xref: /external/chromium_org/third_party/WebKit/Source/wtf/PartitionAlloc.h

/*
 * Copyright (C) 2013 Google Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 *     * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
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 * copyright notice, this list of conditions and the following disclaimer
 * in the documentation and/or other materials provided with the
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 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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#ifndef WTF_PartitionAlloc_h
#define WTF_PartitionAlloc_h

// DESCRIPTION
// partitionAlloc() / partitionAllocGeneric() and partitionFree() /
// partitionFreeGeneric() are approximately analagous to malloc() and free().
//
// The main difference is that a PartitionRoot / PartitionRootGeneric object
// must be supplied to these functions, representing a specific "heap partition"
// that will be used to satisfy the allocation. Different partitions are
// guaranteed to exist in separate address spaces, including being separate from
// the main system heap. If the contained objects are all freed, physical memory
// is returned to the system but the address space remains reserved.
//
// THE ONLY LEGITIMATE WAY TO OBTAIN A PartitionRoot IS THROUGH THE
// SizeSpecificPartitionAllocator / PartitionAllocatorGeneric classes. To
// minimize the instruction count to the fullest extent possible, the
// PartitonRoot is really just a header adjacent to other data areas provided
// by the allocator class.
//
// The partitionAlloc() variant of the API has the following caveats:
// - Allocations and frees against a single partition must be single threaded.
// - Allocations must not exceed a max size, chosen at compile-time via a
// templated parameter to PartitionAllocator.
// - Allocation sizes must be aligned to the system pointer size.
// - Allocations are bucketed exactly according to size.
//
// And for partitionAllocGeneric():
// - Multi-threaded use against a single partition is ok; locking is handled.
// - Allocations of any arbitrary size can be handled (subject to a limit of
// INT_MAX bytes for security reasons).
// - Bucketing is by approximate size, for example an allocation of 4000 bytes
// might be placed into a 4096-byte bucket. Bucket sizes are chosen to try and
// keep worst-case waste to ~10%.
//
// The allocators are designed to be extremely fast, thanks to the following
// properties and design:
// - Just a single (reasonably predicatable) branch in the hot / fast path for
// both allocating and (significantly) freeing.
// - A minimal number of operations in the hot / fast path, with the slow paths
// in separate functions, leading to the possibility of inlining.
// - Each partition page (which is usually multiple physical pages) has a
// metadata structure which allows fast mapping of free() address to an
// underlying bucket.
// - Supports a lock-free API for fast performance in single-threaded cases.
// - The freelist for a given bucket is split across a number of partition
// pages, enabling various simple tricks to try and minimize fragmentation.
// - Fine-grained bucket sizes leading to less waste and better packing.
//
// The following security properties are provided at this time:
// - Linear overflows cannot corrupt into the partition.
// - Linear overflows cannot corrupt out of the partition.
// - Freed pages will only be re-used within the partition.
//   (exception: large allocations > ~1MB)
// - Freed pages will only hold same-sized objects when re-used.
// - Dereference of freelist pointer should fault.
// - Out-of-line main metadata: linear over or underflow cannot corrupt it.
// - Partial pointer overwrite of freelist pointer should fault.
// - Rudimentary double-free detection.
// - Large allocations (> ~1MB) are guard-paged at the beginning and end.
//
// The following security properties could be investigated in the future:
// - Per-object bucketing (instead of per-size) is mostly available at the API,
// but not used yet.
// - No randomness of freelist entries or bucket position.
// - Better checking for wild pointers in free().
// - Better freelist masking function to guarantee fault on 32-bit.

