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Linux Memory Issues. An introduction to some low-level and some high-level memory management concepts. Some Architecture History. 8080 (late-1970s) 16-bit address (64-KB) 8086 (early-1980s) 20-bit address (1-MB) 80286 (mid-’80s) 24-bit address (16-MB) 80386 (late-’80s) 32-bit address (4-GB)

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Linux memory issues

Linux Memory Issues

An introduction to some low-level and some high-level memory management concepts

Some architecture history
Some Architecture History

  • 8080 (late-1970s) 16-bit address (64-KB)

  • 8086 (early-1980s) 20-bit address (1-MB)

  • 80286 (mid-’80s) 24-bit address (16-MB)

  • 80386 (late-’80s) 32-bit address (4-GB)

  • 80686 (late-’90s) 36-bit address (64-GB)

  • Core2 (mid-2000s) 40-bit address (1-TB)

Backward compatibility
‘Backward Compatibility’

  • Many buyers resist ‘early obsolescence’

  • New processors need to run old programs

  • Early design-decisions leave their legacy

  • 8086 could run recompiled 8080 programs

  • 80x86 can still run most 8086 applications

  • Win95/98 could run most MS-DOS apps

  • But a few areas of incompatibility existed

Linux must accommodate legacy
Linux must accommodate legacy

  • Legacy elements: hardware and firmware

  • CPU: reset-address and interrupt vectors

  • ROM-BIOS: data area and boot location

  • Display Controllers: VRAM & video BIOS

  • Support chipsets: 15MB ‘Memory Window’

  • DMA: 24-bit memory-address bus

  • SMP: combined Local and I/O APICs

Other cpu architectures
Other CPU Architectures

  • Besides IA-32, Linux runs on other CPUs

    (e.g., PowerPC, MC68000, IBM360, Sparc)

  • So must accommodate their differences

    • Memory-Mapped I/O

    • Wider address-buses

    • Non-Uniform Memory Access (NUMA)

Nodes zones and pages
Nodes, Zones, and Pages

  • Nodes: to accommodate NUMA systems

  • However 80x86 doesn’t support NUMA

  • So on 80x86 Linux uses just one ‘node’

  • Zones: to accommodate distinct regions

  • Three ‘zones’ on 80x86:

    • ZONE_DMA (memory below 16-MB)

    • ZONE_NORMAL (from 16-MB to 896-MB)

    • ZONE_HIGHMEM (memory above 896-MB)

Zones divided into pages
Zones divided into Pages

  • 80x86 supports 4-KB page-frames

  • Linux uses an array of ‘page descriptors’

  • Array of page descriptors: ‘mem_map[]’

  • physical memory is ‘mapped’ by CPU

How 80x86 addresses ram
How 80x86 Addresses RAM

  • Two-stages: ‘segmentation’ plus ‘paging’:

    • First: logical address  linear address

    • Then: linear address  physical address

  • CPU employs special caches:

    • Segment-registers contain ‘hidden’ fields

    • Paging uses ‘Translation Lookaside Buffer’

Logical to linear
Logical to Linear

virtual address-space




global descriptor table





and segment-limit


Segment descriptor format
Segment Descriptor Format




[19..16 ]

Base[ 31..24 ]

Base[ 23..16 ]

Base[ 15..0 ]

Limit[ 15..0 ]

Linear to physical
Linear to Physical

linear address

physical address-space






page frame




Cr3 and cr4
CR3 and CR4

  • Register CR3 holds the physical address of the current task’s page-directory table

  • Register CR4 was added in the 80486 so software would have the option of “turning on” certain advanced new CPU features, yet older software still could execute (by just leaving the new features “turned off”)

Example page size extensions
Example: Page-Size Extensions

  • 80586 can map either 4KB or 4MB pages

  • With 4MB pages: middle table is omitted

  • Entire 4GB address-space is subdivided

    into 1024 4MB-pages

    Demo-module: ‘cr3.c’ creates a pseudo-file showing the values in CR3 and in CR4

Linear to physical1
Linear to Physical

linear address

physical address-space





page frame


4-MB page-frames

Pagetable entry format
PageTable Entry Format





Frame Address

Frame attributes

Some Frame Attributes:

P : (present=1, not-present=0)

R/W : (writable=1, readonly=0)

U/S : (user=1, supervisor=0)

D : (dirty=1, clean=0)

A : (accessed=1, not-accessed=0)

S : (size 4MB = 1, size 4KB = 0)

Visualizing memory
Visualizing Memory

  • Our ‘pgdir.c’ module creates a pseudo-file that lets users see a visual depiction of the CPU’s current ‘map’ of ‘virtual memory’

  • Virtual address-space (4-GB)

    • subdivided into 4MB pages (1024 pels)

    • Text characters: 16 rows by 64 columns

Virtual memory visualization
Virtual Memory Visualization

  • Shows which addresses are ‘mapped’

  • Display granularity is 4MB

  • Data is gotten from task’s page-directory

  • Page-Directory location is in register CR3

  • Legend:

    ‘-’ = frame not mapped

    ‘3’ = r/w by supervisor

    ‘7’ = r/w by user

Assigning memory to tasks
Assigning memory to tasks

  • Each Linux process has a ‘process descriptor’

    with a pointer inside it named ‘mm’:

    struct task_struct {

    pid_t pid;

    char comm[16];

    struct mm_struct *mm;

    /* plus many additional fields */ };

Struct mm struct
struct mm_struct

  • It describes the task’s ‘memory map’

  • Where’s the code, the data, the stack?

  • Where are the ‘shared libraries’?

  • What are attributes of each memory area?

  • How many distinct memory areas?

  • Where is the task’s ‘page directory’?

Demo mm c
Demo: ‘mm.c’

  • It creates a pseudo-file: ‘/proc/mm’

  • Allows users to see values stored in some of the ‘mm_struct’ object’s important fields

Virtual memory areas
Virtual Memory Areas

  • Inside ‘mm_struct’ is a pointer to a list

  • Name of this pointer is ‘mmap’

    struct mm_struct {

    struct vm_area_struct *mmap;

    /* plus many other fields */ };

Linked list of vmas
Linked List of VMAs

  • Each ‘vm_area_struct’ points to another

    struct vm_area_struct {

    unsigned long vm_start;

    unsigned long vm_end;

    unsigned long vm_flags;

    struct vm_area_struct *vm_next;

    /* plus some other fields */ };

Structure relationships
Structure relationships

The ‘process descriptor’ for a task


Task’s mm structure









Linked list of ‘vm_area_struct’ structures

Demo vma c module
Demo ‘vma.c’ module

  • It creates a pseudo-file: /proc/vma

  • Lets user see the list of VMAs for a task

  • Also shows the ‘pgd’ field in ‘mm_struct’


  • Compare our demo to ‘/proc/self/maps’

In class exercise 2
In-class exercise #2

  • Try running our ‘domalloc.cpp’ demo

  • It lets you see how a call to the ‘malloc()’ function would affect an application list of ‘vm_area_struct’ objects

  • NOTE: You have to install our ‘vma.ko’ kernel-object before running ‘domalloc’