpanInternal(slot_size);
- }
- template <bool thread_safe>
- void PartitionBucket<thread_safe>::Init(uint32_t new_slot_size) {
- slot_size = new_slot_size;
- slot_size_reciprocal = kReciprocalMask / new_slot_size + 1;
- active_slot_spans_head =
- SlotSpanMetadata<thread_safe>::get_sentinel_slot_span();
- empty_slot_spans_head = nullptr;
- decommitted_slot_spans_head = nullptr;
- num_full_slot_spans = 0;
- bool prefer_smaller_slot_spans =
- #if defined(PA_PREFER_SMALLER_SLOT_SPANS)
- true
- #else
- false
- #endif
- ;
- num_system_pages_per_slot_span =
- ComputeSystemPagesPerSlotSpan(slot_size, prefer_smaller_slot_spans);
- }
- template <bool thread_safe>
- PA_ALWAYS_INLINE SlotSpanMetadata<thread_safe>*
- PartitionBucket<thread_safe>::AllocNewSlotSpan(PartitionRoot<thread_safe>* root,
- unsigned int flags,
- size_t slot_span_alignment) {
- PA_DCHECK(!(reinterpret_cast<uintptr_t>(root->next_partition_page) %
- PartitionPageSize()));
- PA_DCHECK(!(reinterpret_cast<uintptr_t>(root->next_partition_page_end) %
- PartitionPageSize()));
- size_t num_partition_pages = get_pages_per_slot_span();
- size_t slot_span_reservation_size = num_partition_pages
- << PartitionPageShift();
- size_t slot_span_committed_size = get_bytes_per_span();
- PA_DCHECK(num_partition_pages <= NumPartitionPagesPerSuperPage());
- PA_DCHECK(slot_span_committed_size % SystemPageSize() == 0);
- PA_DCHECK(slot_span_committed_size <= slot_span_reservation_size);
- uintptr_t adjusted_next_partition_page =
- base::bits::AlignUp(root->next_partition_page, slot_span_alignment);
- if (PA_UNLIKELY(adjusted_next_partition_page + slot_span_reservation_size >
- root->next_partition_page_end)) {
- // AllocNewSuperPage() may crash (e.g. address space exhaustion), put data
- // on stack.
- PA_DEBUG_DATA_ON_STACK("slotsize", slot_size);
- PA_DEBUG_DATA_ON_STACK("spansize", slot_span_reservation_size);
- // In this case, we can no longer hand out pages from the current super page
- // allocation. Get a new super page.
- if (!AllocNewSuperPage(root, flags)) {
- return nullptr;
- }
- // AllocNewSuperPage() updates root->next_partition_page, re-query.
- adjusted_next_partition_page =
- base::bits::AlignUp(root->next_partition_page, slot_span_alignment);
- PA_CHECK(adjusted_next_partition_page + slot_span_reservation_size <=
- root->next_partition_page_end);
- }
- auto* gap_start_page =
- PartitionPage<thread_safe>::FromAddr(root->next_partition_page);
- auto* gap_end_page =
- PartitionPage<thread_safe>::FromAddr(adjusted_next_partition_page);
- for (auto* page = gap_start_page; page < gap_end_page; ++page) {
- PA_DCHECK(!page->is_valid);
- page->has_valid_span_after_this = 1;
- }
- root->next_partition_page =
- adjusted_next_partition_page + slot_span_reservation_size;
- uintptr_t slot_span_start = adjusted_next_partition_page;
- auto* slot_span = &gap_end_page->slot_span_metadata;
- InitializeSlotSpan(slot_span);
- // Now that slot span is initialized, it's safe to call FromSlotStart.
- PA_DCHECK(slot_span ==
- SlotSpanMetadata<thread_safe>::FromSlotStart(slot_span_start));
- // System pages in the super page come in a decommited state. Commit them
- // before vending them back.
- // If lazy commit is enabled, pages will be committed when provisioning slots,
- // in ProvisionMoreSlotsAndAllocOne(), not here.
- if (!kUseLazyCommit) {
- PA_DEBUG_DATA_ON_STACK("slotsize", slot_size);
- PA_DEBUG_DATA_ON_STACK("spansize", slot_span_reservation_size);
- PA_DEBUG_DATA_ON_STACK("spancmt", slot_span_committed_size);
- root->RecommitSystemPagesForData(
- slot_span_start, slot_span_committed_size,
- PageAccessibilityDisposition::kRequireUpdate);
- }
- PA_CHECK(get_slots_per_span() <=
- SlotSpanMetadata<ThreadSafe>::kMaxSlotsPerSlotSpan);
- // Double check that we had enough space in the super page for the new slot
- // span.