#include "wtf/Assertions.h"
#include "wtf/BitwiseOperations.h"
#include "wtf/ByteSwap.h"
#include "wtf/CPU.h"
#include "wtf/PageAllocator.h"
#include "wtf/SpinLock.h"

#include <limits.h>

#if defined(MEMORY_TOOL_REPLACES_ALLOCATOR)
#include <stdlib.h>
#endif

#ifndef NDEBUG
#include <string.h>
#endif

namespace WTF {

// Maximum size of a partition's mappings. 2046MB. Note that the total amount of
// bytes allocatable at the API will be smaller. This is because things like
// guard pages, metadata, page headers and wasted space come out of the total.
// The 2GB is not necessarily contiguous in virtual address space.
static const size_t kMaxPartitionSize = 2046u * 1024u * 1024u;

// Allocation granularity of sizeof(void*) bytes.
static const size_t kAllocationGranularity = sizeof(void*);
static const size_t kAllocationGranularityMask = kAllocationGranularity - 1;
static const size_t kBucketShift = (kAllocationGranularity == 8) ? 3 : 2;

// Underlying partition storage pages are a power-of-two size. It is typical
// for a partition page to be based on multiple system pages. Most references to
// "page" refer to partition pages.
// We also have the concept of "super pages" -- these are the underlying system
// allocations we make. Super pages contain multiple partition pages inside them
// and include space for a small amount of metadata per partition page.
// Inside super pages, we store "slot spans". A slot span is a continguous range
// of one or more partition pages that stores allocations of the same size.
// Slot span sizes are adjusted depending on the allocation size, to make sure
// the packing does not lead to unused (wasted) space at the end of the last
// system page of the span. For our current max slot span size of 64k and other
// constant values, we pack _all_ partitionAllocGeneric() sizes perfectly up
// against the end of a system page.
static const size_t kPartitionPageShift = 14; // 16KB
static const size_t kPartitionPageSize = 1 << kPartitionPageShift;
static const size_t kPartitionPageOffsetMask = kPartitionPageSize - 1;
static const size_t kPartitionPageBaseMask = ~kPartitionPageOffsetMask;
static const size_t kMaxPartitionPagesPerSlotSpan = 4;

// To avoid fragmentation via never-used freelist entries, we hand out partition
// freelist sections gradually, in units of the dominant system page size.
// What we're actually doing is avoiding filling the full partition page
// (typically 16KB) will freelist pointers right away. Writing freelist
// pointers will fault and dirty a private page, which is very wasteful if we
// never actually store objects there.
static const size_t kNumSystemPagesPerPartitionPage = kPartitionPageSize / kSystemPageSize;
static const size_t kMaxSystemPagesPerSlotSpan = kNumSystemPagesPerPartitionPage * kMaxPartitionPagesPerSlotSpan;

// We reserve virtual address space in 2MB chunks (aligned to 2MB as well).
// These chunks are called "super pages". We do this so that we can store
// metadata in the first few pages of each 2MB aligned section. This leads to
// a very fast free(). We specifically choose 2MB because this virtual address
// block represents a full but single PTE allocation on ARM, ia32 and x64.
static const size_t kSuperPageShift = 21; // 2MB
static const size_t kSuperPageSize = 1 << kSuperPageShift;
static const size_t kSuperPageOffsetMask = kSuperPageSize - 1;
static const size_t kSuperPageBaseMask = ~kSuperPageOffsetMask;
static const size_t kNumPartitionPagesPerSuperPage = kSuperPageSize / kPartitionPageSize;

static const size_t kPageMetadataShift = 5; // 32 bytes per partition page.
static const size_t kPageMetadataSize = 1 << kPageMetadataShift;

