base/allocator/partition_allocator/page_allocator.h

// Copyright 2013 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_ALLOCATOR_PARTITION_ALLOCATOR_PAGE_ALLOCATOR_H_
#define BASE_ALLOCATOR_PARTITION_ALLOCATOR_PAGE_ALLOCATOR_H_

#include <cstddef>
#include <cstdint>

#include "base/allocator/partition_allocator/page_allocator_constants.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_buildflags.h"
#include "build/build_config.h"

namespace partition_alloc {

struct PageAccessibilityConfiguration {
  enum Permissions {
    kInaccessible,
    kRead,
    kReadWrite,
    // This flag is mapped to kReadWrite on systems that
    // don't support MTE.
    kReadWriteTagged,
    // This flag is mapped to kReadExecute on systems
    // that don't support Arm's BTI.
    kReadExecuteProtected,
    kReadExecute,
    // This flag is deprecated and will go away soon.
    // TODO(bbudge) Remove this as soon as V8 doesn't need RWX pages.
    kReadWriteExecute,
  };

#if BUILDFLAG(ENABLE_PKEYS)
  constexpr explicit PageAccessibilityConfiguration(Permissions permissions)
      : permissions(permissions), pkey(0) {}
  constexpr PageAccessibilityConfiguration(Permissions permissions, int pkey)
      : permissions(permissions), pkey(pkey) {}
#else
  constexpr explicit PageAccessibilityConfiguration(Permissions permissions)
      : permissions(permissions) {}
#endif  // BUILDFLAG(ENABLE_PKEYS)

  Permissions permissions;
#if BUILDFLAG(ENABLE_PKEYS)
  // Tag the page with a Memory Protection Key. Use 0 for none.
  int pkey;
#endif  // BUILDFLAG(ENABLE_PKEYS)
};

// Use for De/RecommitSystemPages API.
enum class PageAccessibilityDisposition {
  // Enforces permission update (Decommit will set to
  // PageAccessibilityConfiguration::kInaccessible;
  // Recommit will set to whatever was requested, other than
  // PageAccessibilityConfiguration::kInaccessible).
  kRequireUpdate,
  // Will not update permissions, if the platform supports that (POSIX & Fuchsia
  // only).
  kAllowKeepForPerf,
};

// macOS supports tagged memory regions, to help in debugging. On Android,
// these tags are used to name anonymous mappings.
enum class PageTag {
  kFirst = 240,           // Minimum tag value.
  kSimulation = 251,      // Memory simulator tool.
  kBlinkGC = 252,         // Blink GC pages.
  kPartitionAlloc = 253,  // PartitionAlloc, no matter the partition.
  kChromium = 254,        // Chromium page.
  kV8 = 255,              // V8 heap pages.
  kLast = kV8             // Maximum tag value.
};

// See
// https://github.com/apple-oss-distributions/xnu/blob/5c2921b07a2480ab43ec66f5b9e41cb872bc554f/osfmk/mach/vm_statistics.h#L687
static_assert(
    static_cast<int>(PageTag::kLast) < 256,
    "Tags are only 1 byte long on macOS, see vm_statistics.h in XNU.");

PA_COMPONENT_EXPORT(PARTITION_ALLOC)
uintptr_t NextAlignedWithOffset(uintptr_t ptr,
                                uintptr_t alignment,
                                uintptr_t requested_offset);