- PA_DCHECK(root->next_partition_page <= root->next_partition_page_end);
- #if defined(PA_USE_MTE_CHECKED_PTR_WITH_64_BITS_POINTERS)
- PA_DCHECK(root->next_tag_bitmap_page);
- uintptr_t next_tag_bitmap_page =
- base::bits::AlignUp(reinterpret_cast<uintptr_t>(
- PartitionTagPointer(root->next_partition_page)),
- SystemPageSize());
- if (root->next_tag_bitmap_page < next_tag_bitmap_page) {
- #if BUILDFLAG(PA_DCHECK_IS_ON)
- uintptr_t super_page =
- reinterpret_cast<uintptr_t>(slot_span) & kSuperPageBaseMask;
- uintptr_t tag_bitmap = super_page + PartitionPageSize();
- PA_DCHECK(next_tag_bitmap_page <= tag_bitmap + ActualTagBitmapSize());
- PA_DCHECK(next_tag_bitmap_page > tag_bitmap);
- #endif
- SetSystemPagesAccess(root->next_tag_bitmap_page,
- next_tag_bitmap_page - root->next_tag_bitmap_page,
- PageAccessibilityConfiguration::kReadWrite);
- root->next_tag_bitmap_page = next_tag_bitmap_page;
- }
- #endif // defined(PA_USE_MTE_CHECKED_PTR_WITH_64_BITS_POINTERS)
- return slot_span;
- }
- template <bool thread_safe>
- PA_ALWAYS_INLINE uintptr_t PartitionBucket<thread_safe>::AllocNewSuperPage(
- PartitionRoot<thread_safe>* root,
- unsigned int flags) {
- // Need a new super page. We want to allocate super pages in a contiguous
- // address region as much as possible. This is important for not causing
- // page table bloat and not fragmenting address spaces in 32 bit
- // architectures.
- uintptr_t requested_address = root->next_super_page;
- // Allocate from GigaCage. Route to the appropriate GigaCage pool based on
- // BackupRefPtr support.
- pool_handle pool = root->ChoosePool();
- uintptr_t super_page =
- ReserveMemoryFromGigaCage(pool, requested_address, kSuperPageSize);
- if (PA_UNLIKELY(!super_page)) {
- if (flags & AllocFlags::kReturnNull)
- return 0;
- // Didn't manage to get a new uncommitted super page -> address space issue.
- ScopedUnlockGuard unlock{root->lock_};
- PartitionOutOfMemoryMappingFailure(root, kSuperPageSize);
- }
- *ReservationOffsetPointer(super_page) = kOffsetTagNormalBuckets;
- root->total_size_of_super_pages.fetch_add(kSuperPageSize,
- std::memory_order_relaxed);
- root->next_super_page = super_page + kSuperPageSize;
- // TODO(crbug.com/1307514): Add direct map support.
- uintptr_t state_bitmap = super_page + PartitionPageSize() +
- (is_direct_mapped() ? 0 : ReservedTagBitmapSize());
- PA_DCHECK(SuperPageStateBitmapAddr(super_page) == state_bitmap);
- const size_t state_bitmap_reservation_size =
- root->IsQuarantineAllowed() ? ReservedStateBitmapSize() : 0;
- const size_t state_bitmap_size_to_commit =
- root->IsQuarantineAllowed() ? CommittedStateBitmapSize() : 0;
- PA_DCHECK(state_bitmap_reservation_size % PartitionPageSize() == 0);
- PA_DCHECK(state_bitmap_size_to_commit % SystemPageSize() == 0);
- PA_DCHECK(state_bitmap_size_to_commit <= state_bitmap_reservation_size);
- uintptr_t payload = state_bitmap + state_bitmap_reservation_size;
- root->next_partition_page = payload;
- root->next_partition_page_end = root->next_super_page - PartitionPageSize();
- PA_DCHECK(payload ==
- SuperPagePayloadBegin(super_page, root->IsQuarantineAllowed()));
- PA_DCHECK(root->next_partition_page_end == SuperPagePayloadEnd(super_page));
- // Keep the first partition page in the super page inaccessible to serve as a
- // guard page, except an "island" in the middle where we put page metadata and
- // also a tiny amount of extent metadata.
- {
- ScopedSyscallTimer timer{root};
- RecommitSystemPages(
- super_page + SystemPageSize(),
- #if BUILDFLAG(PUT_REF_COUNT_IN_PREVIOUS_SLOT)
- // If PUT_REF_COUNT_IN_PREVIOUS_SLOT is on, and if the BRP pool is used,
- // allocate 2 SystemPages, one for SuperPage metadata and the other for
- // RefCount bitmap.
- (pool == GetBRPPool()) ? SystemPageSize() * 2 : SystemPageSize(),
- #else
- SystemPageSize(),
- #endif
- PageAccessibilityConfiguration::kReadWrite,
- PageAccessibilityDisposition::kRequireUpdate);
- }
- // If we were after a specific address, but didn't get it, assume that
- // the system chose a lousy address. Here most OS'es have a default
- // algorithm that isn't randomized. For example, most Linux
- // distributions will allocate the mapping directly before the last
- // successful mapping, which is far from random. So we just get fresh
- // randomness for the next mapping attempt.