// The following kGeneric* constants apply to the generic variants of the API.
// The "order" of an allocation is closely related to the power-of-two size of
// the allocation. More precisely, the order is the bit index of the
// most-significant-bit in the allocation size, where the bit numbers starts
// at index 1 for the least-significant-bit.
// In terms of allocation sizes, order 0 covers 0, order 1 covers 1, order 2
// covers 2->3, order 3 covers 4->7, order 4 covers 8->15.
static const size_t kGenericMinBucketedOrder = 4; // 8 bytes.
static const size_t kGenericMaxBucketedOrder = 20; // Largest bucketed order is 1<<(20-1) (storing 512KB -> almost 1MB)
static const size_t kGenericNumBucketedOrders = (kGenericMaxBucketedOrder - kGenericMinBucketedOrder) + 1;
static const size_t kGenericNumBucketsPerOrderBits = 3; // Eight buckets per order (for the higher orders), e.g. order 8 is 128, 144, 160, ..., 240
static const size_t kGenericNumBucketsPerOrder = 1 << kGenericNumBucketsPerOrderBits;
static const size_t kGenericSmallestBucket = 1 << (kGenericMinBucketedOrder - 1);
static const size_t kGenericMaxBucketSpacing = 1 << ((kGenericMaxBucketedOrder - 1) - kGenericNumBucketsPerOrderBits);
static const size_t kGenericMaxBucketed = (1 << (kGenericMaxBucketedOrder - 1)) + ((kGenericNumBucketsPerOrder - 1) * kGenericMaxBucketSpacing);
static const size_t kGenericMinDirectMappedDownsize = kGenericMaxBucketed + 1; // Limit when downsizing a direct mapping using realloc().
static const size_t kGenericMaxDirectMapped = INT_MAX - kSystemPageSize;
static const size_t kBitsPerSizet = sizeof(void*) * CHAR_BIT;

// Constants for the memory reclaim logic.
static const size_t kMaxFreeableSpans = 16;

#ifndef NDEBUG
// These two byte values match tcmalloc.
static const unsigned char kUninitializedByte = 0xAB;
static const unsigned char kFreedByte = 0xCD;
static const uint32_t kCookieValue = 0xDEADBEEFu;
static const size_t kCookieSize = 16; // Handles alignment up to XMM instructions on Intel.
#endif

struct PartitionBucket;
struct PartitionRootBase;

struct PartitionFreelistEntry {
    PartitionFreelistEntry* next;
};

// Some notes on page states. A page can be in one of three major states:
// 1) Active.
// 2) Full.
// 3) Free.
// An active page has available free slots. A full page has no free slots. A
// free page has had its backing memory released back to the system.
// There are two linked lists tracking the pages. The "active page" list is an
// approximation of a list of active pages. It is an approximation because both
// free and full pages may briefly be present in the list until we next do a
// scan over it. The "free page" list is an accurate list of pages which have
// been returned back to the system.
// The significant page transitions are:
// - free() will detect when a full page has a slot free()'d and immediately
// return the page to the head of the active list.
// - free() will detect when a page is fully emptied. It _may_ add it to the
// free list and it _may_ leave it on the active list until a future list scan.
// - malloc() _may_ scan the active page list in order to fulfil the request.
// If it does this, full and free pages encountered will be booted out of the
// active list. If there are no suitable active pages found, a free page (if one
// exists) will be pulled from the free list on to the active list.
struct PartitionPage {
    PartitionFreelistEntry* freelistHead;
    PartitionPage* nextPage;
    PartitionBucket* bucket;
    int16_t numAllocatedSlots; // Deliberately signed, -1 for free page, -n for full pages.
    uint16_t numUnprovisionedSlots;
    uint16_t pageOffset;
    int16_t freeCacheIndex; // -1 if not in the free cache.
};

struct PartitionBucket {
    PartitionPage* activePagesHead; // Accessed most in hot path => goes first.
    PartitionPage* freePagesHead;
    uint32_t slotSize;
    uint16_t numSystemPagesPerSlotSpan;
    uint16_t numFullPages;
};