// Allocates one or more pages.
//
// The requested |address| is just a hint; the actual address returned may
// differ. The returned address will be aligned to |align_offset| modulo |align|
// bytes.
//
// |length|, |align| and |align_offset| are in bytes, and must be a multiple of
// |PageAllocationGranularity()|. |length| and |align| must be non-zero.
// |align_offset| must be less than |align|. |align| must be a power of two.
//
// If |address| is 0/nullptr, then a suitable and randomized address will be
// chosen automatically.
//
// |accessibility| controls the permission of the allocated pages.
// PageAccessibilityConfiguration::kInaccessible means uncommitted.
//
// |page_tag| is used on some platforms to identify the source of the
// allocation. Use PageTag::kChromium as a catch-all category.
//
// |file_descriptor_for_shared_alloc| is only used in mapping the shadow
// pools to the same physical address as the real one in
// PartitionAddressSpace::Init(). It should be ignored in other cases.
//
// This call will return 0/nullptr if the allocation cannot be satisfied.
PA_COMPONENT_EXPORT(PARTITION_ALLOC)
uintptr_t AllocPages(size_t length,
                     size_t align,
                     PageAccessibilityConfiguration accessibility,
                     PageTag page_tag,
                     int file_descriptor_for_shared_alloc = -1);
PA_COMPONENT_EXPORT(PARTITION_ALLOC)
uintptr_t AllocPages(uintptr_t address,
                     size_t length,
                     size_t align,
                     PageAccessibilityConfiguration accessibility,
                     PageTag page_tag);
PA_COMPONENT_EXPORT(PARTITION_ALLOC)
void* AllocPages(void* address,
                 size_t length,
                 size_t align,
                 PageAccessibilityConfiguration accessibility,
                 PageTag page_tag);
PA_COMPONENT_EXPORT(PARTITION_ALLOC)
uintptr_t AllocPagesWithAlignOffset(
    uintptr_t address,
    size_t length,
    size_t align,
    size_t align_offset,
    PageAccessibilityConfiguration page_accessibility,
    PageTag page_tag,
    int file_descriptor_for_shared_alloc = -1);

// Frees one or more pages starting at |address| and continuing for |length|
// bytes.
//
// |address| and |length| must match a previous call to |AllocPages|. Therefore,
// |address| must be aligned to |PageAllocationGranularity()| bytes, and
// |length| must be a multiple of |PageAllocationGranularity()|.
PA_COMPONENT_EXPORT(PARTITION_ALLOC)
void FreePages(uintptr_t address, size_t length);
PA_COMPONENT_EXPORT(PARTITION_ALLOC)
void FreePages(void* address, size_t length);

// Marks one or more system pages, starting at |address| with the given
// |page_accessibility|. |length| must be a multiple of |SystemPageSize()|
// bytes.
//
// Returns true if the permission change succeeded. In most cases you must
// |CHECK| the result.
[[nodiscard]] PA_COMPONENT_EXPORT(PARTITION_ALLOC) bool TrySetSystemPagesAccess(
    uintptr_t address,
    size_t length,
    PageAccessibilityConfiguration page_accessibility);
[[nodiscard]] PA_COMPONENT_EXPORT(PARTITION_ALLOC) bool TrySetSystemPagesAccess(
    void* address,
    size_t length,
    PageAccessibilityConfiguration page_accessibility);

// Marks one or more system pages, starting at |address| with the given
// |page_accessibility|. |length| must be a multiple of |SystemPageSize()|
// bytes.
//
// Performs a CHECK that the operation succeeds.
PA_COMPONENT_EXPORT(PARTITION_ALLOC)
void SetSystemPagesAccess(uintptr_t address,
                          size_t length,
                          PageAccessibilityConfiguration page_accessibility);
PA_COMPONENT_EXPORT(PARTITION_ALLOC)
void SetSystemPagesAccess(void* address,
                          size_t length,
                          PageAccessibilityConfiguration page_accessibility);

