Virtual Memory Management

Basic Concepts

Virtual memory management is a technology used by computer systems to manage memory. Each process has a continuous virtual address space. The size of the virtual address space is determined by the number of CPU bits. The maximum addressing space for a 32-bit hardware platform ranges from 0 GiB to 4 GiB. The 4 GiB space is divided into two parts: 3 GiB higher-address space for the LiteOS-A kernel and 1 GiB lower-address space for user-mode processes. The virtual address space of each process space is independent, and the code and data do not affect each other.

The system divides the virtual memory into memory blocks called virtual pages. The size of a virtual page is generally 4 KiB or 64 KiB. The virtual page of the LiteOS-A kernel is 4 KiB by default. You can configure memory management units (MMUs) as required. The minimum unit of the virtual memory management is a page. A virtual address region in the LiteOS-A kernel can contain one virtual page or multiple virtual pages with contiguous addresses. Similarly, the physical memory is also divided by page, and each memory block is called page frame. The virtual address space is divided as follows: 3 GiB (0x40000000 to 0xFFFFFFFF) for the kernel space and 1 GiB (0x01000000 to 0x3F000000) for the user space. The following tables describe the virtual address plan. You can view or configure virtual addresses in los_vm_zone.h.

Table 1 Kernel-mode addresses

Zone	Description	Property
DMA zone	Addresses for direct memory access (DMA) of I/O devices.	Uncache
Normal zone	Addresses for loading the kernel code segment, data segment, heap, and stack.	Cache
high mem zone	Addresses for allocating contiguous virtual memory. The mapped physical memory blocks may not be contiguous.	Cache

Table 2 User-mode virtual addresses

Zone	Description	Property
Code segment	User-mode code segment address range	Cache
Heap	User-mode heap address range	Cache
Stack	User-mode stack address range	Cache
Shared library	Address range for loading the user-mode shared library, including the address range mapped by mmap.	Cache

Working Principles

In virtual memory management, the virtual address space is contiguous, but the mapped physical memory is not necessarily contiguous, as depicted in the following figure. When an executable program is loaded and runs, the CPU accesses the code or data in the virtual address space in the following two cases:

If the page (for example, V0) containing the virtual address accessed by the CPU is mapped to a physical page (for example, P0), the CPU locates the page table entry corresponding to the process (for details, see Virtual-to-Physical Mapping"), accesses the physical memory based on the physical address information in the page table entry, and returns the content.
If the page (for example, V2) containing the virtual address accessed by the CPU is not mapped to a physical page, the system triggers a page missing fault, requests a physical page, copies the corresponding information to the physical page, and updates the start address of the physical page to the page table entry. Then, the CPU can access specific code or data by executing the instruction for accessing the virtual memory again.

Figure 1 Mapping between the virtual and physical memory addresses

Development Guidelines

Available APIs

Table 3 APIs of the virtual memory management module

Function	API	Description
Obtaining process memory space	LOS_CurrSpaceGet	Obtains the pointer to the current process space structure.
	LOS_SpaceGet	Obtains the pointer to the process space structure corresponding to the virtual address.
	LOS_GetKVmSpace	Obtains the pointer to the kernel process space structure.
	LOS_GetVmallocSpace	Obtains the pointer to the vmalloc space structure.
	LOS_GetVmSpaceList	Obtains the pointer to the process space linked list.
Operations related to the virtual address region	LOS_RegionFind	Searches for and returns the virtual address region corresponding to the specified address in the process space.
	LOS_RegionRangeFind	Searches for and returns the virtual address region corresponding to the specified address range in the process space.
	LOS_IsRegionFileValid	Checks whether the virtual address region is mapped to a file.
	LOS_RegionAlloc	Requests a free virtual address region.
	LOS_RegionFree	Releases a specific region in the process space.
	LOS_RegionEndAddr	Obtains the end address of the specified address region.
	LOS_RegionSize	Obtains the size of a region.
	LOS_IsRegionTypeFile	Checks whether the address region is a file memory mapping.
	LOS_IsRegionPermUserReadOnly	Checks whether the address region is read-only in the user space.
	LOS_IsRegionFlagPrivateOnly	Checks whether the address region has private attributes.
	LOS_SetRegionTypeFile	Sets the file memory mapping attribute.
	LOS_IsRegionTypeDev	Checks whether the address region is device memory mapping.
	LOS_SetRegionTypeDev	Sets the device memory mapping attribute.
	LOS_IsRegionTypeAnon	Checks whether the address region is an anonymous mapping.
	LOS_SetRegionTypeAnon	Sets the anonymous mapping attribute.
Verifying address	LOS_IsUserAddress	Checks whether the address is in the user space.
	LOS_IsUserAddressRange	Checks whether the address region is in the user space.
	LOS_IsKernelAddress	Checks whether the address is in the kernel space.
	LOS_IsKernelAddressRange	Checks whether the address region is in the kernel space.
	LOS_IsRangeInSpace	Checks whether the address region is in the process space.
vmalloc operations	LOS_VMalloc	Requests memory using vmalloc.
	LOS_VFree	Releases memory using vmalloc.
	LOS_IsVmallocAddress	Checks whether the address is requested by using vmalloc.
Requesting memory	LOS_KernelMalloc	Allocates memory from the heap memory pool if the requested memory is less than 16 KiB; allocates multiple contiguous physical pages for memory allocation if the requested memory is greater than 16 KiB.
	LOS_KernelMallocAlign	Allocates memory with alignment attributes. The allocation rule is the same as that of the LOS_KernelMalloc API.
	LOS_KernelFree	Releases the memory requested by LOS_KernelMalloc and LOS_KernelMallocAlign.
	LOS_KernelRealloc	Reallocates the memory requested by LOS_KernelMalloc and LOS_KernelMallocAlign.
Others	LOS_PaddrQuery	Obtains the physical address based on the virtual address.
	LOS_VmSpaceFree	Releases the process space, including the virtual memory region and page table.
	LOS_VmSpaceReserve	Reserves a memory space in the process space.
	LOS_VaddrToPaddrMmap	Maps the physical address region with the specified length to a virtual address region. You need to request the physical address region before the operation.

How to Develop

To use APIs related to virtual memory:

Obtain the process space structure using the APIs for obtaining the process space, and access the structure information.
Perform the following operations on the virtual address region:
- Call LOS_RegionAlloc to request a virtual address region.
- Call LOS_RegionFind and LOS_RegionRangeFind to check whether the corresponding address region exists.
- Call LOS_RegionFree to release a virtual address region.
Call vmalloc and memory requesting APIs to apply for memory in the kernel as demanded.

NOTE: The physical memory requested by using the memory requesting APIs must be contiguous. If the system cannot provide a large number of contiguous memory blocks, the request fails. Therefore, the memory requesting APIs are recommended for requesting small memory blocks. vmalloc is recommended for requesting non-contiguous physical memory. However, the memory is allocated in the unit of pages (4096 bytes/page in the current system). If you want memory that is an integer multiple of a page, you can use vmalloc. For example, you can use vmalloc to request memory for file reading in a file system, which demands a large cache.