// An "extent" is a span of consecutive superpages. We link to the partition's
// next extent (if there is one) at the very start of a superpage's metadata
// area.
struct PartitionSuperPageExtentEntry {
    PartitionRootBase* root;
    char* superPageBase;
    char* superPagesEnd;
    PartitionSuperPageExtentEntry* next;
};

struct WTF_EXPORT PartitionRootBase {
    size_t totalSizeOfCommittedPages;
    size_t totalSizeOfSuperPages;
    unsigned numBuckets;
    unsigned maxAllocation;
    bool initialized;
    char* nextSuperPage;
    char* nextPartitionPage;
    char* nextPartitionPageEnd;
    PartitionSuperPageExtentEntry* currentExtent;
    PartitionSuperPageExtentEntry* firstExtent;
    PartitionPage* globalEmptyPageRing[kMaxFreeableSpans];
    size_t globalEmptyPageRingIndex;
    uintptr_t invertedSelf;

    static int gInitializedLock;
    static bool gInitialized;
    static PartitionPage gSeedPage;
    static PartitionBucket gPagedBucket;
};

// Never instantiate a PartitionRoot directly, instead use PartitionAlloc.
struct PartitionRoot : public PartitionRootBase {
    // The PartitionAlloc templated class ensures the following is correct.
    ALWAYS_INLINE PartitionBucket* buckets() { return reinterpret_cast<PartitionBucket*>(this + 1); }
    ALWAYS_INLINE const PartitionBucket* buckets() const { return reinterpret_cast<const PartitionBucket*>(this + 1); }
};

// Never instantiate a PartitionRootGeneric directly, instead use PartitionAllocatorGeneric.
struct PartitionRootGeneric : public PartitionRootBase {
    int lock;
    // Some pre-computed constants.
    size_t orderIndexShifts[kBitsPerSizet + 1];
    size_t orderSubIndexMasks[kBitsPerSizet + 1];
    // The bucket lookup table lets us map a size_t to a bucket quickly.
    // The trailing +1 caters for the overflow case for very large allocation sizes.
    // It is one flat array instead of a 2D array because in the 2D world, we'd
    // need to index array[blah][max+1] which risks undefined behavior.
    PartitionBucket* bucketLookups[((kBitsPerSizet + 1) * kGenericNumBucketsPerOrder) + 1];
    PartitionBucket buckets[kGenericNumBucketedOrders * kGenericNumBucketsPerOrder];
};

// Flags for partitionAllocGenericFlags.
enum PartitionAllocFlags {
    PartitionAllocReturnNull = 1 << 0,
};

WTF_EXPORT void partitionAllocInit(PartitionRoot*, size_t numBuckets, size_t maxAllocation);
WTF_EXPORT bool partitionAllocShutdown(PartitionRoot*);
WTF_EXPORT void partitionAllocGenericInit(PartitionRootGeneric*);
WTF_EXPORT bool partitionAllocGenericShutdown(PartitionRootGeneric*);

WTF_EXPORT NEVER_INLINE void* partitionAllocSlowPath(PartitionRootBase*, int, size_t, PartitionBucket*);
WTF_EXPORT NEVER_INLINE void partitionFreeSlowPath(PartitionPage*);
WTF_EXPORT NEVER_INLINE void* partitionReallocGeneric(PartitionRootGeneric*, void*, size_t);

#ifndef NDEBUG
WTF_EXPORT void partitionDumpStats(const PartitionRoot&);
#endif

ALWAYS_INLINE PartitionFreelistEntry* partitionFreelistMask(PartitionFreelistEntry* ptr)
{
    // We use bswap on little endian as a fast mask for two reasons:
    // 1) If an object is freed and its vtable used where the attacker doesn't
    // get the chance to run allocations between the free and use, the vtable
    // dereference is likely to fault.
    // 2) If the attacker has a linear buffer overflow and elects to try and
    // corrupt a freelist pointer, partial pointer overwrite attacks are
    // thwarted.
    // For big endian, similar guarantees are arrived at with a negation.
#if CPU(BIG_ENDIAN)
    uintptr_t masked = ~reinterpret_cast<uintptr_t>(ptr);
#else
    uintptr_t masked = bswapuintptrt(reinterpret_cast<uintptr_t>(ptr));
#endif
    return reinterpret_cast<PartitionFreelistEntry*>(masked);
}

