memory mapping

This article is talking about user space Memory mmapping; it’s not limitted to mmap(2) system call.
Understanding memory mapping
TLPI:chapter 49 and LSP: Chapter 8


BSD 4.2
1990 SunOS 4.1
A Must-read: The applications programmer gains access to the facilities of the VM system through several sets of system calls.

Memory mappings

What are memory mappings? - Landley

A memory mapping is a set of page table entries describing the properties
of a consecutive virtual address range. Each memory mapping has a
start address and length, permissions (such as whether the program can
read, write, or execute from that memory), and associated resources (such
as physical pages, swap pages, file contents, and so on).


vma’s unit is PAGE_SIZE;


commit 5846fc6c31162234e88bdfd91548b1cf0d2cebbd
Author: Andrew Morton
Date: Tue Sep 17 06:35:47 2002 -0700
[PATCH] consolidate the VMA splitting code
new_below means the place where the old vma go to! Bad naming!
0 means the old will save the head part. 1 means tail part.

Release memory resources

exit_mm exit_mmap

Shared memory mapping

Chapter 12 Shared Memory Virtual Filesystem:

This is a very clean interface that is conceptually easy to understand but it does not help anonymous pages as there is no file backing. To keep this nice interface, Linux creates an artifical file-backing for anonymous pages using a RAM-based filesystem where each VMA is backed by a “file” in this filesystem. Every inode in the filesystem is placed on a linked list called shmem_inodes so that they may always be easily located. This allows the same file-based interface to be used without treating anonymous pages as a special case.

Firo: every time you create a shared memory via mmap(2), you create a inode with same name dev/zero in the hidden shm_mnt fs;
The name dev/zero is only a name. It has nothing related to /dev/zero in drivers/char/mem.c. And /dev/shm is only a tmpfs; it has nothing related shmemfs, but POSIX’s shm_open uses /dev/shm.

Shared anonymouse mappings

vmscan: limit VM_EXEC protection to file pages
If someone may take advange of reclaimation code by mmap(…, VM_EXEC, SHRED|ANON), OOM may occur since the old code protect it from reclaiming by add it back to the active list. Great patch. However, program running in tmpfs will also penalized.
page_is_file_cache < !PageAnon
* onset - mmap
do_mmap -> mmap_region -> vma_link -> (__shmem_file_setup) && __vma_link_file: into i_mmap interval_tree.
* nuclus - share fault
Read: do_read_fault
Write: do_shared_fault -> shmem_getpage_gfp shmem_add_to_page_cache
WP: do_wp_page -> wp_page_shared or wp_page_reuse
b)IPC using a shared file mapping


late 70s - IPC: see TLPI: Chapter 45 INTRODUCTION TO SYSTEM V IPC
they first appear together in Columbus UNIX, a Bell UNIX for database and efficient transaction processing
1983 - IPC See TLPI or wikipedia shared mmeory.
they land together in System V that made them popular in mainstream UNIX-es, hence the name

1983 - BSD mmap with shared vs private memory mapping
BSD 4.2: The system supports sharing of data between processes by allowing pages to be mapped into memory. These mapped pages may be shared with other processes or private to the process.

1984 Jan - BSD mmap with file memory mapping support by SunOS
The mmap seems firstly implemented by SunOS 1.1
N.B. This call is not completely implemented In 4.2(BSD).
More sunos docs:

SunOS 4 introduced Unix’s mmap, which permitted programs “to map files into memory.”
One paper found in OSTEP: Memory Coherence in Shared Virtual Memory Systems

Shared memory in kernel

Initial version

history: commit 9cb9f18b5d26bf176e13edbc0c248d121217c6b3
Refs: <0.99.10>
Author: Linus Torvalds
AuthorDate: Fri Nov 23 15:09:11 2007 -0500
[PATCH] Linux-0.99.10 (June 7, 1993)
Firo: search ‘shm_swap’

Ramfs based

history: commit 4d372877c63baaaf4c1c3325cae43f6b9782e59e
Refs: <2.4.0-test13pre3>
Author: Linus Torvalds
AuthorDate: Fri Nov 23 15:40:55 2007 -0500
The shmfs cleanup should be unnoticeable except to users who use SAP with
huge shared memory segments, where Christoph Rohlands work not only
makes the code much more readable, it should also make it dependable..
- Christoph Rohland: shmfs for shared memory handling