- if (requested_address && requested_address != super_page)
- root->next_super_page = 0;
- // We allocated a new super page so update super page metadata.
- // First check if this is a new extent or not.
- auto* latest_extent = PartitionSuperPageToExtent<thread_safe>(super_page);
- // By storing the root in every extent metadata object, we have a fast way
- // to go from a pointer within the partition to the root object.
- latest_extent->root = root;
- // Most new extents will be part of a larger extent, and these two fields
- // are unused, but we initialize them to 0 so that we get a clear signal
- // in case they are accidentally used.
- latest_extent->number_of_consecutive_super_pages = 0;
- latest_extent->next = nullptr;
- latest_extent->number_of_nonempty_slot_spans = 0;
- PartitionSuperPageExtentEntry<thread_safe>* current_extent =
- root->current_extent;
- const bool is_new_extent = super_page != requested_address;
- if (PA_UNLIKELY(is_new_extent)) {
- if (PA_UNLIKELY(!current_extent)) {
- PA_DCHECK(!root->first_extent);
- root->first_extent = latest_extent;
- } else {
- PA_DCHECK(current_extent->number_of_consecutive_super_pages);
- current_extent->next = latest_extent;
- }
- root->current_extent = latest_extent;
- latest_extent->number_of_consecutive_super_pages = 1;
- } else {
- // We allocated next to an existing extent so just nudge the size up a
- // little.
- PA_DCHECK(current_extent->number_of_consecutive_super_pages);
- ++current_extent->number_of_consecutive_super_pages;
- PA_DCHECK(payload > SuperPagesBeginFromExtent(current_extent) &&
- payload < SuperPagesEndFromExtent(current_extent));
- }
- #if defined(PA_USE_MTE_CHECKED_PTR_WITH_64_BITS_POINTERS)
- // `root->next_partition_page` currently points at the start of the
- // super page payload. We point `root->next_tag_bitmap_page` to the
- // corresponding point in the tag bitmap and let the caller
- // (slot span allocation) take care of the rest.
- root->next_tag_bitmap_page =
- base::bits::AlignDown(reinterpret_cast<uintptr_t>(
- PartitionTagPointer(root->next_partition_page)),
- SystemPageSize());
- PA_DCHECK(root->next_tag_bitmap_page >= super_page + PartitionPageSize())
- << "tag bitmap can never intrude on metadata partition page";
- #endif // defined(PA_USE_MTE_CHECKED_PTR_WITH_64_BITS_POINTERS)
- // If PCScan is used, commit the state bitmap. Otherwise, leave it uncommitted
- // and let PartitionRoot::RegisterScannableRoot() commit it when needed. Make
- // sure to register the super-page after it has been fully initialized.
- // Otherwise, the concurrent scanner may try to access |extent->root| which
- // could be not initialized yet.
- if (root->IsQuarantineEnabled()) {
- {
- ScopedSyscallTimer timer{root};
- RecommitSystemPages(state_bitmap, state_bitmap_size_to_commit,
- PageAccessibilityConfiguration::kReadWrite,
- PageAccessibilityDisposition::kRequireUpdate);
- }
- PCScan::RegisterNewSuperPage(root, super_page);
- }
- return payload;
- }
- template <bool thread_safe>
- PA_ALWAYS_INLINE void PartitionBucket<thread_safe>::InitializeSlotSpan(
- SlotSpanMetadata<thread_safe>* slot_span) {
- new (slot_span) SlotSpanMetadata<thread_safe>(this);
- slot_span->Reset();
- uint16_t num_partition_pages = get_pages_per_slot_span();
- auto* page = reinterpret_cast<PartitionPage<thread_safe>*>(slot_span);
- for (uint16_t i = 0; i < num_partition_pages; ++i, ++page) {
- PA_DCHECK(i <= PartitionPage<thread_safe>::kMaxSlotSpanMetadataOffset);
- page->slot_span_metadata_offset = i;
- page->is_valid = true;
- }
- }
- template <bool thread_safe>
- PA_ALWAYS_INLINE uintptr_t
- PartitionBucket<thread_safe>::ProvisionMoreSlotsAndAllocOne(
- PartitionRoot<thread_safe>* root,
- SlotSpanMetadata<thread_safe>* slot_span) {
- PA_DCHECK(slot_span !=
- SlotSpanMetadata<thread_safe>::get_sentinel_slot_span());
- size_t num_slots = slot_span->num_unprovisioned_slots;
- PA_DCHECK(num_slots);
- PA_DCHECK(num_slots <= get_slots_per_span());
- // We should only get here when _every_ slot is either used or unprovisioned.
- // (The third possible state is "on the freelist". If we have a non-empty
- // freelist, we should not get here.)
