Specifications

This is the Title of the Book, eMatter Edition
Copyright © 2005 O’Reilly & Associates, Inc. All rights reserved.
Memory Management in Linux
|
419
Virtual Memory Areas
The virtual memory area (VMA) is the kernel data structure used to manage distinct
regions of a process’s address space. A VMA represents a homogeneous region in the
virtual memory of a process: a contiguous range of virtual addresses that have the
same permission flags and are backed up by the same object (a file, say, or swap
space). It corresponds loosely to the concept of a “segment,” although it is better
described as “a memory object with its own properties.” The memory map of a pro-
cess is made up of (at least) the following areas:
• An area for the program’s executable code (often called text)
• Multiple areas for data, including initialized data (that which has an explicitly
assigned value at the beginning of execution), uninitialized data (BSS),
*
and the
program stack
• One area for each active memory mapping
The memory areas of a process can be seen by looking in /proc/<pid/maps> (in which
pid, of course, is replaced by a process ID). /proc/self is a special case of /proc/pid,
because it always refers to the current process. As an example, here are a couple of
memory maps (to which we have added short comments in italics):
# cat /proc/1/maps look at init
08048000-0804e000 r-xp 00000000 03:01 64652 /sbin/init text
0804e000-0804f000 rw-p 00006000 03:01 64652 /sbin/init data
0804f000-08053000 rwxp 00000000 00:00 0 zero-mapped BSS
40000000-40015000 r-xp 00000000 03:01 96278 /lib/ld-2.3.2.so text
40015000-40016000 rw-p 00014000 03:01 96278 /lib/ld-2.3.2.so data
40016000-40017000 rw-p 00000000 00:00 0 BSS for ld.so
42000000-4212e000 r-xp 00000000 03:01 80290 /lib/tls/libc-2.3.2.so text
4212e000-42131000 rw-p 0012e000 03:01 80290 /lib/tls/libc-2.3.2.so data
42131000-42133000 rw-p 00000000 00:00 0 BSS for libc
bffff000-c0000000 rwxp 00000000 00:00 0 Stack segment
ffffe000-fffff000 ---p 00000000 00:00 0 vsyscall page
# rsh wolf cat /proc/self/maps #### x86-64 (trimmed)
00400000-00405000 r-xp 00000000 03:01 1596291 /bin/cat text
00504000-00505000 rw-p 00004000 03:01 1596291 /bin/cat data
00505000-00526000 rwxp 00505000 00:00 0 bss
3252200000-3252214000 r-xp 00000000 03:01 1237890 /lib64/ld-2.3.3.so
3252300000-3252301000 r--p 00100000 03:01 1237890 /lib64/ld-2.3.3.so
3252301000-3252302000 rw-p 00101000 03:01 1237890 /lib64/ld-2.3.3.so
7fbfffe000-7fc0000000 rw-p 7fbfffe000 00:00 0 stack
ffffffffff600000-ffffffffffe00000 ---p 00000000 00:00 0 vsyscall
The fields in each line are:
start-end perm offset major:minor inode image
* The name BSS is a historical relic from an old assembly operator meaning “block started by symbol.” The
BSS segment of executable files isn’t stored on disk, and the kernel maps the zero page to the BSS address
range.
,ch15.13676 Page 419 Friday, January 21, 2005 11:04 AM