src/runtime/mem.go

// Copyright 2022 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package runtime

import "unsafe"

// OS memory management abstraction layer
//
// Regions of the address space managed by the runtime may be in one of four
// states at any given time:
// 1) None - Unreserved and unmapped, the default state of any region.
// 2) Reserved - Owned by the runtime, but accessing it would cause a fault.
//               Does not count against the process' memory footprint.
// 3) Prepared - Reserved, intended not to be backed by physical memory (though
//               an OS may implement this lazily). Can transition efficiently to
//               Ready. Accessing memory in such a region is undefined (may
//               fault, may give back unexpected zeroes, etc.).
// 4) Ready - may be accessed safely.
//
// This set of states is more than is strictly necessary to support all the
// currently supported platforms. One could get by with just None, Reserved, and
// Ready. However, the Prepared state gives us flexibility for performance
// purposes. For example, on POSIX-y operating systems, Reserved is usually a
// private anonymous mmap'd region with PROT_NONE set, and to transition
// to Ready would require setting PROT_READ|PROT_WRITE. However the
// underspecification of Prepared lets us use just MADV_FREE to transition from
// Ready to Prepared. Thus with the Prepared state we can set the permission
// bits just once early on, we can efficiently tell the OS that it's free to
// take pages away from us when we don't strictly need them.
//
// This file defines a cross-OS interface for a common set of helpers
// that transition memory regions between these states. The helpers call into
// OS-specific implementations that handle errors, while the interface boundary
// implements cross-OS functionality, like updating runtime accounting.

// sysAlloc transitions an OS-chosen region of memory from None to Ready.
// More specifically, it obtains a large chunk of zeroed memory from the
// operating system, typically on the order of a hundred kilobytes
// or a megabyte. This memory is always immediately available for use.
//
// sysStat must be non-nil.
//
// Don't split the stack as this function may be invoked without a valid G,
// which prevents us from allocating more stack.
//
//go:nosplit
func sysAlloc(n uintptr, sysStat *sysMemStat) unsafe.Pointer {
	sysStat.add(int64(n))
	gcController.mappedReady.Add(int64(n))
	return sysAllocOS(n)
}

// sysUnused transitions a memory region from Ready to Prepared. It notifies the
// operating system that the physical pages backing this memory region are no
// longer needed and can be reused for other purposes. The contents of a
// sysUnused memory region are considered forfeit and the region must not be
// accessed again until sysUsed is called.
func sysUnused(v unsafe.Pointer, n uintptr) {
	gcController.mappedReady.Add(-int64(n))
	sysUnusedOS(v, n)
}

// sysUsed transitions a memory region from Prepared to Ready. It notifies the
// operating system that the memory region is needed and ensures that the region
// may be safely accessed. This is typically a no-op on systems that don't have
// an explicit commit step and hard over-commit limits, but is critical on
// Windows, for example.
//
// This operation is idempotent for memory already in the Prepared state, so
// it is safe to refer, with v and n, to a range of memory that includes both
// Prepared and Ready memory. However, the caller must provide the exact amount
// of Prepared memory for accounting purposes.
func sysUsed(v unsafe.Pointer, n, prepared uintptr) {
	gcController.mappedReady.Add(int64(prepared))
	sysUsedOS(v, n)
}

// sysHugePage does not transition memory regions, but instead provides a
// hint to the OS that it would be more efficient to back this memory region
// with pages of a larger size transparently.
func sysHugePage(v unsafe.Pointer, n uintptr) {
	sysHugePageOS(v, n)
}

// sysFree transitions a memory region from any state to None. Therefore, it
// returns memory unconditionally. It is used if an out-of-memory error has been
// detected midway through an allocation or to carve out an aligned section of
// the address space. It is okay if sysFree is a no-op only if sysReserve always
// returns a memory region aligned to the heap allocator's alignment
// restrictions.
//
// sysStat must be non-nil.
//
// Don't split the stack as this function may be invoked without a valid G,
// which prevents us from allocating more stack.
//
//go:nosplit
func sysFree(v unsafe.Pointer, n uintptr, sysStat *sysMemStat) {
	sysStat.add(-int64(n))
	gcController.mappedReady.Add(-int64(n))
	sysFreeOS(v, n)
}

// sysFault transitions a memory region from Ready to Reserved. It
// marks a region such that it will always fault if accessed. Used only for
// debugging the runtime.
//
// TODO(mknyszek): Currently it's true that all uses of sysFault transition
// memory from Ready to Reserved, but this may not be true in the future
// since on every platform the operation is much more general than that.
// If a transition from Prepared is ever introduced, create a new function
// that elides the Ready state accounting.
func sysFault(v unsafe.Pointer, n uintptr) {
	gcController.mappedReady.Add(-int64(n))
	sysFaultOS(v, n)
}

// sysReserve transitions a memory region from None to Reserved. It reserves
// address space in such a way that it would cause a fatal fault upon access
// (either via permissions or not committing the memory). Such a reservation is
// thus never backed by physical memory.
//
// If the pointer passed to it is non-nil, the caller wants the
// reservation there, but sysReserve can still choose another
// location if that one is unavailable.
//
// NOTE: sysReserve returns OS-aligned memory, but the heap allocator
// may use larger alignment, so the caller must be careful to realign the
// memory obtained by sysReserve.
func sysReserve(v unsafe.Pointer, n uintptr) unsafe.Pointer {
	return sysReserveOS(v, n)
}

// sysMap transitions a memory region from Reserved to Prepared. It ensures the
// memory region can be efficiently transitioned to Ready.
//
// sysStat must be non-nil.
func sysMap(v unsafe.Pointer, n uintptr, sysStat *sysMemStat) {
	sysStat.add(int64(n))
	sysMapOS(v, n)
}