Memory functionality

memory
Processes
virtual memory
memory mapping
demand paging and swap
logical memory
Page Allocator
pages

The kernel has full access to the system's memory and allows processes to safely access this memory as they require it. Often the first step in doing this is virtual addressing, usually achieved by paging and/or segmentation. Virtual addressing allows the kernel to make a given physical address appear to be another address, the virtual address. Virtual address spaces may be different for different processes; the memory that one process accesses at a particular (virtual) address may be different memory from what another process accesses at the same address. This allows every program to behave as if it is the only one (apart from the kernel) running and thus prevents applications from crashing each other.

On many systems, a program's virtual address may refer to data which is not currently in memory. The layer of indirection provided by virtual addressing allows the operating system to use other data stores, like a hard drive, to store what would otherwise have to remain in main random-access memory (RAM). As a result, operating systems can allow programs to use more memory than the system has physically available. When a program needs data which is not currently in RAM, the MMU signals to the kernel that this has happened, and the kernel responds by writing the contents of an inactive memory block to disk (if necessary) and replacing it with the data requested by the program. The program can then be resumed from the point where it was stopped. This scheme is generally known as demand paging.

Virtual addressing also allows creation of virtual partitions of memory in two disjointed areas, one being reserved for the kernel (kernel space) and the other for the applications (user space). The applications are not permitted by the processor to address kernel memory, thus preventing an application from damaging the running kernel. This fundamental partition of memory space has contributed much to the current designs of actual general-purpose kernels and is almost universal in such systems, Linux being one of them.

⚲ Shell interface

cat /proc/meminfo

man 1 free

man 8 vmstat

Processes

Process is a running user space program. Kernel starts the first process /sbin/init in function run_init_process_idusing kernel_execve_id. Processes occupy system resources, like memory, CPU time. System calls sys_fork_id and sys_execve_id are used to create new processes from user space. The process exit with an sys_exit_id system call.

Linux inherits from Unix its basic process management system calls (⚲ API ↪ ⚙️ implementations):

man 2 fork ↪ kernel_clone_id creates a new process by duplicating the process invoking it.

man 2 _exit ↪ do_exit_id terminates the calling process "immediately". Any open file descriptors belonging to the process are closed.

man 2 wait ↪ kernel_waitid_id suspends the execution of the calling process until one of its children processes terminates.

man 2 execve ↪ do_execve_id runs an executable file in the context of current process, replacing the previous executable. This system call is used by family of functions of libc man 3 exec

Linux enhances the traditional Unix process API with its own system calls man 2 clone. Clone creates a child process that may share parts of its execution context with the parent. It is often used to implement threads (though programmers will typically use a higher-level interface such as man 7 pthreads, implemented on top of clone).

PID - Process identifier defined as pid_t_id is unique sequential number. man 1 ps -A lists current processes. Syscall man 2 getpid ↪ task_tgid_vnr_id return PID of the current process which internally is called TGID - thread group id. A process can contain many threads. man 2 gettid ↪ task_pid_vnr_id returns thread id. Which internal historically is called PID. ⚠️ Warning: confusion. User space PID ≠ kernel space PID. man 1 ps -AF lists current processes and thread as LWP. For a single thread process all these IDs are equal.

⚲ API

⚙️ Internals

syscalls:

man 2 set_tid_address – set pointer to thread ID

man 2 fork – create a child process

man 2 vfork – create a child process and block parent

man 2 clone – create a child process

man 2 unshare – disassociate parts of the process execution context

kernel/sys.c_src

syscalls:

man 2 prctl – operations on a process or thread

kernel/pid.c_src

syscalls:

man 2 pidfd_open – obtain a file descriptor that refers to a process

man 2 pidfd_getfd – obtain a duplicate of another process's file descriptor

syscalls:

man 2 pidfd_open – obtain a file descriptor that refers to a process

man 2 pidfd_getfd – obtain a duplicate of another process's file descriptor

kernel/exit.c_src

syscalls:

man 2 exit – terminate the calling process

man 2 exit_group – exit all threads in a process

man 2 waitid – wait for process to change state

man 2 waitpid – wait for process to change state

📖 References

Inter-process communication

Inter-process communication (IPC) refers specifically to the mechanisms an operating system provides to allow processes it manages to share data. Methods for achieving IPC are divided into categories which vary based on software requirements, such as performance and modularity requirements, and system circumstances. Linux inherited from Unix the following IPC mechanisms:

