CS395T Fall 2010 Lab 2: Memory Management

In this lab, you will write the memory management code for your operating system. Memory management has two components.

The first component is a physical memory allocator for the kernel, so that the kernel can allocate memory and later free it. Your allocator will operate in units of 4096 bytes, called pages. Your task will be to maintain data structures that record which physical pages are free and which are allocated, and how many processes are sharing each allocated page. You will also write the routines to allocate and free pages of memory.

The second component of memory management is virtual memory, which maps the virtual addresses used by kernel and user software to addresses in physical memory. The x86 hardware's memory management unit (MMU) performs the mapping when instructions use memory, consulting a set of page tables. You will modify JOS to set up the MMU's page tables according to a specification we provide.

Getting started

In this and future labs you will progressively build up your kernel. We will also provide you with some additional source. To fetch that source, use Git to commit your Lab 1 source, fetch the latest version of the course repository, and then create a local branch called lab2 based on our lab2 branch, origin/lab2:

The git checkout -b command shown above actually does two things: it first creates a local branch lab2 that is based on the origin/lab2 branch provided by the course staff, and second, it changes the contents of your lab directory to reflect the files stored on the lab2 branch. Git allows switching between existing branches using git checkout branch-name, though you should commit any outstanding changes on one branch before switching to a different one.

You will now need to merge the changes you made in your lab1 branch into the lab2 branch, as follows:

In some cases, Git may not be able to figure out how to merge your changes with the new lab assignment (e.g. if you modified some of the code that is changed in the second lab assignment). In that case, the git merge command will tell you which files are conflicted, and you should first resolve the conflict (by editing the relevant files) and then commit the resulting files with git commit -a.

memlayout.h describes the layout of the virtual address space that you must implement by modifying pmap.c. memlayout.h and pmap.h define the Page structure that you'll use to keep track of which pages of physical memory are free. kclock.c and kclock.h manipulate the PC's battery-backed clock and CMOS RAM hardware, in which the BIOS records the amount of physical memory the PC contains, among other things. The code in pmap.c needs to read this device hardware in order to figure out how much physical memory there is, but that part of the code is done for you: you do not need to know the details of how the CMOS hardware works.

Pay particular attention to memlayout.h and pmap.h, since this lab requires you to use and understand many of the definitions they contain. You may want to reviewinc/mmu.h, too, as it also contains a number of definitions that will be useful for this lab.

Lab Requirements

In this lab and subsequent labs, do all of the regular exercises described in the lab and at least one challenge problem. (Some challenge problems are more challenging than others, of course!) Additionally, write up brief answers to the questions posed in the lab and a short (e.g., one or two paragraph) description of what you did to solve your chosen challenge problem. If you implement more than one challenge problem, you only need to describe one of them in the write-up, though of course you are welcome to do more. Place the write-up in a file called answers.txt (plain text) or answers.html (HTML format) in the top level of your lab directory before handing in your work.

Hand-In Procedure

When you are ready to hand in your lab code and write-up, create a file called slack.txt noting how many slack hours you have used both for this assignment and in total. (This is to help us agree on the number that you have used.) Then run make turnin in the lab directory. This will first do a make clean to clean out any .o files and executables, and then create a tar file called lab2-handin.tar.gz with the entire contents of your lab directory and submit it via the CS turnin utility. If you submit multiple times, we will take the latest submission and count slack hours accordingly.

As before, we will be grading your solutions with a grading program. You can run make grade in the lab directory to test your kernel with the grading program. You may change any of the kernel source and header files you need to in order to complete the lab, but needless to say you must not change or otherwise subvert the grading code.

Part 1: Physical Page Management

The operating system must keep track of which parts of physical RAM are free and which are currently in use. JOS manages the PC's physical memory with page granularity so that it can use the MMU to map and protect each piece of allocated memory.

You'll now write the physical page allocator. It keeps track of which pages are free with a linked list of struct Page objects, each corresponding to a physical page. You need to write the physical page allocator before you can write the rest of the virtual memory implementation, because your page table management code will need to allocate physical memory in which to store page tables.

