This section is about the process address space and how it’s implemented in Linux.

## Introduction

The process address space is the virtual memory addressable by a process [1, P. 305].

Each process is given a flat 32 or 64-bit address space. Normally the address space is unique to each process, although it can be shared between processes (e.g. with threads).

A process doesn’t have permission to access all memory. The area of legal addresses are called memory areas. A process can dynamically add and remove memory areas via the kernel [1, P. 306].

Memory areas have permissions associated with them, such as readable, writeable, and executable. If a process accesses memory that it doesn’t have permission to access, the kernel kills the process with a segmentation fault [1, P. 306].

Memory areas can contain:

• A memory map of the executable file’s code (the text section).
• A memory map of the executable file’s initial global variables (the data section).
• A memory map of the zero page, containing uninitialized variables (the bss section).
• A memory map of the zero page used for the process’s user space stack.
• A text, data, and bss section for each shared library.
• Any memory mapped files.
• Any shared memory segments.
• Any anonymous memory mappings, like this associated with malloc().

[1, Pp. 306-7]

Memory areas don’t overlap.

## The memory descriptor

A process’s address space is represented by a memory descriptor [1, P. 307].

The memory descriptor is represented with the mm_struct struct:

struct mm_struct {
struct vm_area_struct *mmap; /* list of memory areas */
struct rb_root mm_rb; /* red-black tree of VMAs */
struct vm_area_struct *mmap_cache; /* last used memory area */
unsigned long free_area_cache; /* 1st address space hole */
pgd_t *pgd; /* page global directory */
atomic_t mm_users; /* address space users */
atomic_t mm_count; /* primary usage counter */
int map_count; /* number of memory areas */
struct rw_semaphore mmap_sem; /* memory area semaphore */
spinlock_t page_table_lock; /* page table lock */
struct list_head mmlist; /* list of all mm_structs */
unsigned long start_code; /* start address of code */
unsigned long end_code; /* final address of code */
unsigned long start_data; /* start address of data */
unsigned long end_data; /* final address of data */
unsigned long start_brk; /* start address of heap */
unsigned long brk; /* final address of heap */
unsigned long start_stack; /* start address of stack */
unsigned long arg_start; /* start of arguments */
unsigned long arg_end; /* end of arguments */
unsigned long env_start; /* start of environment */
unsigned long env_end; /* end of environment */
unsigned long rss; /* pages allocated */
unsigned long total_vm; /* total number of pages */
unsigned long locked_vm; /* number of locked pages */
unsigned long saved_auxv[AT_VECTOR_SIZE]; /* saved auxv */
mm_context_t context; /* arch-specific data */
unsigned long flags; /* status flags */
int core_waiters; /* thread core dump waiters */
struct core_state *core_state; /* core dump support */
spinlock_t ioctx_lock; /* AIO I/O list lock */
struct hlist_head ioctx_list; /* AIO I/O list */
};


mm_users is the number of users sharing this address space. For example, if two threads share the address space then mm_users is 2 [1, P. 308].

mm_count represents whether the address space is used or not. It’s kept at 1 until all threads using an address space exit, when it is decremented to 0, and the object is freed [1, P. 308].

The mmap and mm_rb fields both contain all memory areas in the address space. mmap stores the address spaces in a linked list, whereas mm_rb stores them in a red black tree [1, P. 308].

The linked list makes it easy to traverse the entire address space, whereas the red black tree is useful for searching for a given element [1, P. 308].

All mm_struct objects are linked together in a linked list. The initial list element is the init_mm memory descriptor, which describes the init process’s address space.

## Allocating a Memory Descriptor

A task’s memory descriptor is stored in the task_struct mm field. The mm_struct is allocated from the mm_cachep slab cache via the allocate_mm() macro. “Normally, each process receives a unique mm_struct and thus a unique process address space” [1, P. 308].

Processes can share their address spaces with their children by passing the CLONE_VM flag to clone. If CLONE_VM is set, then allocate_mm() isn’t called, and the process’s mm field points to its parents memory descriptor. You can see this in copy_mm():

if (clone_flags & CLONE_VM) {
/*
* current is the parent process and
* tsk is the child process during a fork()
*/
atomic_inc(&current->mm->mm_users);
tsk->mm = current->mm;


When a process associated with an address space exits, exit_mm() is called. exit_mm() calls mmput(), which decrements the mm_struct mm_users count. When mm_users reaches 0, mmdrop() is called to decrement the mm_count counter. When mm_count is decremented to 0, and the free_mm() macro is invoked, to return the mm_struct to the mm_cachep [1, P. 309].

Kernel threads don’t have a process address space, so their mm value is NULL.

Kernel threads never access user space memory, and so they don’t have a memory descriptor or page tables. However, kernel threads do need access to some data, such as page tables. To provide kernel threads with page tables to access kernel memory, kernel threads use the memory descriptor of whatever task ran previously (active_mm) [1, P. 309].