- PA_DCHECK(num_slots + slot_span->num_allocated_slots == get_slots_per_span());
- // Similarly, make explicitly sure that the freelist is empty.
- PA_DCHECK(!slot_span->get_freelist_head());
- PA_DCHECK(!slot_span->is_full());
- uintptr_t slot_span_start =
- SlotSpanMetadata<thread_safe>::ToSlotSpanStart(slot_span);
- // If we got here, the first unallocated slot is either partially or fully on
- // an uncommitted page. If the latter, it must be at the start of that page.
- uintptr_t return_slot =
- slot_span_start + (slot_size * slot_span->num_allocated_slots);
- uintptr_t next_slot = return_slot + slot_size;
- uintptr_t commit_start = base::bits::AlignUp(return_slot, SystemPageSize());
- PA_DCHECK(next_slot > commit_start);
- uintptr_t commit_end = base::bits::AlignUp(next_slot, SystemPageSize());
- // If the slot was partially committed, |return_slot| and |next_slot| fall
- // in different pages. If the slot was fully uncommitted, |return_slot| points
- // to the page start and |next_slot| doesn't, thus only the latter gets
- // rounded up.
- PA_DCHECK(commit_end > commit_start);
- // The slot being returned is considered allocated.
- slot_span->num_allocated_slots++;
- // Round down, because a slot that doesn't fully fit in the new page(s) isn't
- // provisioned.
- size_t slots_to_provision = (commit_end - return_slot) / slot_size;
- slot_span->num_unprovisioned_slots -= slots_to_provision;
- PA_DCHECK(slot_span->num_allocated_slots +
- slot_span->num_unprovisioned_slots <=
- get_slots_per_span());
- // If lazy commit is enabled, meaning system pages in the slot span come
- // in an initially decommitted state, commit them here.
- // Note, we can't use PageAccessibilityDisposition::kAllowKeepForPerf, because
- // we have no knowledge which pages have been committed before (it doesn't
- // matter on Windows anyway).
- if (kUseLazyCommit) {
- // TODO(lizeb): Handle commit failure.
- root->RecommitSystemPagesForData(
- commit_start, commit_end - commit_start,
- PageAccessibilityDisposition::kRequireUpdate);
- }
- if (PA_LIKELY(slot_size <= kMaxMemoryTaggingSize)) {
- // Ensure the MTE-tag of the memory pointed by |return_slot| is unguessable.
- TagMemoryRangeRandomly(return_slot, slot_size);
- }
- #if defined(PA_USE_MTE_CHECKED_PTR_WITH_64_BITS_POINTERS)
- NormalBucketPartitionTagSetValue(return_slot, slot_size,
- root->GetNewPartitionTag());
- #endif // defined(PA_USE_MTE_CHECKED_PTR_WITH_64_BITS_POINTERS)
- // Add all slots that fit within so far committed pages to the free list.
- PartitionFreelistEntry* prev_entry = nullptr;
- uintptr_t next_slot_end = next_slot + slot_size;
- size_t free_list_entries_added = 0;
- while (next_slot_end <= commit_end) {
- void* next_slot_ptr;
- if (PA_LIKELY(slot_size <= kMaxMemoryTaggingSize)) {
- // Ensure the MTE-tag of the memory pointed by other provisioned slot is
- // unguessable. They will be returned to the app as is, and the MTE-tag
- // will only change upon calling Free().
- next_slot_ptr = TagMemoryRangeRandomly(next_slot, slot_size);
- } else {
- // No MTE-tagging for larger slots, just cast.
- next_slot_ptr = reinterpret_cast<void*>(next_slot);
- }
- #if defined(PA_USE_MTE_CHECKED_PTR_WITH_64_BITS_POINTERS)
- NormalBucketPartitionTagSetValue(next_slot, slot_size,
- root->GetNewPartitionTag());
- #endif // defined(PA_USE_MTE_CHECKED_PTR_WITH_64_BITS_POINTERS)
- auto* entry = PartitionFreelistEntry::EmplaceAndInitNull(next_slot_ptr);
- if (!slot_span->get_freelist_head()) {
- PA_DCHECK(!prev_entry);
- PA_DCHECK(!free_list_entries_added);
- slot_span->SetFreelistHead(entry);
- } else {
- PA_DCHECK(free_list_entries_added);
- prev_entry->SetNext(entry);
- }
- next_slot = next_slot_end;
- next_slot_end = next_slot + slot_size;
- prev_entry = entry;
- #if BUILDFLAG(PA_DCHECK_IS_ON)
- free_list_entries_added++;
- #endif
- }
- #if BUILDFLAG(PA_DCHECK_IS_ON)
- // The only provisioned slot not added to the free list is the one being
- // returned.