Exercise 1. In the file kern/pmap.c, you must implement code for the following functions.

        boot_alloc()
	page_init()
	page_alloc()
	page_free()

You also need to add some code to i386_vm_init() in pmap.c, as indicated by comments there. For now, just add the code needed before the call to check_page_alloc().

You probably want to work on boot_alloc(), then i386_vm_init(), then page_init(), page_alloc(), and page_free().

check_page_alloc() tests your physical page allocator. You should boot JOS and see whether check_page_alloc() reports success. Fix your code so that it passes. You may find it helpful to add your own assert()s to verify that your assumptions are correct.

This lab, and all the CS395T labs, will require you to do a bit of detective work to figure out exactly what you need to do. This assignment does not describe all the details of the code you'll have to add to JOS. Look for comments in the parts of the JOS source that you have to modify; those comments often contain specifications and hints. You will also need to look at related parts of JOS, at the Intel manuals, and perhaps at your notes from previous Operating Systems courses.

Part 2: Virtual Memory

Before doing anything else, familiarize yourself with the x86's protected-mode memory management architecture: namely segmentation and page translation.

Exercise 2. Read chapters 5 and 6 of the Intel 80386 Reference Manual, if you haven't done so already. Although JOS relies most heavily on page translation, you will also need a basic understanding of how segmentation works in protected mode to understand what's going on in JOS.

Virtual, Linear, and Physical Addresses

In x86 terminology, a virtual address consists of a segment selector and an offset within the segment. A linear address is what you get after segment translation but before page translation. A physical address is what you finally get after both segment and page translation. Be sure you understand the difference between these three types or "levels" of addresses!

Exercise 3. While GDB can only access QEMU's memory by virtual address, it's often useful to be able to inspect physical memory while setting up virtual memory. Review the QEMU monitor commands, especially the xp command, which lets you inspect physical memory. To access the QEMU monitor, press Ctrl-a c in the terminal (the same binding returns to the serial console), or Ctrl-Alt-2 in the VGA window (Ctrl-Alt-1 returns to the VGA console).

Use the xp command in the QEMU monitor and the x command in GDB to inspect memory at corresponding physical and virtual addresses and make sure you see the same data.

QEMU's info mem command may also prove useful in the lab. We've also added a info pg command to our patched version of QEMU that prints out the current page table.

The JOS kernel often needs to manipulate addresses as opaque values or as integers, without dereferencing them, for example in the physical memory allocator. Sometimes these are virtual addresses, and sometimes they are physical addresses. To help document the code, the JOS source distinguishes the two cases: the type uintptr_t represents virtual addresses, and physaddr_t represents physical addresses. Both these types are really just synonyms for 32-bit integers (uint32_t), so the compiler won't stop you from assigning one type to another! Since they are integer types (not pointers), the compiler will complain if you try to dereference them.

The JOS kernel can dereference a uintptr_t by first casting it to a pointer type. In contrast, the kernel can't sensibly dereference a physical address, since the MMU translates all memory references. If you cast a physaddr_t to a pointer and dereference it, you may be able to load and store to the resulting address (the hardware will interpret it as a virtual address), but you probably won't get the memory location you intended.

C type	Address type
`T*`	Virtual
`uintptr_t`	Virtual
`physaddr_t`	Physical

Question

Assuming that the following JOS kernel code is correct, what type should variable x have, uintptr_t or physaddr_t?
```
	mystery_t x;
	char* value = return_a_pointer();
	*value = 10;
	x = (mystery_t) value;
```

In Part 3 of Lab 1 we noted that the boot loader sets up the x86 segmentation hardware so that the kernel runs at its link address of 0xf0100000, even though it is actually loaded in physical memory just above the ROM BIOS at 0x00100000. In other words, the kernel's virtual starting address at this point is 0xf0100000, but its linear and physical starting addresses are both 0x00100000. The kernel's virtual and linear addresses differ because of the segmentation hardware, while its linear and physical addresses are the same because we have not yet enabled page translation.