## Virtual Memory Areas

VMAs (Virtual Memory Areas) are represented with the vm_area_struct struct. The struct describes a single memory area that covers a contiguous interval in a given address space. Each memory area has certain properties, like permissions, and a set of associated operations [1, P. 310].

You can see the struct:

struct vm_area_struct {
struct mm_struct *vm_mm; /* associated mm_struct */
unsigned long vm_start; /* VMA start, inclusive */
unsigned long vm_end; /* VMA end , exclusive */
struct vm_area_struct *vm_next; /* list of VMA’s */
pgprot_t vm_page_prot; /* access permissions */
unsigned long vm_flags; /* flags */
struct rb_node vm_rb; /* VMA’s node in the tree */
struct {
void *parent;
} vm_set;
struct prio_tree_node prio_tree_node;
} shared;
struct list_head anon_vma_node; /* anon_vma entry */
struct anon_vma *anon_vma; /* anonymous VMA object */
struct vm_operations_struct *vm_ops; /* associated ops */
unsigned long vm_pgoff; /* offset within file */
struct file *vm_file; /* mapped file, if any */
void *vm_private_data; /* private data */
};


vm_start is the lowest memory address of the area, vm_end is the first byte after the highest memory address of the area. $vm_end - vm_start$ is the total bytes of the memory area [1, P. 310].

The vm_flags field contains bit flags. Unlike the permissions associated with an physical page which the hardware is responsible for, the VMA flags specify behavior that the kernel is responsible for maintaining. You can see a full list of the flags in the following table:

Flag Effect on the VMA and Its Pages
VM_READ Pages can be read from.
VM_WRITE Pages can be written to.
VM_EXEC Pages can be executed.
VM_SHARED Pages are shared.
VM_MAYREAD The VM_READ flag can be set.
VM_MAYWRITE The VM_WRITE flag can be set.
VM_MAYEXEC The VM_EXEC flag can be set.
VM_MAYSHARE The VM_SHARE flag can be set.
VM_GROWSDOWN The area can grow downward.
VM_GROWSUP The area can grow upward.
VM_SHM The area is used for shared memory.
VM_DENYWRITE The area maps an unwritable file.
VM_EXECUTABLE The area maps an executable file.
VM_LOCKED The pages in this area are locked.
VM_IO The area maps a device’s I/O space.
VM_SEQ_READ The pages seem to be accessed sequentially.
VM_RAND_READ The pages seem to be accessed randomly.
VM_DONTCOPY This area must not be copied on fork().
VM_DONTEXPAND This area cannot grow via mremap().
VM_RESERVED This area must not be swapped out.
VM_ACCOUNT This area is an accounted VM object.
VM_HUGETLB This area uses hugetlb pages.
VM_NONLINEAR This area is a nonlinear mapping.

[1, P. 311]

The vm_ops field points to the object of operations associated with a given memory area. The operations object is different for different types of memory area. [1, P. 312]

The methods object is represented by operations_struct:

struct vm_operations_struct {
void (*open) (struct vm_area_struct *);
void (*close) (struct vm_area_struct *);
int (*fault) (struct vm_area_struct *, struct vm_fault *);
int (*page_mkwrite) (struct vm_area_struct *vma, struct vm_fault *vmf);
int (*access) (struct vm_area_struct *, unsigned long , void *, int, int);
};

• open is invoked when the memory area is added to an address space.
• close is invoked when the memory area is removed from an address space.
• fault is invoked by the page fault handler when a page not present in physical memory is accessed.
• page_mkwrite is “invoked by the page fault handler when a page that was read-only is made writable”.
• access “is invoked by access_process_vm when get_user_pages fails”.

[1, P. 313]

mmap links together all memory area objects is a singly linked list. each vm_area_struct is linked in via its vm_next_field. The areas are sorted by ascending address [1, P. 313].

The find_vma() function finds a VMA in which an address resides:

struct vm_area_struct * find_vma(struct mm_struct *mm, unsigned long addr);


It searches for the first memory area whose vm_end value is greater than the addr. It can return a VMA that starts at an address greater than the addr [1, P. 316].

You can see the implementation of find_vma():

struct vm_area_struct * find_vma(struct mm_struct *mm, unsigned long addr)
{
struct vm_area_struct *vma = NULL;

if (mm) {
vma = mm->mmap_cache;
struct rb_node *rb_node;

rb_node = mm->mm_rb.rb_node;
vma = NULL;
while (rb_node) {
struct vm_area_struct * vma_tmp;

vma_tmp = rb_entry(rb_node, struct vm_area_struct, vm_rb);
vma = vma_tmp;
break;
rb_node = rb_node->rb_left;
} else
rb_node = rb_node->rb_right;
}
if (vma)
mm->mmap_cache = vma;
}
}

return vma;
}


find_vma_prev() works the same as find_vma() but it returns the last vma before addr:

struct vm_area_struct * find_vma_prev(struct mm_struct *mm, unsigned long addr,
struct vm_area_struct **pprev)


The pprev argument stores a pointer to the VMA preceding addr [1, P. 317].