Signals (⚲ API ↪ ⚙️ implementations):

man 2 kill sends signal to a process

man 2 tgkill ↪ do_tkill_id sends a signal to a thread

man 2 process_vm_readv ↪ process_vm_rw_id - zero-copy data transfer between process address spaces

🔧 TODO: man 2 sigaction man 2 signal man 2 sigaltstack man 2 sigpending man 2 sigprocmask man 2 sigsuspend man 2 sigwaitinfo man 2 sigtimedwait

kernel/signal.c_src

Anonymous pipes and named pipes (FIFOs) man 2 mknod ↪ do_mknodat_id S_IFIFO_id

Express Data Path PF_XDP_id

Unix domain socket PF_UNIX_id

Memory-mapped files man 2 mmap ⤑ ksys_mmap_pgoff_id

Sys V IPC:

Message queues

Semaphores

Shared memory: man 2 shmget, man 2 shmctl, man 2 shmat, man 2 shmdt

📖 References

Inter-process communication

man 7 sysvipc

Memory management Linux API

⚲ man 2 brk ↪ sys_brk_id, do_brk_flags_id dynamically changes data segment size of the calling process.

The change is made by resetting the program break of the process, which determines the maximum space that can be allocated. The program break is the address of the first location beyond the current end of the data region, and determines the maximum space that can be allocated by the process. The amount of available space increases as the break value increases. The added available space is initialized to a value of zero.

⚲ man 2 mmap ↪ ksys_mmap_pgoff_id maps files or devices into memory.

It is a method of memory-mapped file I/O. It naturally implements demand paging, because file contents are not read from disk initially and do not use physical RAM at all. The actual reads from disk are performed in a "lazy" manner, after a specific location is accessed. After the memory is no longer needed it is important to man 2 unmmap the pointers to it. Protection information can be managed using man 2 mprotect ↪ do_mprotect_pkey_id and special treatment can be enforced using man 2 madvise ↪ do_madvise_id. In Linux, man 2 mmap can create several types of mappings, such as anonymous mappings, shared mappings and private mappings. Using the MAP_ANONYMOUS flag mmap() can map a specific area of the process's virtual memory not backed by any file, whose contents are initialized to zero.

These functions are typically called from a higher-level memory management library function such as C standard library man 3 malloc or C++ new operator.

💾 History: Two basic related to memory management system calls brk and mmap Linux inherits from Unix.

BTW: On Linux, man 2 sbrk is not a separate system call, but a C library function that also calls to sys_brk_id and keeps some internal state to return the previous break value.

📚 References

Memory Management APIs_doc

x86_64 Memory Management_doc

sbrk

mmap

Memory management

⚙️ Internals:

sys_brk_id ↯ call hierarchy:

do_brk_flags_id

vm_area_alloc_id

kmem_cache_alloc_id

__kmem_cache_alloc_lru_id

Virtual memory

🔧 TODO:

Virtually contiguous memory on top of physical and swapped memory pages.

🗝️ Acronyms:

VPFN - Virtual Page Frame Number

PFN - Physical Page Frame Number

pgd - Page Directory

pmd - Page Middle Directory

pud - Page Upper Directory

pte - Page table Entry

TLB - Translation Lookaside Buffer

MMU - Memory Management Unit

⚲ API:

linux/vmalloc.h_inc

vmalloc_id vfree_id

linux/cleanup.h_inc – scope-based cleanup helpers

⚙️ Internals:

📚 References

Virtually Contiguous Mappings_doc

Page Tables_doc

Scope-based Cleanup Helpers_doc

Data structures

⚲ API:

linux/types.h_inc

linux/kref.h_inc

list_head_id – common double linked list

linux/list.h_inc - basic list_head_id operations

list_head_id list_add_id ...

linux/klist.h_inc – some klist_node_id->kref_id helpers

klist_add_tail_id ...

linux/kobject.h_inc

linux/circ_buf.h_inc

linux/kfifo.h_inc – generic kernel FIFO

kfifo_in_id ...

linux/rbtree.h_inc – Red-black trees

rb_node_id

linux/scatterlist.h_inc

scatterlist_id

👁 Example: samples/kfifo/dma-example.c_src

linux/idr.h_inc – ID allocation

linux/bitmap.h_inc – bit arrays that consume one or more unsigned longs

linux/sbitmap.h_inc – fast and scalable bitmaps

📚 References:

