9.2. 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 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 of User Mode stack
unsigned long	arg_start	Initial address of command-line arguments
unsigned long	arg_end	Final address of command-line arguments
unsigned long	env_start	Initial address of environment variables
unsigned long	env_end	Final address of environment variables
unsigned long	rss	Number of page frames allocated to the process
unsigned long	anon_rss	Number of page frames assigned to anonymous memory mappings
unsigned long	total_vm	Size of the process address space (number of pages)
unsigned long	locked_vm	Number of "locked" pages that cannot be swapped out (see Chapter 17)
unsigned long	shared_vm	Number of pages in shared file memory mappings
unsigned long	exec_vm	Number of pages in executable memory mappings
unsigned long	stack_vm	Number of pages in the User Mode stack
unsigned long	reserved_vm	Number of pages in reserved or special memory regions
unsigned long	def_flags	Default access flags of the memory regions
unsigned long	nr_ptes	Number of Page Tables of this process
unsigned long []	saved_auxv	Used when starting the execution of an ELF program (see Chapter 20)
unsigned int	dumpable	Flag that specifies whether the process can produce a core dump of the memory
cpumask_t	cpu_vm_mask	Bit mask for lazy TLB switches (see Chapter 2)
mm_context_t	context	Pointer to table for architecture-specific information (e.g., LDT's address in 80 — 86 platforms)
unsigned long	swap_token_time	When this process will become eligible for having the swap token (see the section "The Swap Token" in Chapter 17)
char	recent_pagein	Flag set if a major Page Fault has recently occurred
int	core_waiters	Number of lightweight processes that are dumping the contents of the process address space to a core file (see the section "Deleting a Process Address Space" later in this chapter)
struct completion *	core_startup_done	Pointer to a completion used when creating a core file (see the section "Completions" in Chapter 5)
struct completion	core_done	Completion used when creating a core file
rwlock_t	ioctx_list_lock	Lock used to protect the list of asynchronous I/O contexts (see Chapter 16)
struct kioctx *	ioctx_list	List of asynchronous I/O contexts (see Chapter 16)
struct kioctx	default_kioctx	Default asynchronous I/O context (see Chapter 16)
unsigned long	hiwater_rss	Maximum number of page frames ever owned by the process
unsigned long	hiwater_vm	Maximum number of pages ever included in the memory regions of the process