Virtual_Memory_Management.md

Virtual Memory Management

Table of Contents

1. Process Memory Map

Process Memory Map

We are going to analyze the memory map associated with the dyn_ctypes example program used in the Processor Architecture section.

> gdb dyn_ctypes
(gdb) break dyn_incr.c:23
Breakpoint 1 at 0x8f2: file dyn_incr.c, line 23.
(gdb) run
Starting program: /home/andi/cyber/Computing_Systems/dyn_ctypes 
Breakpoint 1, incr (v64=4822678189205112, v32=<optimized out>, v16=52446, v8=<optimized out>, n=<optimized out>) at dyn_incr.c:24
24	}

With the run command the debugger starts the dyn_ctypes process. Its process identifier or PID can be determined with the followng gdb command:

(gdb) info inferior
  Num  Description       Executable        
* 1    process 7028      /home/andi/cyber/Computing_Systems/dyn_ctypes 

Because of the breakpoint the dyn_ctypes process is waiting to continue. Thus we open a second console terminal and have a look at the /proc directory which offers an interface to the kernel. Information on all running processes is available, among them the current memory map of the process:

cat /proc/7028/maps
555555554000-555555555000 r-xp 00000000 103:05 4462815  dyn_ctypes  # text & rodata
555555754000-555555755000 r--p 00000000 103:05 4462815  dyn_ctypes
555555755000-555555756000 rw-p 00001000 103:05 4462815  dyn_ctypes  # data & bss
555555756000-555555777000 rw-p 00000000  00:00       0  [heap]
7ffff79e4000-7ffff7bcb000 r-xp 00000000 103:05  437812  /lib/x86_64-linux-gnu/libc-2.27.so
7ffff7bcb000-7ffff7dcb000 ---p 001e7000 103:05  437812  /lib/x86_64-linux-gnu/libc-2.27.so
7ffff7dcb000-7ffff7dcf000 r--p 001e7000 103:05  437812  /lib/x86_64-linux-gnu/libc-2.27.so
7ffff7dcf000-7ffff7dd1000 rw-p 001eb000 103:05  437812  /lib/x86_64-linux-gnu/libc-2.27.so
7ffff7dd1000-7ffff7dd5000 rw-p 00000000  00:00       0 
7ffff7dd5000-7ffff7dfc000 r-xp 00000000 103:05  393251  /lib/x86_64-linux-gnu/ld-2.27.so
7ffff7fd5000-7ffff7fd7000 rw-p 00000000  00:00       0 
7ffff7ff7000-7ffff7ffa000 r--p 00000000  00:00       0  [vvar]
7ffff7ffa000-7ffff7ffc000 r-xp 00000000  00:00       0  [vdso]
7ffff7ffc000-7ffff7ffd000 r--p 00027000 103:05  393251  /lib/x86_64-linux-gnu/ld-2.27.so
7ffff7ffd000-7ffff7ffe000 rw-p 00028000 103:05  393251  /lib/x86_64-linux-gnu/ld-2.27.so
7ffff7ffe000-7ffff7fff000 rw-p 00000000  00:00       0 
7ffffffde000-7ffffffff000 rw-p 00000000  00:00       0  [stack]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall]

We see that the memory map of process 7028 consists of 17 user space entries and one kernel entry, some of them with a size of 4096 bytes and some of them larger. We can compare this map with the Virtual Memory Map that we constructed earlier for the dyn_ctypes program based on gdb information:

• 0x555555554000 contains the read-only and executable text and rodata segments
• 0x555555755000 contains the writable but not executable data and bss segments
• 0x555555756000 contains the writable but not executable Heap segment
• 0x7ffffffde000 contains the writable but not executable Stack segment

Detailed information on each maps entry can be extracted from smaps:

cat /proc/7028/smaps
...
7ffffffde000-7ffffffff000 rw-p 00000000 00:00 0  [stack]
Size:                132 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Rss:                   8 kB
Pss:                   8 kB
Shared_Clean:          0 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:         8 kB
Referenced:            8 kB
Anonymous:             8 kB
LazyFree:              0 kB
AnonHugePages:         0 kB
ShmemPmdMapped:        0 kB
Shared_Hugetlb:        0 kB
Private_Hugetlb:       0 kB
Swap:                  0 kB
SwapPss:               0 kB
Locked:                0 kB
VmFlags: rd wr mr mw me gd ac
...

We see from the Rss field of the stack entry that only two pages of physical memory (8 kB of RAM) of the 132 kB Stack segment have been allocated.

Author: Andreas Steffen CC BY 4.0