In the virtual memory layout you are going to set up for JOS in this lab, we will switch from using the x86 segmentation hardware for virtual memory to using page translation instead. Using page translation, we will accomplish the same virtual memory layout we currently use segmentation for, plus much more. While we can't actually disable the segmentation hardware, we will stop using it for anything interesting, effectively disabling it by giving it segments with zero offsets. After you finish this lab and the JOS kernel successfully enables paging and "disables" segmentation, the kernel's virtual and linear addresses will be the same, while its linear and physical addresses will differ because of page translation.

However, the JOS kernel sometimes needs to read or modify memory for which it only knows the physical address. For example, adding a mapping to a page table may require allocating physical memory to store a page directory and then initializing that memory. However, the kernel, like any other software, cannot bypass virtual memory translation and thus cannot directly load and store to physical addresses. One reason JOS remaps of all of physical memory starting from physical address 0 at virtual address 0xf0000000 is to help the kernel read and write memory for which it knows just the physical address. In order to translate a physical address into a virtual address that the kernel can actually read and write, the kernel must add 0xf0000000 to the physical address to find its corresponding virtual address in the remapped region. You should use KADDR(pa) to do that addition.

The JOS kernel also sometimes needs to be able to find a physical address given the virtual address of the memory in which a kernel data structure is stored. The kernel addresses its global variables and memory that boot_alloc() allocates, with addresses in the region where the kernel was loaded, starting at 0xf0000000, the very region where we mapped all of physical memory. Thus, to turn a virtual address in this region into a physical address, the kernel can simply subtract 0xf0000000. You should use PADDR(va) to do that subtraction.

Page Table Management

Now you'll write a set of routines to manage page tables: to insert and remove linear-to-physical mappings, and to create page table pages when needed.

In future labs you will often map the same physical page at multiple virtual addresses (or in the address spaces of multiple environments), so you will keep a count of the number of times each physical page is mapped in the corresponding Page's pp_ref. The count should be equal to the number of references to the physical page in the page table(s) below UTOP (the mappings above UTOP are mostly just set up at boot time by the kernel and are not tracked by the reference-counting system). When the count goes to zero, the physical page can be freed.

Exercise 4. In the file kern/pmap.c, you must implement code for the following functions.

        pgdir_walk()
        boot_map_segment()
        page_lookup()
        page_remove()
        page_insert()

page_check(), called from i386_vm_init(), tests your page table management routines. You should make sure it reports success before proceeding.

Part 3: Kernel Address Space

JOS divides the processor's 32-bit linear address space into two parts. User environments (processes), which we will begin loading and running in lab 3, will have control over the layout and contents of the lower part, while the kernel always maintains complete control over the upper part. The dividing line is defined somewhat arbitrarily by the symbol ULIM in inc/memlayout.h, reserving approximately 256MB of linear (and therefore virtual) address space for the kernel. This explains why we needed to give the kernel such a high link address in lab 1: otherwise there would not be enough room in the kernel's virtual address space to map in a user environment below it at the same time.

Permissions and Fault Isolation

Since kernel and user memory are both present in each environment's address space, we will have to use permission bits in our x86 page tables to allow user code access only to the user part of the address space. Otherwise bugs in user code might overwrite kernel data, causing a crash or more subtle malfunction; user code might also be able to steal other environments' private data.

The user environment will have no permission to any of the memory above ULIM, while the kernel will be able to read and write this memory. For the address range (UTOP,ULIM], both the kernel and the user environment have the same permission: they can read but not write this address range. This range of address is used to expose certain kernel data structures read-only to the user environment. Lastly, the address space below UTOP is for the user environment to use; the user environment will set permissions for accessing this memory.

Initializing the Kernel Address Space

Now you'll set up the address space above UTOP: the kernel part of the address space. inc/memlayout.h shows the layout you should use. You'll use the functions you just wrote to set up the appropriate linear to physical mappings.

Exercise 5. Fill in the missing code in i386_vm_init() after the call to page_check().

Your code should now pass the check_boot_pgdir() check.

Question

What entries (rows) in the page directory have been filled in at this point? What addresses do they map and where do they point? In other words, fill out this table as much as possible:

Entry	Base Virtual Address	Points to (logically):
1023	?	Page table for top 4MB of phys memory
1022	?	?
.	?	?
.	?	?
.	?	?
2	0x00800000	?
1	0x00400000	?
0	0x00000000	[see next question?]