- PA_DCHECK(slots_to_provision == free_list_entries_added + 1);
- // We didn't necessarily provision more than one slot (e.g. if |slot_size|
- // is large), meaning that |slot_span->freelist_head| can be nullptr.
- if (slot_span->get_freelist_head()) {
- PA_DCHECK(free_list_entries_added);
- slot_span->get_freelist_head()->CheckFreeList(slot_size);
- }
- #endif
- // We had no free slots, and created some (potentially 0) in sorted order.
- slot_span->set_freelist_sorted();
- return return_slot;
- }
- template <bool thread_safe>
- bool PartitionBucket<thread_safe>::SetNewActiveSlotSpan() {
- SlotSpanMetadata<thread_safe>* slot_span = active_slot_spans_head;
- if (slot_span == SlotSpanMetadata<thread_safe>::get_sentinel_slot_span())
- return false;
- SlotSpanMetadata<thread_safe>* next_slot_span;
- // The goal here is to find a suitable slot span in the active list. Suitable
- // slot spans are |is_active()|, i.e. they either have (a) freelist entries,
- // or (b) unprovisioned free space. The first case is preferable, since it
- // doesn't cost a system call, and doesn't cause new memory to become dirty.
- //
- // While looking for a new slot span, active list maintenance is performed,
- // that is:
- // - Empty and decommitted slot spans are moved to their respective lists.
- // - Full slot spans are removed from the active list but are not moved
- // anywhere. They could be tracked in a separate list, but this would
- // increase cost non trivially. Indeed, a full slot span is likely to become
- // non-full at some point (due to a free() hitting it). Since we only have
- // space in the metadata for a single linked list pointer, removing the
- // newly-non-full slot span from the "full" list would require walking it
- // (to know what's before it in the full list).
- //
- // Since we prefer slot spans with provisioned freelist entries, maintenance
- // happens in two stages:
- // 1. Walk the list to find candidates. Each of the skipped slot span is moved
- // to either:
- // - one of the long-lived lists: empty, decommitted
- // - the temporary "active slots spans with no freelist entry" list
- // - Nowhere for full slot spans.
- // 2. Once we have a candidate:
- // - Set it as the new active list head
- // - Reattach the temporary list
- //
- // Note that in most cases, the whole list will not be walked and maintained
- // at this stage.
- SlotSpanMetadata<thread_safe>* to_provision_head = nullptr;
- SlotSpanMetadata<thread_safe>* to_provision_tail = nullptr;
- for (; slot_span; slot_span = next_slot_span) {
- next_slot_span = slot_span->next_slot_span;
- PA_DCHECK(slot_span->bucket == this);
- PA_DCHECK(slot_span != empty_slot_spans_head);
- PA_DCHECK(slot_span != decommitted_slot_spans_head);
- if (slot_span->is_active()) {
- // Has provisioned slots.
- if (slot_span->get_freelist_head()) {
- // Will use this slot span, no need to go further.
- break;
- } else {
- // Keeping head and tail because we don't want to reverse the list.
- if (!to_provision_head)
- to_provision_head = slot_span;
- if (to_provision_tail)
- to_provision_tail->next_slot_span = slot_span;
- to_provision_tail = slot_span;
- slot_span->next_slot_span = nullptr;
- }
- } else if (slot_span->is_empty()) {
- slot_span->next_slot_span = empty_slot_spans_head;
- empty_slot_spans_head = slot_span;
- } else if (PA_LIKELY(slot_span->is_decommitted())) {
- slot_span->next_slot_span = decommitted_slot_spans_head;
- decommitted_slot_spans_head = slot_span;
- } else {
- PA_DCHECK(slot_span->is_full());
- // Move this slot span... nowhere, and also mark it as full. We need it
- // marked so that free'ing can tell, and move it back into the active
- // list.
- slot_span->marked_full = 1;
- ++num_full_slot_spans;
- // Overflow. Most likely a correctness issue in the code. It is in theory
- // possible that the number of full slot spans really reaches (1 << 24),
- // but this is very unlikely (and not possible with most GigaCage
- // settings).
- PA_CHECK(num_full_slot_spans);
- // Not necessary but might help stop accidents.
- slot_span->next_slot_span = nullptr;
- }
- }
- bool usable_active_list_head = false;
- // Found an active slot span with provisioned entries on the freelist.
- if (slot_span) {
- usable_active_list_head = true;
- // We have active slot spans with unprovisioned entries. Re-attach them into
- // the active list, past the span with freelist entries.
- if (to_provision_head) {
- auto* next = slot_span->next_slot_span;
- slot_span->next_slot_span = to_provision_head;
- to_provision_tail->next_slot_span = next;
- }
- active_slot_spans_head = slot_span;
- } else if (to_provision_head) {
- usable_active_list_head = true;
- // Need to provision new slots.
- active_slot_spans_head = to_provision_head;
- } else {
- // Active list is now empty.