ALWAYS_INLINE size_t partitionCookieSizeAdjustAdd(size_t size)
{
#ifndef NDEBUG
    // Add space for cookies, checking for integer overflow.
    ASSERT(size + (2 * kCookieSize) > size);
    size += 2 * kCookieSize;
#endif
    return size;
}

ALWAYS_INLINE size_t partitionCookieSizeAdjustSubtract(size_t size)
{
#ifndef NDEBUG
    // Remove space for cookies.
    ASSERT(size >= 2 * kCookieSize);
    size -= 2 * kCookieSize;
#endif
    return size;
}

ALWAYS_INLINE void* partitionCookieFreePointerAdjust(void* ptr)
{
#ifndef NDEBUG
    // The value given to the application is actually just after the cookie.
    ptr = static_cast<char*>(ptr) - kCookieSize;
#endif
    return ptr;
}

ALWAYS_INLINE void partitionCookieWriteValue(void* ptr)
{
#ifndef NDEBUG
    uint32_t* cookiePtr = reinterpret_cast<uint32_t*>(ptr);
    for (size_t i = 0; i < kCookieSize / sizeof(kCookieValue); ++i, ++cookiePtr)
        *cookiePtr = kCookieValue;
#endif
}

ALWAYS_INLINE void partitionCookieCheckValue(void* ptr)
{
#ifndef NDEBUG
    uint32_t* cookiePtr = reinterpret_cast<uint32_t*>(ptr);
    for (size_t i = 0; i < kCookieSize / sizeof(kCookieValue); ++i, ++cookiePtr)
        ASSERT(*cookiePtr == kCookieValue);
#endif
}

ALWAYS_INLINE char* partitionSuperPageToMetadataArea(char* ptr)
{
    uintptr_t pointerAsUint = reinterpret_cast<uintptr_t>(ptr);
    ASSERT(!(pointerAsUint & kSuperPageOffsetMask));
    // The metadata area is exactly one system page (the guard page) into the
    // super page.
    return reinterpret_cast<char*>(pointerAsUint + kSystemPageSize);
}

ALWAYS_INLINE PartitionPage* partitionPointerToPageNoAlignmentCheck(void* ptr)
{
    uintptr_t pointerAsUint = reinterpret_cast<uintptr_t>(ptr);
    char* superPagePtr = reinterpret_cast<char*>(pointerAsUint & kSuperPageBaseMask);
    uintptr_t partitionPageIndex = (pointerAsUint & kSuperPageOffsetMask) >> kPartitionPageShift;
    // Index 0 is invalid because it is the metadata area and the last index is invalid because it is a guard page.
    ASSERT(partitionPageIndex);
    ASSERT(partitionPageIndex < kNumPartitionPagesPerSuperPage - 1);
    PartitionPage* page = reinterpret_cast<PartitionPage*>(partitionSuperPageToMetadataArea(superPagePtr) + (partitionPageIndex << kPageMetadataShift));
    // Many partition pages can share the same page object. Adjust for that.
    size_t delta = page->pageOffset << kPageMetadataShift;
    page = reinterpret_cast<PartitionPage*>(reinterpret_cast<char*>(page) - delta);
    return page;
}

ALWAYS_INLINE void* partitionPageToPointer(PartitionPage* page)
{
    uintptr_t pointerAsUint = reinterpret_cast<uintptr_t>(page);
    uintptr_t superPageOffset = (pointerAsUint & kSuperPageOffsetMask);
    ASSERT(superPageOffset > kSystemPageSize);
    ASSERT(superPageOffset < kSystemPageSize + (kNumPartitionPagesPerSuperPage * kPageMetadataSize));
    uintptr_t partitionPageIndex = (superPageOffset - kSystemPageSize) >> kPageMetadataShift;
    // Index 0 is invalid because it is the metadata area and the last index is invalid because it is a guard page.
    ASSERT(partitionPageIndex);
    ASSERT(partitionPageIndex < kNumPartitionPagesPerSuperPage - 1);
    uintptr_t superPageBase = (pointerAsUint & kSuperPageBaseMask);
    void* ret = reinterpret_cast<void*>(superPageBase + (partitionPageIndex << kPartitionPageShift));
    return ret;
}