List Management Functions_doc

FIFO Buffer_doc

Data structures and low-level utilities_doc

Everything you never wanted to know about kobjects, ksets, and ktypes_doc

Adding reference counters (krefs) to kernel objects_doc

Generic Associative Array Implementation_doc

XArray_doc

ID Allocation_doc

Circular Buffers_doc

Red-black Trees (rbtree) in Linux_doc

Generic radix trees/sparse arrays_doc

Generic bitfield packing and unpacking functions_doc

How to access I/O mapped memory from within device drivers_doc

this_cpu operations_doc

The errseq_t datatype_doc

Data Structures in the Linux Kernel

Memory mapping

🔧 TODO

Key items:

man 2 mmap man 2 mprotect man 2 mmap2 man 2 mincore man 2 ksys_mmap_pgoff

do_mmap_id mm_struct_id vm_area_struct_id vm_struct_id remap_pfn_range_id SetPageReserved_id ClearPageReserved_id free_mmap_pages alloc_mmap_pages free_mmap_pages_id

⚲ API:

⚙️ Internals:

📚 References:

Memory mapping, linux-kernel-labs

Swap

🔧 TODO

⚲ API:

cat /proc/sys/vm/swappiness ↪ vm_swappiness_id

linux/swap.h_inc

man 2 swapon ↪ enable_swap_slots_cache_id

man 2 swapoff

man 2 mlock ↪ do_mlock_id

man 2 shmctl ↪ shmctl_do_lock_id

⚙️ Internals:

mm/swapfile.c_src

mm/vmscan.c_src

mm/mlock.c_src

VM_LOCKED_id swap_info_struct_id si_swapinfo_id swap_info_id handle_pte_fault_id do_swap_page_id wakeup_kswapd_id kswapd_id

📚 References:

Memory paging

https://wiki.archlinux.org/title/swap

Logical memory

⚲ kmalloc_id is the normal method of allocating memory in the kernel for objects smaller than the page size. It is defined in linux/slab.h_inc. The first argument size is the size (in bytes) of the block of memory to be allocated. The second argument flags are the allocation flags or GFP flags, a set of macros that the caller provides to control the type of requested memory. The most commonly used values for flags are GFP_KERNEL and GFP_ATOMIC, but there is more to be considered.

Memory-allocation requests in the kernel are always qualified by a set of GFP flags ("GFP" initially came from "get free page") describing what can and cannot be done to satisfy the request. The most commonly used flags are GFP_ATOMIC and GFP_KERNEL, though they are actually built up from lower-level flags. The full set of flags is huge; they can be found in the linux/gfp.h_inc header file.

⚲ API:

↯ RAII allocation functions hierarchy from linux/device.h_inc:

devm_kcalloc_id - zeroed array

devm_kmalloc_array_id

devm_kmalloc_id - common allocation

devm_kzalloc_id - zeroed allocation

devm_kmalloc_id - common allocation

Classic direct API:

linux/slab.h_inc

kmalloc_id, kfree_id

Slab allocation

Slab allocation is a memory management algorithm intended for the efficient memory allocation of kernel objects. It eliminates fragmentation caused by allocations and deallocations. The technique is used to retain allocated memory that contains a data object of a certain type for reuse upon subsequent allocations of objects of the same type.

Basics

This section is about SLAB and SLUB allocator implementations

A slab can be thought of as an array of objects of certain type or with the same size, spanning through one or more contiguous pages of memory; for example, the slab named "task_struct" holds objects of struct task_struct type, used by the scheduling subsystem. Other slabs store objects used by other subsystems, and there is also slabs for dynamic allocations inside the kernel, such as the "kmalloc-64" slab that holds up to 64-byte chunks requested via kmalloc() calls. In a slab, each object can be allocated and freed separately.

The primary motivation for slab allocation is that the initialization and destruction of kernel data objects can actually outweigh the cost of allocating memory for them. As object creation and deletion are widely employed by the kernel, overhead costs of initialization can result in significant performance drops. The notion of object caching was therefore introduced in order to avoid the invocation of functions used to initialize object state.

