“Virtual” Memory How does the OS kernel provide “private” and “shared” memory areas to multiple concurrent processes?

Hardware/Firmware/Software The PC ‘s memory management system is a cooperative venture involving hardware, software, and firmware The hardware and firmware would be the same no matter which Operating System we choose to run (i.e., Linux or Windows) An operating system utilizes the hardware (in its own unique way) to provide multiple processes with protected memory-spaces

The Linux/x86 scheme Let’s focus on the particular approach that Linux takes in utilizing the Intel x86 CPU’s memory-addressing capabilities There are two conceptual levels (because “processor independence” is a goal Linux strives for): much of the Linux ‘mm’ code runs on almost any CPU, yet there’s some code is (unavoidably) specific to the x86

Source-code organization High-level ‘mm’ code is processor-independent (It’s in the ‘/usr/src/linux/mm’ subdirectory) Low-level ‘mm’ code is processor-specific (It’s in the ‘/usr/src/linux/arch/’ subdirectories) i386/mmppc/mmmips/mmalpha/mm...

Our machines have 1GB of RAM Main Memory (1 GB) Intel Pentium CPU 0x x3FFFFFFF system bus NICVGA FIFO (4 KB) VRAM (32 MB) some devices have RAM, too

Booting up At power-on, the CPU sees only 1-MB of the physical memory (including the ROMs) The OS initialization code must “activate” the processor’s “protected-mode” MMU so that all the ‘Extended Memory’ (including device-memory) will become addressable Finally the OS enables “virtual memory” to allow the illusion of a 4-GB address-space

Kernel memory-mapping Virtual Memory 0x xFFFFFFFF 4-GB Physical memory 0x x3FFFFFFF 1-GB 896-MB 0xC display memory device memory mappings

Application memory-mapping kernel space user space text data stack text data stack physical memory virtual memory mappings 3-GB

‘mm_struct’ Each application-process uses a different mapping of physical memory regions into the Pentium’s “virtual” address-space, so one process can’t access memory owned by another process (i.e., private regions) The OS switches these mapping when it suspends one process to resume another The ‘map’ for each task is in ‘task_struct’

How Linux tracks mappings task_struct mm mm_struct pgd start, end, etc vm_area_struct start, end, etc vm_area_struct start, end, etc vm_area_struct start, end, etc vm_area_struct vma mapping-tables

When a program ‘forks()’ We already know how new processes get created in the UNIX/Linux environment (i.e., via the ‘fork()’ function or system-call) A child-process gets constructed using the same code and data as its parent-process Yet each task’s memory-regions are kept ‘private’ (i.e., accessible to it alone) So how exactly is this feat achieved?

user space Similar memory-mappings kernel space user space text data stack parent-process kernel space text data stack child-process text data stack physical memory virtual memory

Some Linux demos We have written a Linux kernel module to display some info in a task’s ‘mm_struct’ (using a pseudo-file named ‘/proc/mm’) We also wrote a program (‘mmfork.cpp’) that forks a child process, so each task can read and display its ‘mm_struct’ info A user can compare the contents of the mm_struct’s info for the parent and child

More demos We wrote a Linux module (named ‘vma.c’) that displays information from a task’s list of ‘vm_area_struct’ data-structures You could write a program, similar to our ‘mmfork.cpp’ demo, that would let a user compare the list of memory-regions which a parent-process owns and the list that its child-process has inherited

In-class exercise Write an application program that allows a user to compare BOTH the ‘mm_struct’ and the ‘vm_area_struct’ information that belong to a parent-process and to its child You can use our ‘mm.c’ and ‘vma.c’ Linux kernel modules, and you can imitate the programming ideas we used in ‘mmfork’ TURN IN printouts of your code and output