ALWAYS_INLINE PartitionPage* partitionPointerToPage(void* ptr)
{
    PartitionPage* page = partitionPointerToPageNoAlignmentCheck(ptr);
    // Checks that the pointer is a multiple of bucket size.
    ASSERT(!((reinterpret_cast<uintptr_t>(ptr) - reinterpret_cast<uintptr_t>(partitionPageToPointer(page))) % page->bucket->slotSize));
    return page;
}

ALWAYS_INLINE PartitionRootBase* partitionPageToRoot(PartitionPage* page)
{
    PartitionSuperPageExtentEntry* extentEntry = reinterpret_cast<PartitionSuperPageExtentEntry*>(reinterpret_cast<uintptr_t>(page) & kSystemPageBaseMask);
    return extentEntry->root;
}

ALWAYS_INLINE bool partitionPointerIsValid(void* ptr)
{
    PartitionPage* page = partitionPointerToPage(ptr);
    PartitionRootBase* root = partitionPageToRoot(page);
    return root->invertedSelf == ~reinterpret_cast<uintptr_t>(root);
}

ALWAYS_INLINE void* partitionBucketAlloc(PartitionRootBase* root, int flags, size_t size, PartitionBucket* bucket)
{
    PartitionPage* page = bucket->activePagesHead;
    ASSERT(page->numAllocatedSlots >= 0);
    void* ret = page->freelistHead;
    if (LIKELY(ret != 0)) {
        // If these asserts fire, you probably corrupted memory.
        ASSERT(partitionPointerIsValid(ret));
        PartitionFreelistEntry* newHead = partitionFreelistMask(static_cast<PartitionFreelistEntry*>(ret)->next);
        page->freelistHead = newHead;
        ASSERT(!ret || partitionPointerIsValid(ret));
        page->numAllocatedSlots++;
    } else {
        ret = partitionAllocSlowPath(root, flags, size, bucket);
    }
#ifndef NDEBUG
    if (!ret)
        return 0;
    // Fill the uninitialized pattern. and write the cookies.
    page = partitionPointerToPage(ret);
    size_t bucketSize = page->bucket->slotSize;
    memset(ret, kUninitializedByte, bucketSize);
    partitionCookieWriteValue(ret);
    partitionCookieWriteValue(reinterpret_cast<char*>(ret) + bucketSize - kCookieSize);
    // The value given to the application is actually just after the cookie.
    ret = static_cast<char*>(ret) + kCookieSize;
#endif
    return ret;
}

ALWAYS_INLINE void* partitionAlloc(PartitionRoot* root, size_t size)
{
#if defined(MEMORY_TOOL_REPLACES_ALLOCATOR)
    void* result = malloc(size);
    RELEASE_ASSERT(result);
    return result;
#else
    size = partitionCookieSizeAdjustAdd(size);
    ASSERT(root->initialized);
    size_t index = size >> kBucketShift;
    ASSERT(index < root->numBuckets);
    ASSERT(size == index << kBucketShift);
    PartitionBucket* bucket = &root->buckets()[index];
    return partitionBucketAlloc(root, 0, size, bucket);
#endif // defined(MEMORY_TOOL_REPLACES_ALLOCATOR)
}

