Lab 2: Memory Management¶
- Handed out: Tuesday, January 15, 2019
- Due: 4:00pm (before class) Thursday, January, 24, 2019
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.
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 changes you’ve made since handing in lab 1 (if any),
fetch the latest version of the course repository, and then create a
local branch called
lab2 based on our lab2 branch,
$ cd ~/jos $ git pull Already up-to-date. $ git checkout -b lab2 origin/lab2 Branch lab2 set up to track remote branch refs/remotes/origin/lab2. Switched to a new branch "lab2" $
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
lab2 branch, as follows:
$ git merge lab1 Merge made by recursive. kern/kdebug.c | 11 +++++++++-- kern/monitor.c | 19 +++++++++++++++++++ lib/printfmt.c | 7 +++---- 3 files changed, 31 insertions(+), 6 deletions(-) $
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.
Lab 2 contains the following new source files, which you should browse through:
memlayout.h describes the layout of the virtual address space that
you must implement by modifying
pmap.h define the
PageInfo structure that you’ll use to keep
track of which pages of physical memory are free.
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
pmap.h, since this
lab requires you to use and understand many of the definitions they
contain. You may want to review
inc/mmu.h, too, as it also contains
a number of definitions that will be useful for this lab.
In this lab and subsequent labs, do all of the regular exercises
described in the lab and provide a writeup that briefly answers
to the questions posed in the lab.
Please place the write-up in a file called
answers-lab2.txt in the top level of your
directory before handing in your work.
When you are ready to hand in your lab code and write-up, add your
answers-lab2.txt to the Git repository, commit your changes, and
then tag your commit with
lab2-final as the final to submit the lab.
$ git add answers-lab2.txt $ git commit -am "my answer to lab2" [lab2 a823de9] my answer to lab2 4 files changed, 87 insertions(+), 10 deletions(-) $ make grade # check your result! $ git tag lab2-final $ git push $ git push origin --tags
As before, we will be grading your solutions with a grading program. You
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 PageInfo objects (which,
unlike xv6, are not embedded in the free pages themselves), 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.
In the file
kern/pmap.c, you must implement code for the
following functions (probably in the order given).
boot_alloc() mem_init() (only up to the call to check_page_free_list(1)) page_init() page_alloc() page_free()
check_page_alloc() test 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() to verify that
your assumptions are correct.
This lab, and all the cs519 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.
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. Look at chapters 5 and 6 of the Intel 80386 Reference Manual, if you haven’t done so already. Read the sections about page translation and page-based protection closely (5.2 and 6.4). We recommend that you also skim the sections about segmentation; while JOS uses paging for virtual memory and protection, segment translation and segment-based protection cannot be disabled on the x86, so you will need a basic understanding of it.
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 and what ultimately goes out on the hardware bus to your RAM.
Selector +--------------+ +-----------+ ---------->| | | | | Segmentation | | Paging | Software | |-------->| |----------> RAM Offset | Mechanism | | Mechanism | ---------->| | | | +--------------+ +-----------+ Virtual Linear Physical
A C pointer is the “offset” component of the virtual address. In
boot/boot.S, we installed a Global Descriptor Table (GDT) that
effectively disabled segment translation by setting all segment base
addresses to 0 and limits to
0xffffffff. Hence the “selector” has no
effect and the linear address always equals the offset of the virtual
address. In lab 3, we’ll have to interact a little more with
segmentation to set up privilege levels, but as for memory translation,
we can ignore segmentation throughout the JOS labs and focus solely on
Recall that in part 3 of lab 1, we installed a simple page table so that the kernel could run at its link address of 0xf0100000, even though it is actually loaded in physical memory just above the ROM BIOS at 0x00100000. This page table mapped only 4MB of memory. In the virtual memory layout you are going to set up for JOS in this lab, we’ll expand this to map the first 256MB of physical memory starting at virtual address 0xf0000000 and to map a number of other regions of virtual memory.
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
from the lab tools guide,
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).
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.
From code executing on the CPU, once we’re in protected mode (which we
entered first thing in
boot/boot.S, there’s no way to directly use
a linear or physical address. All memory references are interpreted as
virtual addresses and translated by the MMU, which means all pointers in
C are virtual addresses.
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 opaque
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
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
C type Address type
Assuming that the following JOS kernel code is correct, what type
mystery_t x; char* value = return_a_pointer(); *value = 10; x = (mystery_t) value;
The JOS kernel sometimes needs to read or modify memory for which it
knows only 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. Kernel global variables and memory allocated by
boot_alloc() are in the region where the kernel was loaded, starting
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
to do that subtraction.
In future labs you will often have the same physical page mapped at
multiple virtual addresses simultaneously (or in the address spaces of
multiple environments). You will keep a count of the number of
references to each physical page in the
pp_ref field of the
struct PageInfo corresponding to the physical page. When this count
goes to zero for a physical page, that page can be freed because it is
no longer used. In general, this count should equal to the number of
times the physical page appears below ``UTOP`` in all page tables (the
UTOP are mostly set up at boot time by the kernel and
should never be freed, so there’s no need to reference count them).
We’ll also use it to keep track of the number of pointers we keep to the
page directory pages and, in turn, of the number of references the page
directories have to page table pages.
Be careful when using
page_alloc. The page it returns will always
have a reference count of 0, so
pp_ref should be incremented as soon
as you’ve done something with the returned page (like inserting it into
a page table). Sometimes this is handled by other functions (for
page_insert) and sometimes the function calling
page_alloc must do it directly.
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 the file
kern/pmap.c, you must implement code for the
pgdir_walk() boot_map_region() page_lookup() page_remove() page_insert()
check_page(), called from
mem_init(), tests your page table
management routines. You should make sure it reports success before
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
inc/memlayout.h, reserving approximately 256MB of
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.
You’ll find it helpful to refer to the JOS memory layout diagram in
inc/memlayout.h both for this part and for later labs.
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
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.
Fill in the missing code in
mem_init() after the call to
Your code should now pass the
Questions 2. 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]
- 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?
- Revisit the page table setup in
kern/entrypgdir.c. Immediately after we turn on paging, EIP is still a low number (a little over 1MB). At what point do we transition to running at an EIP above KERNBASE? What makes it possible for us to continue executing at a low EIP between when we enable paging and when we begin running at an EIP above KERNBASE? Why is this transition necessary?
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
- 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.
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
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
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.
This completes the lab. Make sure you pass all of the make grade
tests and don’t forget to write up your answers to the questions in
Commit your changes (including adding
answers-lab2.txt) and run
git tag lab2-final,
git push, and
git push origin --tags
in the top directory of your lab repository to hand in your lab.