- active_slot_spans_head =
- SlotSpanMetadata<thread_safe>::get_sentinel_slot_span();
- }
- return usable_active_list_head;
- }
- template <bool thread_safe>
- void PartitionBucket<thread_safe>::MaintainActiveList() {
- SlotSpanMetadata<thread_safe>* slot_span = active_slot_spans_head;
- if (slot_span == SlotSpanMetadata<thread_safe>::get_sentinel_slot_span())
- return;
- SlotSpanMetadata<thread_safe>* new_active_slot_spans_head = nullptr;
- SlotSpanMetadata<thread_safe>* new_active_slot_spans_tail = nullptr;
- SlotSpanMetadata<thread_safe>* next_slot_span;
- for (; slot_span; slot_span = next_slot_span) {
- next_slot_span = slot_span->next_slot_span;
- if (slot_span->is_active()) {
- // Ordering in the active slot span list matters, don't reverse it.
- if (!new_active_slot_spans_head)
- new_active_slot_spans_head = slot_span;
- if (new_active_slot_spans_tail)
- new_active_slot_spans_tail->next_slot_span = slot_span;
- new_active_slot_spans_tail = slot_span;
- slot_span->next_slot_span = nullptr;
- } else if (slot_span->is_empty()) {
- // For the empty and decommitted lists, LIFO ordering makes sense (since
- // it would lead to reusing memory which has been touched relatively
- // recently, which only matters for committed spans though).
- slot_span->next_slot_span = empty_slot_spans_head;
- empty_slot_spans_head = slot_span;
- } else if (slot_span->is_decommitted()) {
- slot_span->next_slot_span = decommitted_slot_spans_head;
- decommitted_slot_spans_head = slot_span;
- } else {
- // Full slot spans are not tracked, just accounted for.
- PA_DCHECK(slot_span->is_full());
- slot_span->marked_full = 1;
- ++num_full_slot_spans;
- PA_CHECK(num_full_slot_spans); // Overflow.
- slot_span->next_slot_span = nullptr;
- }
- }
- if (!new_active_slot_spans_head) {
- new_active_slot_spans_head =
- SlotSpanMetadata<thread_safe>::get_sentinel_slot_span();
- }
- active_slot_spans_head = new_active_slot_spans_head;
- }
- template <bool thread_safe>
- void PartitionBucket<thread_safe>::SortSlotSpanFreelists() {
- for (auto* slot_span = active_slot_spans_head; slot_span;
- slot_span = slot_span->next_slot_span) {
- // No need to sort the freelist if it's already sorted. Note that if the
- // freelist is sorted, this means that it didn't change at all since the
- // last call. This may be a good signal to shrink it if possible (if an
- // entire OS page is free, we can decommit it).
- //
- // Besides saving CPU, this also avoids touching memory of fully idle slot
- // spans, which may required paging.
- if (slot_span->num_allocated_slots > 0 && !slot_span->freelist_is_sorted())
- slot_span->SortFreelist();
- }
- }
- PA_COMPONENT_EXPORT(PARTITION_ALLOC)
- bool CompareSlotSpans(SlotSpanMetadata<ThreadSafe>* a,
- SlotSpanMetadata<ThreadSafe>* b) {
- auto criteria_tuple = [](SlotSpanMetadata<ThreadSafe> const* a) {
- size_t freelist_length = a->GetFreelistLength();
- // The criteria are, in order (hence the lexicographic comparison below):
- // 1. Prefer slot spans with freelist entries. The ones without freelist
- // entries would be skipped in SetNewActiveSlotSpan() anyway.
- // 2. Then the ones with the fewest freelist entries. They are either close
- // to being full (for the provisioned memory), or close to being pushed
- // at the end of the list (since they would not have freelist entries
- // anymore, and would either fall into the first case, or be skipped by
- // SetNewActiveSlotSpan()).
- // 3. The ones with the fewer unprovisioned slots, meaning that they are
- // close to being completely full.
- //
- // Note that this sorting order is not necessarily the best one when slot
- // spans are partially provisioned. From local testing, in steady-state,
- // most slot spans are entirely provisioned (or decommitted), which may be a
- // consequence of the lack of partial slot span decommit, or of fairly
- // effective fragmentation avoidance heuristics. Make sure to evaluate
- // whether an alternative sorting order (sorting according to freelist size
- // + unprovisioned slots) makes more sense.
- return std::tuple<bool, size_t, size_t>{
- freelist_length == 0, freelist_length, a->num_unprovisioned_slots};
- };
- return criteria_tuple(a) < criteria_tuple(b);
- }
- template <bool thread_safe>
- void PartitionBucket<thread_safe>::SortActiveSlotSpans() {
- // Sorting up to |kMaxSlotSpansToSort| slot spans. This is capped for two
- // reasons:
- // - Limiting execution time
- // - Current code cannot allocate.
- //
- // In practice though, it's rare to have that many active slot spans.