After check_boot_pgdir(), i386_vm_init() maps the first four MB of virtual address space to the first four MB of physical memory, then deletes this mapping at the end of the function. Why is this mapping necessary? What would happen if it were omitted? Does this actually limit our kernel to be 4MB? What must be true if our kernel were larger than 4MB?
We have placed the kernel and user environment in the same address space. Why will user programs not be able to read or write the kernel's memory? What specific mechanisms protect the kernel memory?
What is the maximum amount of physical memory that this operating system can support? Why?
How much space overhead is there for managing memory, if we actually had the maximum amount of physical memory? How is this overhead broken down?

Challenge! We consumed many physical pages to hold the page tables for the KERNBASE mapping. Do a more space-efficient job using the PTE_PS ("Page Size") bit in the page directory entries. This bit was not supported in the original 80386, but is supported on more recent x86 processors. You will therefore have to refer to Volume 3 of the current Intel manuals. Make sure you design the kernel to use this optimization only on processors that support it!

Challenge! Extend the JOS kernel monitor with commands to:

Display in a useful and easy-to-read format all of the physical page mappings (or lack thereof) that apply to a particular range of virtual/linear addresses in the currently active address space. For example, you might enter 'showmappings 0x3000 0x5000' to display the physical page mappings and corresponding permission bits that apply to the pages at virtual addresses 0x3000, 0x4000, and 0x5000.
Explicitly set, clear, or change the permissions of any mapping in the current address space.
Dump the contents of a range of memory given either a virtual or physical address range. Be sure the dump code behaves correctly when the range extends across page boundaries!
Do anything else that you think might be useful later for debugging the kernel. (There's a good chance it will be!)

Address Space Layout Alternatives

The address space layout we use in JOS is not the only one possible. An operating system might map the kernel at low linear addresses while leaving the upper part of the linear address space for user processes. x86 kernels generally do not take this approach, however, because one of the x86's backward-compatibility modes, known as virtual 8086 mode, is "hard-wired" in the processor to use the bottom part of the linear address space, and thus cannot be used at all if the kernel is mapped there.

It is even possible, though much more difficult, to design the kernel so as not to have to reserve any fixed portion of the processor's linear or virtual address space for itself, but instead effectively to allow allow user-level processes unrestricted use of the entire 4GB of virtual address space - while still fully protecting the kernel from these processes and protecting different processes from each other!

Challenge! Write up an outline of how a kernel could be designed to allow user environments unrestricted use of the full 4GB virtual and linear address space. Hint: the technique is sometimes known as "follow the bouncing kernel." In your design, be sure to address exactly what has to happen when the processor transitions between kernel and user modes, and how the kernel would accomplish such transitions. Also describe how the kernel would access physical memory and I/O devices in this scheme, and how the kernel would access a user environment's virtual address space during system calls and the like. Finally, think about and describe the advantages and disadvantages of such a scheme in terms of flexibility, performance, kernel complexity, and other factors you can think of.

Challenge! Since our JOS kernel's memory management system only allocates and frees memory on page granularity, we do not have anything comparable to a general-purpose malloc/free facility that we can use within the kernel. This could be a problem if we want to support certain types of I/O devices that require physically contiguous buffers larger than 4KB in size, or if we want user-level environments, and not just the kernel, to be able to allocate and map 4MB superpages for maximum processor efficiency. (See the earlier challenge problem about PTE_PS.)

Generalize the kernel's memory allocation system to support pages of a variety of power-of-two allocation unit sizes from 4KB up to some reasonable maximum of your choice. Be sure you have some way to divide larger allocation units into smaller ones on demand, and to coalesce multiple small allocation units back into larger units when possible. Think about the issues that might arise in such a system.

Challenge! Extend the JOS kernel monitor with commands to allocate and free pages explicitly, and display whether or not any given page of physical memory is currently allocated. For example:

	K> alloc_page
		0x13000
	K> page_status 0x13000
		allocated
	K> free_page 0x13000
	K> page_status 0x13000
		free

Think of other commands or extensions to these commands that may be useful for debugging, and add them.