// Decommits one or more system pages starting at |address| and continuing for
// |length| bytes. |address| and |length| must be aligned to a system page
// boundary.
//
// This API will crash if the operation cannot be performed!
//
// If disposition is PageAccessibilityDisposition::kRequireUpdate (recommended),
// the decommitted pages will be made inaccessible before the call returns.
// While it is always a programming error to access decommitted pages without
// first recommitting them, callers may use
// PageAccessibilityDisposition::kAllowKeepForPerf to allow the implementation
// to skip changing permissions (use with care), for performance reasons (see
// crrev.com/c/2567282 and crrev.com/c/2563038 for perf regressions encountered
// in the past). Implementations may choose to always modify permissions, hence
// accessing those pages may or may not trigger a fault.
//
// Decommitting means that physical resources (RAM or swap/pagefile) backing the
// allocated virtual address range may be released back to the system, but the
// address space is still allocated to the process (possibly using up page table
// entries or other accounting resources). There is no guarantee that the pages
// are zeroed, unless |DecommittedMemoryIsAlwaysZeroed()| is true.
//
// This operation may not be atomic on some platforms.
//
// Note: "Committed memory" is a Windows Memory Subsystem concept that ensures
// processes will not fault when touching a committed memory region. There is
// no analogue in the POSIX & Fuchsia memory API where virtual memory pages are
// best-effort allocated resources on the first touch. If
// PageAccessibilityDisposition::kRequireUpdate disposition is used, this API
// behaves in a platform-agnostic way by simulating the Windows "decommit" state
// by both discarding the region (allowing the OS to avoid swap operations)
// *and* changing the page protections so accesses fault.
PA_COMPONENT_EXPORT(PARTITION_ALLOC)
void DecommitSystemPages(
    uintptr_t address,
    size_t length,
    PageAccessibilityDisposition accessibility_disposition);
PA_COMPONENT_EXPORT(PARTITION_ALLOC)
void DecommitSystemPages(
    void* address,
    size_t length,
    PageAccessibilityDisposition accessibility_disposition);

// Decommits one or more system pages starting at |address| and continuing for
// |length| bytes. |address| and |length| must be aligned to a system page
// boundary.
//
// In contrast to |DecommitSystemPages|, this API guarantees that the pages are
// zeroed and will always mark the region as inaccessible (the equivalent of
// setting them to PageAccessibilityConfiguration::kInaccessible).
//
// This API will crash if the operation cannot be performed.
PA_COMPONENT_EXPORT(PARTITION_ALLOC)
void DecommitAndZeroSystemPages(uintptr_t address, size_t length);
PA_COMPONENT_EXPORT(PARTITION_ALLOC)
void DecommitAndZeroSystemPages(void* address, size_t length);

// Whether decommitted memory is guaranteed to be zeroed when it is
// recommitted. Do not assume that this will not change over time.
constexpr PA_COMPONENT_EXPORT(
    PARTITION_ALLOC) bool DecommittedMemoryIsAlwaysZeroed() {
#if BUILDFLAG(IS_APPLE)
  return false;
#else
  return true;
#endif
}

// (Re)Commits one or more system pages, starting at |address| and continuing
// for |length| bytes with the given |page_accessibility| (must not be
// PageAccessibilityConfiguration::kInaccessible). |address| and |length|
// must be aligned to a system page boundary.
//
// This API will crash if the operation cannot be performed!
//
// If disposition is PageAccessibilityConfiguration::kRequireUpdate, the calls
// updates the pages to |page_accessibility|. This can be used regardless of
// what disposition was used to decommit the pages.
// PageAccessibilityConfiguration::kAllowKeepForPerf allows the implementation
// to leave the page permissions, if that improves performance. This option can
// only be used if the pages were previously accessible and decommitted with
// that same option.
//
// The memory will be zeroed when it is committed for the first time. However,
// there is no such guarantee when memory is recommitted, unless
// |DecommittedMemoryIsAlwaysZeroed()| is true.
//
// This operation may not be atomic on some platforms.
PA_COMPONENT_EXPORT(PARTITION_ALLOC)
void RecommitSystemPages(
    uintptr_t address,
    size_t length,
    PageAccessibilityConfiguration page_accessibility,
    PageAccessibilityDisposition accessibility_disposition);

// Like RecommitSystemPages(), but returns false instead of crashing.
[[nodiscard]] PA_COMPONENT_EXPORT(PARTITION_ALLOC) bool TryRecommitSystemPages(
    uintptr_t address,
    size_t length,
    PageAccessibilityConfiguration page_accessibility,
    PageAccessibilityDisposition accessibility_disposition);