With slab allocation, memory chunks suitable to fit data objects of certain type or size are preallocated. The slab allocator keeps track of these chunks, known as caches kmalloc_caches_id, so that when a request to allocate memory for a data object of a certain type is received, it can instantly satisfy the request with an already allocated slot slab_alloc_id.

Deallocatoion of the object with kfree_id does not free up the memory, but only opens a slot which is put in the list of free slots kmem_cache_cpu_id by the slab allocator. The next call to allocate memory of the same size will return the now unused memory slot. See slab_alloc_id//___slab_alloc_id/get_freelist_id. This process eliminates the need to search for suitable memory space and greatly alleviates memory fragmentation. In this context, a slab is one or more contiguous pages in the memory containing pre-allocated memory chunks.

Slab allocation provides a kind of front-end to the zoned buddy allocator for those sections of the kernel that require more flexible memory allocation than the standard 4KB page size.

⚲ Interface:

sudo cat /proc/slabinfo

linux/slab.h_inc

kmem_cache_alloc_id, kmem_cache_free_id

man 1 slabtop

⚙️ Internals:

mm/slab_common.c_src

mm_init_id is called from start_kernel_id

kmem_cache_init_id

create_kmalloc_caches_id

____kasan_kmalloc_id

SLUB allocator – default Unqueued allocator

SLUB is the iteration of the original SLAB allocator that replaced it and became the Linux default allocator since 2.6.23.

⚙️ Internals: mm/slub.c_src

📚 References:

CONFIG_SLUB_id

SLOB allocator – Simple List Of Blocks for 🤖 embedded devices

Unfortunately, SLAB and SLUB allocators consume a big amount of memory allocating their slabs, which is a serious drawback in small systems with memory constraints, such as embedded systems. To overcome it, the SLOB (Simple List Of Blocks) allocator was designed in January 2006 by Matt Mackall as a simpler method to allocate kernel objects.

SLOB allocator uses a first-fit algorithm, which chooses the first available space for memory. This algorithm reduces memory consumption, but a major limitation of this method is that it suffers greatly from internal fragmentation.

The SLOB allocator is also used as a fall back by the kernel build system when no slab allocator is defined (when the CONFIG_SLAB_id flag is disabled).

⚙️ Internals: mm/slob.c_src, slob_alloc_id

SLOB

SLAB allocator

💾 History: SLAB allocator is the original implementation Slab allocation. It was the default allocator since kernel 2.2 until 2.6.23, when SLUB allocator became the default, but it is still available as an option.

SLAB is the name given to the first slab allocation implementation in the kernel to distinguish it from later allocators that use the same interface. It's heavily based on Jeff Bonwick's paper "The Slab Allocator: An Object-Caching Kernel Memory Allocator" (1994) describing the first slab allocator implemented in the Solaris 5.4 kernel.

⚙️ Internals: mm/slab.c_src

📚 References for Slab allocation:

KASAN - KernelAddressSANitizer_doc - dynamic memory safety error detector designed to find out-of-bound and use-after-free bugs

Video "SL[AUO]B: Kernel memory allocator design and philosophy" Christopher Lameter (Linux.conf.au 2015 conference) Slides

Page Allocator

The page allocator (or "zoned buddy allocator") is a low-level allocator that deals with physical memory. It delivers physical pages (usually with a size of 4096 bytes) of free memory to high-level memory consumers such as the slab allocators and kmalloc(). As the ultimate source of memory in the system, the page allocator must ensure that memory is always available, since a failure providing memory to a critical kernel subsystem can lead to a general system failure or a kernel panic.

The page allocator divides physical memory into "zones", each of which corresponds to zone_type_id with specific characteristics. ZONE_DMA contains memory at the bottom of the address range for use by severely challenged devices, for example, while ZONE_NORMAL_id may contain most memory on the system. 32-bit systems have a ZONE_HIGHMEM for memory that is not directly mapped into the kernel's address space. Depending on the characteristics of any given allocation request, the page allocator will search the available zones in a specific priority order. For the curious, /proc/zoneinfo gives a lot of information about the zones in use on any given system.

Within a zone, memory is grouped into page blocks, each of which can be marked with a migration type - migratetype_id describing how the block should be allocated.

