xref: /third_party/ltp/doc/nommu-notes.txt

-------------
--- Intro ---
-------------

Linux running on processors without a memory management unit place certain
restrictions on the userspace programs.  Here we will provide some guidelines
for people who are not familiar with such systems.

If you are not familiar with virtual memory, you might want to review some
background such as:
	http://en.wikipedia.org/wiki/Virtual_Memory
	/usr/src/linux/Documentation/nommu-mmap.txt

----------------------------
--- No memory protection ---
----------------------------

By virtue of every process getting its own virtual memory space, applications
are protected from each other.  So a bad memory access in one will not affect
the memory of another.  When processors forgo virtual memory, they typically
do not add memory protection back in to the hardware.  There are one or two
exceptions to this rule, but for now, we'll assume no one supports it.

In practical terms, this means you cannot dereference bad pointers directly
and expect the kernel to catch and kill your application.  However, you can
expect the kernel to catch some bad pointers when given to system calls.

For example, this will "work" in the sense that no signal will be sent:
	char *foo = NULL;
	foo[0] = 'a';
	foo[1] = 'b';

However, the kernel should return errors when using "standard" bad pointers
with system calls.  Such as:
	char *foo = NULL;
	write(1, foo, 10);
	-> kernel will return EFAULT or similar
The other bad pointer you can rely on in your tests is -1:
	char *foo = (void *)-1;
	read(0, foo, 10);
	-> kernel will return EFAULT or similar

Otherwise, no bad pointer may reliably be tested, either directly or
indirectly via the kernel.  This tends to be a large part of the UCLINUX
ifdef code that shows up in LTP.

----------------
--- No forks ---
----------------

The ubiquitous fork() function relies completely on the Copy On Write (COW)
functionality provided by virtual memory to share pages between processes.
Since this isn't feasible without virtual memory, there is no fork() function.
You will either get a linker error (undefined reference to fork) or you will
get a runtime failure of ENOSYS.

Typically, fork() is used for very few programming paradigms:
	- daemonization
	- run a program
	- parallelism

For the daemonization functionality, simply use the daemon() function.  This
works under both MMU and NOMMU systems.

To run a program, simply use vfork() followed by an exec-style function.
And change the error handler in the child from exit() to _exit().  This too
works under both MMU and NOMMU systems.  But be aware of vfork() semantics --
since the parent and child share the same memory process, the child has to be
careful in what it does.  This is why the recommended construct is simply:
	pid_t child = vfork();
	if (vfork == 0)
		_exit(execl(....));

For parallelism where processes use IPC to work together, you have to options,
neither of which are easy.  You can rewrite to use threads, or you can re-exec
yourself with special flags to pass along updated runtime state.  This is what
the self_exec() helper function in LTP is designed for.

-------------------------
--- No overcommitting ---
-------------------------

Virtual memory allows people to do malloc(128MiB) and get back a buffer that
big.  But that buffer is only of virtual memory, not physical.  On a NOMMU
system, the memory comes immediately from physical memory and takes it away
from anyone else.

Avoid large mallocs.

---------------------
--- Fragmentation ---
---------------------

On a MMU system, when physical memory gets fragmented, things slow down.  But
they keep working.  This is because every new process gets a clean virtual
memory address space.  While processes can fragment their own virtual address
space, this usually takes quite a long time and a lot of effort, so generally
it is not a problem people hit.

On a NOMMU system, when physical memory gets fragmented, access to large
contiguous blocks becomes unavailable which means requests fail.  Even if your
system has 40MiB _total_ free, the largest contiguous block might only be 1MiB
which means that allocations larger than that will always fail.

Break up your large memory allocations when possible.  Generally speaking,
single allocations under 2MiB aren't a problem.

-----------------
--- No paging ---
-----------------

No virtual memory means you can't mmap() a file and only have the pages read in
(paged) on the fly.  So if you use mmap() on a file, the kernel must allocate
memory for it and read in all the contents immediately.

---------------------
--- No swap space ---
---------------------

See the "No paging" section above.  For the same reason, there is no support
for swap partitions.  Plus, nommu typically means embedded which means flash
based storage which means limited storage space and limited number of times
you can write it.

-------------------------
--- No dynamic stacks ---
-------------------------

No virtual memory means that applications can't all have their stacks at the
top of memory and allowed to grown "indefinitely" downwards.  Stack space is
fixed at process creation time (when it is first executed) and cannot grow.
While the fixed size may be increased, it's best to avoid stack pressure in
the first place.

Avoid the alloca() function and use malloc()/free() instead.

Avoid declaring large buffers on the stack.  Some people like to do things
such as:
	char buf[PATH_MAX];
This will most likely smash the stack on nommu systems !  Use global variables
(the bss), or use malloc()/free() type functions.

-------------------------------
--- No dynamic data segment ---
-------------------------------

No virtual memory means that mappings cannot arbitrarily be extended.  Another
process might have its own mapping right after yours!  This is where the brk()
and sbrk() functions come into play.  These are most often used to dynamically
increase the heap space via the C library, but a few people use these manually.

Best if you simply avoid them, and if you're writing tests to exercise these
functions specifically, make them nops/XFAIL for nommu systems.

-------------------------------
--- Limited shared mappings ---
-------------------------------

No virtual memory means files cannot be mmapped in and have writes to it
written back out to disk on the fly.  So you cannot use MAP_SHARED when
mmapping a file.

-------------------------
--- No fixed mappings ---
-------------------------

The MAP_FIXED option to mmap() is not supported.  It doesn't even really work
all that well under MMU systems.

Best if you simply avoid it, and if you're writing tests to exercise this
option specifically, make them nops/XFAIL for nommu systems.