## mmap and do_mmap

do_mmap() creates a new address interval. If the new VMA created is adjacent an existing VMA that has the same privileges as the new address range, the existing VMA is extended. Otherwise a new VMA is created [1, P. 318].

do_mmap() is declared in <linux/mm.h>:

unsigned long do_mmap(struct file *file, unsigned long addr,
unsigned long len, unsigned long prot,
unsigned long flag, unsigned long offset)


do_mmap() “maps the file specified by file at offset offset for length len”. file can be NULL and offset can be 0, which will mean the mapping does not have an associated file [1, P. 318].

“The prot parameter specifies the access permissions for pages in the memory area. The possible permission flags are defined in <asm/mman.h> and are unique to each supported architecture” [1, P. 318]. In practice each architecture defines the flags listed in the following table:

Flag Effect on the Pages in the New Interval
PROT_READ Corresponds to VM_READ.
PROT_WRITE Corresponds to VM_WRITE.
PROT_EXEC Corresponds to VM_EXEC.
PROT_NONE Cannot access page.

flags specifies flags corresponding to VMA flags. These include the following flags:

Flag Effect on the New Interval
MAP_SHARED The mapping can be shared.
MAP_PRIVATE The mapping cannot be shared.
MAP_FIXED The new interval must start at the given address addr.
MAP_ANONYMOUS The mapping is not file-backed, but is anonymous.
MAP_GROWSDOWN Corresponds to VM_GROWSDOWN.
MAP_DENYWRITE Corresponds to VM_DENYWRITE.
MAP_EXECUTABLE Corresponds to VM_EXECUTABLE.
MAP_LOCKED Corresponds to VM_LOCKED.
MAP_NORESERVE No need to reserve space for the mapping.
MAP_POPULATE Populate (prefault) page tables.
MAP_NONBLOCK Do not block on I/O.

do_mmap() returns the initial address of the newly created address interval. If possible, the interval is merged with an adjacent memory area, otherwise “a new vm_area_struct structure is allocated from the vm_area_cachep slab cache, and the new memory area is added to the address space’s linked list and red-black tree of memory areas via the vma_link function”. The, “the total_vm field in the memory descriptor is updated” [1, P. 319].

do_mmap() is made available to user space via the mmap(), and mmap2() system calls. The system call is implemented as:

void * mmap2(void *start,
size_t length,
int prot,
int flags,
int fd,
off_t pgoff)


do_munmap() removes an address interval from a specified process address space. The function is declared in <linux/mm.h>:

int do_munmap(struct mm_struct *mm, unsigned long start, size_t len)


Users can call the munmap() system call to use do_munmap():

int munmap(void *start, size_t length)


## Page Tables

“Although applications operate on virtual memory mapped to physical addresses, processors operate directly on those physical addresses.” When an application accesses a virtual memory address, it must be translated into a physical page address so that the processor can resolve the request. Translating a virtual memory address to a physical memory address is done using page tables [1, P. 320].

“Page tables work by splitting the virtual address into chunks. Each chunk is used as an index into a table. The table points to either another table or the associated physical page” [1, P. 320].

In Linux, page tables consist of three levels. The multiple levels enable a sparsely populated address space. If page tables were implemented as a static array, their size would be enormous.

The top-level page table is PGD (the Page Global Directory), which consists of an array of pgd_t types. pgd_t is normally an unsigned long. The PGD entries point to entries in the second-level, the page middle directory (PMD), which is an array of pmd_t types. These entries point to entries in the page table entries structure [1, P. 321].

“The final level is called simply the page table and consists of page table entries of type pte_t. Page table entries point to physical pages” [1, P. 321].

“In most architectures, page table lookups are handled (at least to some degree) by hardware. In normal operation, hardware can handle much of the responsibility of using the page tables. The kernel must set things up, however, in such a way that the hardware is happy and can do its thing” [1, P. 321].

Each process has its own page table. “Because nearly every access of a page in virtual memory must be resolved to its corresponding address in physical memory, the performance of the page tables is very critical. Unfortunately, looking up all these addresses in memory can be done only so quickly. To facilitate this, most processors implement a translation lookaside buffer, or simply TLB, which acts as a hardware cache of virtual-to-physical mappings. When accessing a virtual address, the processor first checks whether the mapping is cached in the TLB. If there is a hit, the physical address is immediately returned. Otherwise, if there is a miss, the page tables are consulted for the corresponding physical address” [1, Pp. 321-2].

## References

1. [1] L. R., Linux Kernel Development (Developer’s Library), 3rd ed. Addison-Wesley Professional, 2010.