- SlotSpanMetadata<thread_safe>* active_spans_array[kMaxSlotSpansToSort];
- size_t index = 0;
- SlotSpanMetadata<thread_safe>* overflow_spans_start = nullptr;
- for (auto* slot_span = active_slot_spans_head; slot_span;
- slot_span = slot_span->next_slot_span) {
- if (index < kMaxSlotSpansToSort) {
- active_spans_array[index++] = slot_span;
- } else {
- // Starting from this one, not sorting the slot spans.
- overflow_spans_start = slot_span;
- break;
- }
- }
- // We sort the active slot spans so that allocations are preferably serviced
- // from the fullest ones. This way we hope to reduce fragmentation by keeping
- // as few slot spans as full as possible.
- //
- // With perfect information on allocation lifespan, we would be able to pack
- // allocations and get almost no fragmentation. This is obviously not the
- // case, so we have partially full SlotSpans. Nevertheless, as a heuristic we
- // want to:
- // - Keep almost-empty slot spans as empty as possible
- // - Keep mostly-full slot spans as full as possible
- //
- // The first part is done in the hope that future free()s will make these
- // slot spans completely empty, allowing us to reclaim them. To that end, sort
- // SlotSpans periodically so that the fullest ones are preferred.
- //
- // std::sort() is not completely guaranteed to never allocate memory. However,
- // it may not throw std::bad_alloc, which constrains the implementation. In
- // addition, this is protected by the reentrancy guard, so we would detect
- // such an allocation.
- std::sort(active_spans_array, active_spans_array + index, CompareSlotSpans);
- active_slot_spans_head = overflow_spans_start;
- // Reverse order, since we insert at the head of the list.
- for (int i = index - 1; i >= 0; i--) {
- active_spans_array[i]->next_slot_span = active_slot_spans_head;
- active_slot_spans_head = active_spans_array[i];
- }
- }
- template <bool thread_safe>
- uintptr_t PartitionBucket<thread_safe>::SlowPathAlloc(
- PartitionRoot<thread_safe>* root,
- unsigned int flags,
- size_t raw_size,
- size_t slot_span_alignment,
- bool* is_already_zeroed) {
- PA_DCHECK((slot_span_alignment >= PartitionPageSize()) &&
- base::bits::IsPowerOfTwo(slot_span_alignment));
- // The slow path is called when the freelist is empty. The only exception is
- // when a higher-order alignment is requested, in which case the freelist
- // logic is bypassed and we go directly for slot span allocation.
- bool allocate_aligned_slot_span = slot_span_alignment > PartitionPageSize();
- PA_DCHECK(!active_slot_spans_head->get_freelist_head() ||
- allocate_aligned_slot_span);
- SlotSpanMetadata<thread_safe>* new_slot_span = nullptr;
- // |new_slot_span->bucket| will always be |this|, except when |this| is the
- // sentinel bucket, which is used to signal a direct mapped allocation. In
- // this case |new_bucket| will be set properly later. This avoids a read for
- // most allocations.
- PartitionBucket* new_bucket = this;
- *is_already_zeroed = false;
- // For the PartitionRoot::Alloc() API, we have a bunch of buckets
- // marked as special cases. We bounce them through to the slow path so that
- // we can still have a blazing fast hot path due to lack of corner-case
- // branches.
- //
- // Note: The ordering of the conditionals matter! In particular,
- // SetNewActiveSlotSpan() has a side-effect even when returning
- // false where it sweeps the active list and may move things into the empty or
- // decommitted lists which affects the subsequent conditional.
- if (PA_UNLIKELY(is_direct_mapped())) {
- PA_DCHECK(raw_size > kMaxBucketed);
- PA_DCHECK(this == &root->sentinel_bucket);
- PA_DCHECK(active_slot_spans_head ==
- SlotSpanMetadata<thread_safe>::get_sentinel_slot_span());
- // No fast path for direct-mapped allocations.
- if (flags & AllocFlags::kFastPathOrReturnNull)
- return 0;
- new_slot_span =
- PartitionDirectMap(root, flags, raw_size, slot_span_alignment);
- if (new_slot_span)
- new_bucket = new_slot_span->bucket;
- // Memory from PageAllocator is always zeroed.
- *is_already_zeroed = true;
- } else if (PA_LIKELY(!allocate_aligned_slot_span && SetNewActiveSlotSpan())) {
- // First, did we find an active slot span in the active list?
- new_slot_span = active_slot_spans_head;
- PA_DCHECK(new_slot_span->is_active());
- } else if (PA_LIKELY(!allocate_aligned_slot_span &&
- (empty_slot_spans_head != nullptr ||
- decommitted_slot_spans_head != nullptr))) {
- // Second, look in our lists of empty and decommitted slot spans.
- // Check empty slot spans first, which are preferred, but beware that an
- // empty slot span might have been decommitted.