ALWAYS_INLINE void partitionFreeWithPage(void* ptr, PartitionPage* page)
{
    // If these asserts fire, you probably corrupted memory.
#ifndef NDEBUG
    size_t bucketSize = page->bucket->slotSize;
    partitionCookieCheckValue(ptr);
    partitionCookieCheckValue(reinterpret_cast<char*>(ptr) + bucketSize - kCookieSize);
    memset(ptr, kFreedByte, bucketSize);
#endif
    ASSERT(page->numAllocatedSlots);
    PartitionFreelistEntry* freelistHead = page->freelistHead;
    ASSERT(!freelistHead || partitionPointerIsValid(freelistHead));
    RELEASE_ASSERT(ptr != freelistHead); // Catches an immediate double free.
    ASSERT(!freelistHead || ptr != partitionFreelistMask(freelistHead->next)); // Look for double free one level deeper in debug.
    PartitionFreelistEntry* entry = static_cast<PartitionFreelistEntry*>(ptr);
    entry->next = partitionFreelistMask(freelistHead);
    page->freelistHead = entry;
    --page->numAllocatedSlots;
    if (UNLIKELY(page->numAllocatedSlots <= 0))
        partitionFreeSlowPath(page);
}

ALWAYS_INLINE void partitionFree(void* ptr)
{
#if defined(MEMORY_TOOL_REPLACES_ALLOCATOR)
    free(ptr);
#else
    ptr = partitionCookieFreePointerAdjust(ptr);
    ASSERT(partitionPointerIsValid(ptr));
    PartitionPage* page = partitionPointerToPage(ptr);
    partitionFreeWithPage(ptr, page);
#endif
}

ALWAYS_INLINE PartitionBucket* partitionGenericSizeToBucket(PartitionRootGeneric* root, size_t size)
{
    size_t order = kBitsPerSizet - countLeadingZerosSizet(size);
    // The order index is simply the next few bits after the most significant bit.
    size_t orderIndex = (size >> root->orderIndexShifts[order]) & (kGenericNumBucketsPerOrder - 1);
    // And if the remaining bits are non-zero we must bump the bucket up.
    size_t subOrderIndex = size & root->orderSubIndexMasks[order];
    PartitionBucket* bucket = root->bucketLookups[(order << kGenericNumBucketsPerOrderBits) + orderIndex + !!subOrderIndex];
    ASSERT(!bucket->slotSize || bucket->slotSize >= size);
    ASSERT(!(bucket->slotSize % kGenericSmallestBucket));
    return bucket;
}

ALWAYS_INLINE void* partitionAllocGenericFlags(PartitionRootGeneric* root, int flags, size_t size)
{
#if defined(MEMORY_TOOL_REPLACES_ALLOCATOR)
    void* result = malloc(size);
    RELEASE_ASSERT(result);
    return result;
#else
    ASSERT(root->initialized);
    size = partitionCookieSizeAdjustAdd(size);
    PartitionBucket* bucket = partitionGenericSizeToBucket(root, size);
    spinLockLock(&root->lock);
    void* ret = partitionBucketAlloc(root, flags, size, bucket);
    spinLockUnlock(&root->lock);
    return ret;
#endif
}

ALWAYS_INLINE void* partitionAllocGeneric(PartitionRootGeneric* root, size_t size)
{
    return partitionAllocGenericFlags(root, 0, size);
}

ALWAYS_INLINE void partitionFreeGeneric(PartitionRootGeneric* root, void* ptr)
{
#if defined(MEMORY_TOOL_REPLACES_ALLOCATOR)
    free(ptr);
#else
    ASSERT(root->initialized);

    if (UNLIKELY(!ptr))
        return;

    ptr = partitionCookieFreePointerAdjust(ptr);
    ASSERT(partitionPointerIsValid(ptr));
    PartitionPage* page = partitionPointerToPage(ptr);
    spinLockLock(&root->lock);
    partitionFreeWithPage(ptr, page);
    spinLockUnlock(&root->lock);
#endif
}

