Understanding the Linux Kernel, 3rd Edition

The Memory Descriptor

All information related to the process address space is included in an object called the memory descriptor of type mm_struct. This object is referenced by the mm field of the process descriptor. The fields of a memory descriptor are listed in Table 9-2.

Table 9-2. The fields of the memory descriptor

Type	Field	Description
struct vm_area_struct *	mmap	Pointer to the head of the list of memory region objects
struct rb_root	mm_rb	Pointer to the root of the red-black tree of memory region objects
struct vm_area_struct *	mmap_cache	Pointer to the last referenced memory region object
unsigned long (*)( )	get_unmapped_area	Method that searches an available linear address interval in the process address space
void (*)( )	unmap_area	Method invoked when releasing a linear address interval
unsigned long	mmap_base	Identifies the linear address of the first allocated anonymous memory region or file memory mapping (see the section "Program Segments and Process Memory Regions" in Chapter 20)
unsigned long	free_area_cache	Address from which the kernel will look for a free interval of linear addresses in the process address space
pgd_t *	pgd	Pointer to the Page Global Directory
atomic_t	mm_users	Secondary usage counter
atomic_t	mm_count	Main usage counter
int	map_count	Number of memory regions
struct rw_semaphore	mmap_sem	Memory regions’ read/write semaphore
spinlock_t	page_table_lock	Memory regions’ and Page Tables’ spin lock
struct list_head	mmlist	Pointers ...