What is pfn in linux

In Linux, the full name of pfn is "page frame number", which is the physical memory area number. "Page frame" refers to physical memory. It divides physical memory into page size areas and gives each page a number, and this number is PFN.

#The operating environment of this tutorial: linux5.9.8 system, Dell G3 computer.

1. What is page frame?

One of the most important functions of the operating system is to manage various resources in the computer system. As the most important resource: memory, we must manage it. In the Linux operating system, physical memory is managed according to page size. The specific page size is related to the hardware and Linux system configuration. 4k is the most classic setting. Therefore, for physical memory, we divide it into pages arranged by page size. The memory area of ​​page size in each physical memory is called page frame. We establish a struct page data structure for each physical page frame to track the usage of each physical page: Is it used for the text segment of the kernel? Or is it a page table for a process? Is it used for various file caches or is it in a free state...

Each page frame has a one-to-one corresponding page data structure. The system defines page_to_pfn and pfn_to_page macros to use in the page frame number and page Converting between data structures, the specific method of conversion is related to the memory model. We will describe the three memory models in the Linux kernel in detail in Chapter 3.

2. What is PFN?

For a computer system, its entire physical address space should be a period of address space starting from 0 to the maximum physical space that the actual system can support. In the ARM system, assuming the physical address is 32 bits, then the physical address space is 4G. In the ARM64 system, if the number of supported physical address bits is 48, then the physical address space is 256T. Of course, in fact, not all such a large physical address space is used for memory, some also belong to I/O space (of course, some CPU arches have their own independent io address space). Therefore, the physical address space occupied by the memory should be a limited interval, and it is impossible to cover the entire physical address space. However, now that the memory is getting larger and larger, for 32-bit systems, the 4G physical address space can no longer meet the memory requirements, so there is the concept of high memory, which will be described in detail later.

PFN is the abbreviation of page frame number. The so-called page frame is for physical memory. The physical memory is divided into areas of page size, and each page is numbered. This number is PFN. Assuming that physical memory starts at address 0, then the page frame with PFN equal to 0 is the page that starts at address 0 (physical address). Assuming that physical memory starts at address x, then the first page frame number is (x>>PAGE_SHIFT).

The value range of PFN is 0----(memory size >> 12).

However, due to the relationship between physical memory mapping, the physical memory address 0 corresponds to the physical address 0 on the system. For example: on s3c2440, the memory address starts from 0x30000000. When the ram is connected, the 0 address of the ram is 0x30000000 in the view of s3c2440. Therefore, the value of pfn in the system should be equal to (physical address - memory base address) >> 12.

But in Linux, all virtual addresses are used, so the virtual address must be converted into a physical address first. So there is a macro definition in Linux to convert the virtual address of the kernel linear space and pfn:

#define virt_to_pfn(kaddr)	(__pa(kaddr) >> PAGE_SHIFT)   //__pa() 将虚拟地址转化成物理地址  
#define pfn_to_virt(pfn)	__va((pfn) << PAGE_SHIFT)

So pfn should correspond to the page frame number in the Linux kernel. The corresponding physical address can be found through simple conversion.

For each physical page, the kernel assigns a descriptor to describe it: page. pfn can be converted to page:

#define page_to_pfn __page_to_pfn
#define pfn_to_page __pfn_to_page

All page structures are stored in mem_map for easy management.

Because Linux divides physical memory into 4K pages for management. Therefore, the MMU will also be set up in the software for matching. Described later.

Linux divides the 4G virtual space into user space and kernel space. The user space is 0---3G, and the kernel space is 3G---4G. User space can be accessed from kernel space, but kernel space must be accessed from user space through system calls.

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