// Discard one or more system pages starting at |address| and continuing for
// |length| bytes. |length| must be a multiple of |SystemPageSize()|.
//
// Discarding is a hint to the system that the page is no longer required. The
// hint may:
//   - Do nothing.
//   - Discard the page immediately, freeing up physical pages.
//   - Discard the page at some time in the future in response to memory
//   pressure.
//
// Only committed pages should be discarded. Discarding a page does not decommit
// it, and it is valid to discard an already-discarded page. A read or write to
// a discarded page will not fault.
//
// Reading from a discarded page may return the original page content, or a page
// full of zeroes.
//
// Writing to a discarded page is the only guaranteed way to tell the system
// that the page is required again. Once written to, the content of the page is
// guaranteed stable once more. After being written to, the page content may be
// based on the original page content, or a page of zeroes.
PA_COMPONENT_EXPORT(PARTITION_ALLOC)
void DiscardSystemPages(uintptr_t address, size_t length);
PA_COMPONENT_EXPORT(PARTITION_ALLOC)
void DiscardSystemPages(void* address, size_t length);

// Rounds up |address| to the next multiple of |SystemPageSize()|. Returns
// 0 for an |address| of 0.
PA_ALWAYS_INLINE PAGE_ALLOCATOR_CONSTANTS_DECLARE_CONSTEXPR uintptr_t
RoundUpToSystemPage(uintptr_t address) {
  return (address + internal::SystemPageOffsetMask()) &
         internal::SystemPageBaseMask();
}

// Rounds down |address| to the previous multiple of |SystemPageSize()|. Returns
// 0 for an |address| of 0.
PA_ALWAYS_INLINE PAGE_ALLOCATOR_CONSTANTS_DECLARE_CONSTEXPR uintptr_t
RoundDownToSystemPage(uintptr_t address) {
  return address & internal::SystemPageBaseMask();
}

// Rounds up |address| to the next multiple of |PageAllocationGranularity()|.
// Returns 0 for an |address| of 0.
PA_ALWAYS_INLINE PAGE_ALLOCATOR_CONSTANTS_DECLARE_CONSTEXPR uintptr_t
RoundUpToPageAllocationGranularity(uintptr_t address) {
  return (address + internal::PageAllocationGranularityOffsetMask()) &
         internal::PageAllocationGranularityBaseMask();
}

// Rounds down |address| to the previous multiple of
// |PageAllocationGranularity()|. Returns 0 for an |address| of 0.
PA_ALWAYS_INLINE PAGE_ALLOCATOR_CONSTANTS_DECLARE_CONSTEXPR uintptr_t
RoundDownToPageAllocationGranularity(uintptr_t address) {
  return address & internal::PageAllocationGranularityBaseMask();
}

// Reserves (at least) |size| bytes of address space, aligned to
// |PageAllocationGranularity()|. This can be called early on to make it more
// likely that large allocations will succeed. Returns true if the reservation
// succeeded, false if the reservation failed or a reservation was already made.
PA_COMPONENT_EXPORT(PARTITION_ALLOC) bool ReserveAddressSpace(size_t size);

// Releases any reserved address space. |AllocPages| calls this automatically on
// an allocation failure. External allocators may also call this on failure.
//
// Returns true when an existing reservation was released.
PA_COMPONENT_EXPORT(PARTITION_ALLOC) bool ReleaseReservation();

// Returns true if there is currently an address space reservation.
PA_COMPONENT_EXPORT(PARTITION_ALLOC) bool HasReservationForTesting();

// Returns |errno| (POSIX) or the result of |GetLastError| (Windows) when |mmap|
// (POSIX) or |VirtualAlloc| (Windows) fails.
PA_COMPONENT_EXPORT(PARTITION_ALLOC) uint32_t GetAllocPageErrorCode();

// Returns the total amount of mapped pages from all clients of
// PageAllocator. These pages may or may not be committed. This is mostly useful
// to assess address space pressure.
PA_COMPONENT_EXPORT(PARTITION_ALLOC) size_t GetTotalMappedSize();

#if BUILDFLAG(IS_WIN)
// Sets whether to retry the allocation of pages when a commit failure
// happens. This doesn't cover cases where the system is out of address space,
// or reaches another limit.
PA_COMPONENT_EXPORT(PARTITION_ALLOC)
void SetRetryOnCommitFailure(bool retry_on_commit_failure);
bool GetRetryOnCommitFailure();
#endif  // BUILDFLAG(IS_WIN)

}  // namespace partition_alloc

#endif  // BASE_ALLOCATOR_PARTITION_ALLOCATOR_PAGE_ALLOCATOR_H_