Understanding the Linux Kernel, Second Edition

The Memory Descriptor

All information related to the process address space is included in a data structure called a memory descriptor . This structure of type mm_struct is referenced by the mm field of the process descriptor. The fields of a memory descriptor are listed in Table 8-2.

Table 8-2. 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
rb_root_t	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
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 to adjacent elements in the list of memory descriptors
unsigned long	start_code	Initial address of executable code
unsigned long	end_code	Final address of executable code
unsigned long	start_data	Initial address of initialized data
unsigned long	end_data	Final address of initialized data
unsigned long	start_brk	Initial address of the heap
unsigned long	brk	Current final address of the heap
unsigned long	start_stack	Initial address ...