Reputation: 41

Where is the stack memory allocated from for a Linux process?

We know that when a process is created,one stack is allocated for this process.The size of the stack is typically 8 Mb in linux.My question is that,from where this stack is allocated??From user space or from system space?

Upvotes: 4

Answers (4)

Ciro Santilli OurBigBook.com

Reputation: 384334

First you must understand what paging and page faults are: How does x86 paging work?

Kernel vs process memory

The Linux Kernel reserves two zones of virtual memory:

one for kernel memory
one for programs

The exact split is configured by CONFIG_VMSPLIT_.... By default:

on 32-bit:

the bottom 3/4 is program space: 00000000 to BFFFFFFF
the top 1/4 is kernel memory: C0000000 to FFFFFFFF

Like this:

------------------ FFFFFFFF
Kernel
------------------ C0000000
------------------ BFFFFFFF


Process


------------------ 00000000

on 64-bit: currently only 48-bits are actually used, split into two equally sized disjoint spaces. The Linux kernel just assigns:
- the bottom part to processes 00000000 00000000 to 008FFFFF FFFFFFFF
- the top part to the kernel: FFFF8000 00000000 to FFFFFFFF FFFFFFFF
Like this:
```
------------------ FFFFFFFF FFFFFFFF
Kernel
------------------ FFFF8000 00000000


(not addressable)


------------------ 008FFFFF FFFFFFFF
Process
------------------ 00000000 00000000
```

Process address space

Simplified program virtual memory of a process:

------------------ <--- Top of the process address space
Stack (grows down)
v v v v v v v v v
------------------

(unmapped)

------------------ <--- Maximum stack size.


(unmapped)


-------------------
mmap
-------------------


(unmapped)


-------------------
^ ^ ^ ^ ^ ^ ^ ^ ^ ^
brk (grows up)
-------------------
BSS
-------------------
Data
-------------------
Text
-------------------

------------------- <--- Bottom or process address space.

Stack allocation

The kernel maintains a list of pages that belong to each process, and synchronizes that with the paging.

If the program accesses memory that does not belong to it, the kernel handles a page-fault, and decides what to do:

if it is above the maximum stack size, allocate those pages to the process
otherwise, send a SIGSEGV to the process, which usually kills it

More info at: https://unix.stackexchange.com/questions/145557/how-does-stack-allocation-work-in-linux/239323#239323

brk and mmap

Those system calls allow processes to explicitly request chunks of memory to the kernel instead of just going down the stack and segfaulting.

Here is a practical example of brk: What does brk( ) system call do?

This answer explains the advantage of using the stack when that is possible: What is the function of the push / pop instructions used on registers in x86 assembly?

Physical memory

There is no clear split between kernel and userspace memory: Is there an explict split between userspace and kernel in physical memory on Linux x86-64?

Upvotes: 5

vinc17

Reputation: 3476

As others said, the stack is allocated in user space. But here are more details about this, in particular about its size and its growth.

8 MB is actually not the stack size, but the maximum stack size. A small part is initially allocated and the kernel grows the stack automatically when needed (after a page fault), keeping it below the stack size limit. If you do a memory access above the limit, you'll get a segmentation fault. But even if you do not reach this limit, this means that you might exhaust the physical memory (RAM + swap) just by filling the stack.

Here's a reference I've given in my answer to How does stack allocation work in Linux?: Mel Gorman's paper Understanding The Linux Virtual Memory Manager. See in particular Section 4.6.1 Handling a Page Fault, with the exception "Region not valid but is beside an expandable region like the stack" and the corresponding action "Expand the region and allocate a page". See also D.5.2 Expanding the Stack.

Upvotes: 3