ALWAYS_INLINE bool partitionBucketIsDirectMapped(PartitionBucket* bucket)
{
    return !bucket->numSystemPagesPerSlotSpan;
}

ALWAYS_INLINE size_t partitionDirectMapSize(size_t size)
{
    // Caller must check that the size is not above the kGenericMaxDirectMapped
    // limit before calling. This also guards against integer overflow in the
    // calculation here.
    ASSERT(size <= kGenericMaxDirectMapped);
    return (size + kSystemPageOffsetMask) & kSystemPageBaseMask;
}

ALWAYS_INLINE size_t partitionAllocActualSize(PartitionRootGeneric* root, size_t size)
{
#if defined(MEMORY_TOOL_REPLACES_ALLOCATOR)
    return size;
#else
    ASSERT(root->initialized);
    size = partitionCookieSizeAdjustAdd(size);
    PartitionBucket* bucket = partitionGenericSizeToBucket(root, size);
    if (LIKELY(!partitionBucketIsDirectMapped(bucket))) {
        size = bucket->slotSize;
    } else if (size > kGenericMaxDirectMapped) {
        // Too large to allocate => return the size unchanged.
    } else {
        ASSERT(bucket == &PartitionRootBase::gPagedBucket);
        size = partitionDirectMapSize(size);
    }
    return partitionCookieSizeAdjustSubtract(size);
#endif
}

ALWAYS_INLINE bool partitionAllocSupportsGetSize()
{
#if defined(MEMORY_TOOL_REPLACES_ALLOCATOR)
    return false;
#else
    return true;
#endif
}

ALWAYS_INLINE size_t partitionAllocGetSize(void* ptr)
{
    // No need to lock here. Only 'ptr' being freed by another thread could
    // cause trouble, and the caller is responsible for that not happening.
    ASSERT(partitionAllocSupportsGetSize());
    ptr = partitionCookieFreePointerAdjust(ptr);
    ASSERT(partitionPointerIsValid(ptr));
    PartitionPage* page = partitionPointerToPage(ptr);
    size_t size = page->bucket->slotSize;
    return partitionCookieSizeAdjustSubtract(size);
}

// N (or more accurately, N - sizeof(void*)) represents the largest size in
// bytes that will be handled by a SizeSpecificPartitionAllocator.
// Attempts to partitionAlloc() more than this amount will fail.
template <size_t N>
class SizeSpecificPartitionAllocator {
public:
    static const size_t kMaxAllocation = N - kAllocationGranularity;
    static const size_t kNumBuckets = N / kAllocationGranularity;
    void init() { partitionAllocInit(&m_partitionRoot, kNumBuckets, kMaxAllocation); }
    bool shutdown() { return partitionAllocShutdown(&m_partitionRoot); }
    ALWAYS_INLINE PartitionRoot* root() { return &m_partitionRoot; }
private:
    PartitionRoot m_partitionRoot;
    PartitionBucket m_actualBuckets[kNumBuckets];
};

class PartitionAllocatorGeneric {
public:
    void init() { partitionAllocGenericInit(&m_partitionRoot); }
    bool shutdown() { return partitionAllocGenericShutdown(&m_partitionRoot); }
    ALWAYS_INLINE PartitionRootGeneric* root() { return &m_partitionRoot; }
private:
    PartitionRootGeneric m_partitionRoot;
};

} // namespace WTF

using WTF::SizeSpecificPartitionAllocator;
using WTF::PartitionAllocatorGeneric;
using WTF::PartitionRoot;
using WTF::partitionAllocInit;
using WTF::partitionAllocShutdown;
using WTF::partitionAlloc;
using WTF::partitionFree;
using WTF::partitionAllocGeneric;
using WTF::partitionFreeGeneric;
using WTF::partitionReallocGeneric;
using WTF::partitionAllocActualSize;
using WTF::partitionAllocSupportsGetSize;
using WTF::partitionAllocGetSize;

#endif // WTF_PartitionAlloc_h