⚲ API:

cat /proc/buddyinfo

linux/gfp.h_inc

linux/mmzone.h_inc

alloc_page_id

devm_get_free_pages_id - RAII function, ↯ hierarchy of it:

__get_free_pages_id

alloc_pages_id

alloc_pages_node_id

__alloc_pages_id - the 'heart' of the zoned buddy allocator

⚙️ Internals:

build_all_zonelists_id is called from start_kernel_id, ↯ call hierarchy:

build_all_zonelists_init_id

__build_all_zonelists_id

build_zonelists_id

__alloc_pages_id - the 'heart' of the zoned buddy allocator

struct zone_id

free_area_id

mm/mmzone.c_src

mm/page_alloc.c_src

📚 References:

Get Free Page flags_doc

Buddy memory allocation

Page replacement algorithm

📚 References for logical memory:

Memory Allocation Guide_doc

Selecting memory allocator_doc

Physical memory

Memory Layout

A 32-bit processor can address a maximum of 4GB of memory. Linux kernels split the 4GB address space between user processes and the kernel; under the most common configuration, the first 3GB of the 32-bit range are given over to user space, and the kernel gets the final 1GB starting at 0xc0000000. Sharing the address space gives a number of performance benefits; in particular, the hardware's address translation buffer can be shared between the kernel and user space.

In x86-64 - CONFIG_X86_64_id with 4-level page tables (CONFIG_X86_5LEVEL_id=n), only the least significant 48 bits of a virtual memory address would actually be used in address translation (page table lookup). The remainder bits 48 through 63 of any virtual address must be copies of bit 47, or the processor will raise an exception. Addresses complying with this rule are referred to as "canonical form." Canonical form addresses run from 0 through 00007FFF'FFFFFFFF, and from FFFF8000'00000000 through FFFFFFFF'FFFFFFFF, for a total of 256 TB of usable virtual address space. This is still approximately 64,000 times the virtual address space on 32-bit machines.

Linux takes the higher-addressed half of the address space for itself (kernel space) and leaves the lower-addressed half for user space. The "canonical address" design has, in effect, two memory halves: the lower half starts at 00000000'00000000 and "grows upwards" as more virtual address bits become available, while the higher half is "docked" to the top of the address space and grows downwards.


Start addr	Offset	End addr	Size	VM area description
`0000'8000'0000'0000`	+128 TB	`ffff'7fff'ffff'ffff`		... huge, almost 64 bits wide hole of non-canonical virtual memory addresses up to the -128 TB starting offset of kernel mappings.
`0000'0000'0000'0000`	0	`0000'7fff'ffff'ffff`	128 TB=2⁴⁷	user-space virtual memory, different per mm
`ffff'ffff'ffe0'0000`	-2 MB	`ffff'ffff'ffff'ffff`	2 MB=2²¹	... unused hole
`ffff'ffff'ff60'0000`	-10 MB	`ffff'ffff'ff60'0fff`	4 kB=2¹²	VSYSCALL_ADDR_id - legacy vsyscall ABI
`ffff'ffff'8000'0000`	-2 GB	`ffff'ffff'9fff'ffff`	512 MB=2¹⁹	kernel text mapping, mapped to physical address 0

`ffff'8880'0000'0000`	-119.5 TB	`ffff'c87f'ffff'ffff`	64 TB	page_offset_base_id = __PAGE_OFFSET_BASE_L4_id - direct mapping of all physical memory

`ffff'8000'0000'0000`	-128 TB	`ffff'87ff'ffff'ffff`	8 TB	... guard hole, also reserved for hypervisor

⚲ API:

man 8 setarch --addr-no-randomize cat /proc/self/maps

⚙️ Internals:

arch/x86/include/asm/page_64_types.h_src

arch/x86/mm/init_64.c_src

📚 References:

X86_64 memory nap_doc

Address space layout randomization

Pages

In Linux, different architectures have different page sizes. The original —for x86 architecture— and still most commonly used page size is 4096 bytes (4 KB). The page size (in bytes) of the current architecture is defined by the PAGE_SIZE macro included in arch/x86/include/asm/page_types.h_src header file. User space programs can get this value using the man 2 getpagesize library function. Another related macro is PAGE_SHIFT, that contains the number of bits to shift an address to get its page number —12 bits for 4K pages.

One of the most fundamental kernel data structures relating memory-management is struct page. The kernel keeps track of the status of every page of physical memory present in the system using variables of this type. There are millions of pages in a modern system, and therefore there are millions of these structures in memory.

The full definition of struct page can be found in linux/mm_types.h_inc.

Pages