- while (PA_LIKELY((new_slot_span = empty_slot_spans_head) != nullptr)) {
- PA_DCHECK(new_slot_span->bucket == this);
- PA_DCHECK(new_slot_span->is_empty() || new_slot_span->is_decommitted());
- empty_slot_spans_head = new_slot_span->next_slot_span;
- // Accept the empty slot span unless it got decommitted.
- if (new_slot_span->get_freelist_head()) {
- new_slot_span->next_slot_span = nullptr;
- new_slot_span->ToSuperPageExtent()
- ->IncrementNumberOfNonemptySlotSpans();
- // Re-activating an empty slot span, update accounting.
- size_t dirty_size = base::bits::AlignUp(
- new_slot_span->GetProvisionedSize(), SystemPageSize());
- PA_DCHECK(root->empty_slot_spans_dirty_bytes >= dirty_size);
- root->empty_slot_spans_dirty_bytes -= dirty_size;
- break;
- }
- PA_DCHECK(new_slot_span->is_decommitted());
- new_slot_span->next_slot_span = decommitted_slot_spans_head;
- decommitted_slot_spans_head = new_slot_span;
- }
- if (PA_UNLIKELY(!new_slot_span) &&
- PA_LIKELY(decommitted_slot_spans_head != nullptr)) {
- // Commit can be expensive, don't do it.
- if (flags & AllocFlags::kFastPathOrReturnNull)
- return 0;
- new_slot_span = decommitted_slot_spans_head;
- PA_DCHECK(new_slot_span->bucket == this);
- PA_DCHECK(new_slot_span->is_decommitted());
- decommitted_slot_spans_head = new_slot_span->next_slot_span;
- // If lazy commit is enabled, pages will be recommitted when provisioning
- // slots, in ProvisionMoreSlotsAndAllocOne(), not here.
- if (!kUseLazyCommit) {
- uintptr_t slot_span_start =
- SlotSpanMetadata<thread_safe>::ToSlotSpanStart(new_slot_span);
- // Since lazy commit isn't used, we have a guarantee that all slot span
- // pages have been previously committed, and then decommitted using
- // PageAccessibilityDisposition::kAllowKeepForPerf, so use the
- // same option as an optimization.
- // TODO(lizeb): Handle commit failure.
- root->RecommitSystemPagesForData(
- slot_span_start, new_slot_span->bucket->get_bytes_per_span(),
- PageAccessibilityDisposition::kAllowKeepForPerf);
- }
- new_slot_span->Reset();
- *is_already_zeroed = DecommittedMemoryIsAlwaysZeroed();
- }
- PA_DCHECK(new_slot_span);
- } else {
- // Getting a new slot span is expensive, don't do it.
- if (flags & AllocFlags::kFastPathOrReturnNull)
- return 0;
- // Third. If we get here, we need a brand new slot span.
- // TODO(bartekn): For single-slot slot spans, we can use rounded raw_size
- // as slot_span_committed_size.
- new_slot_span = AllocNewSlotSpan(root, flags, slot_span_alignment);
- // New memory from PageAllocator is always zeroed.
- *is_already_zeroed = true;
- }
- // Bail if we had a memory allocation failure.
- if (PA_UNLIKELY(!new_slot_span)) {
- PA_DCHECK(active_slot_spans_head ==
- SlotSpanMetadata<thread_safe>::get_sentinel_slot_span());
- if (flags & AllocFlags::kReturnNull)
- return 0;
- // See comment in PartitionDirectMap() for unlocking.
- ScopedUnlockGuard unlock{root->lock_};
- root->OutOfMemory(raw_size);
- PA_IMMEDIATE_CRASH(); // Not required, kept as documentation.
- }
- PA_DCHECK(new_bucket != &root->sentinel_bucket);
- new_bucket->active_slot_spans_head = new_slot_span;
- if (new_slot_span->CanStoreRawSize())
- new_slot_span->SetRawSize(raw_size);
- // If we found an active slot span with free slots, or an empty slot span, we
- // have a usable freelist head.
- if (PA_LIKELY(new_slot_span->get_freelist_head() != nullptr)) {
- PartitionFreelistEntry* entry =
- new_slot_span->PopForAlloc(new_bucket->slot_size);
- // We may have set *is_already_zeroed to true above, make sure that the
- // freelist entry doesn't contain data. Either way, it wouldn't be a good
- // idea to let users see our internal data.
- uintptr_t slot_start = entry->ClearForAllocation();
- return slot_start;
- }
- // Otherwise, we need to provision more slots by committing more pages. Build
- // the free list for the newly provisioned slots.
- PA_DCHECK(new_slot_span->num_unprovisioned_slots);
- return ProvisionMoreSlotsAndAllocOne(root, new_slot_span);
- }
- template struct PartitionBucket<ThreadSafe>;
- } // namespace partition_alloc::internal
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