Specifications

This is the Title of the Book, eMatter Edition

Memory Management in Linux

421

VMAs are as follows (note the similarity between these fields and the /proc output we

just saw):

unsigned long vm_start;

unsigned long vm_end;

The virtual address range covered by this VMA. These fields are the first two

fields shown in /proc/*/maps.

struct file *vm_file;

A pointer to the struct file structure associated with this area (if any).

unsigned long vm_pgoff;

The offset of the area in the file, in pages. When a file or device is mapped, this is

the file position of the first page mapped in this area.

unsigned long vm_flags;

A set of flags describing this area. The flags of the most interest to device driver

writers are

VM_IO and VM_RESERVED. VM_IO marks a VMA as being a memory-

mapped I/O region. Among other things, the

VM_IO flag prevents the region from

being included in process core dumps.

VM_RESERVED tells the memory manage-

ment system not to attempt to swap out this VMA; it should be set in most

device mappings.

struct vm_operations_struct *vm_ops;

A set of functions that the kernel may invoke to operate on this memory area. Its

presence indicates that the memory area is a kernel “object,” like the

struct file

we have been using throughout the book.

void *vm_private_data;

A field that may be used by the driver to store its own information.

struct vm_area_struct, the vm_operations_struct is defined in <linux/mm.h>;it

includes the operations listed below. These operations are the only ones needed to

handle the process’s memory needs, and they are listed in the order they are

declared. Later in this chapter, some of these functions are implemented.

void (*open)(struct vm_area_struct *vma);

The open method is called by the kernel to allow the subsystem implementing

the VMA to initialize the area. This method is invoked any time a new reference

to the VMA is made (when a process forks, for example). The one exception

happens when the VMA is first created by mmap; in this case, the driver’s mmap

method is called instead.

void (*close)(struct vm_area_struct *vma);

When an area is destroyed, the kernel calls its close operation. Note that there’s

no usage count associated with VMAs; the area is opened and closed exactly

once by each process that uses it.

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