# Problem set 3: WeensyOS

In this assignment, you implement process memory isolation, virtual memory, and some system calls in a tiny operating system. This will introduce you to virtual memory and operating system design.

• Due Saturday 10/24 11:59:59pm any time zone (extension 1 day later).

You may want to read Chapter 9 of the text. The 64-bit x86 virtual memory architecture is described in Section 9.7. The PTE_P, PTE_W, and PTE_U bits are shown in Figure 9.23 and discussed in Section 9.7.1.

## Get the code

First, ensure that your repository has a handout remote. Type

$git remote show handout If this reports an error, run$ git remote add handout git://github.com/cs61/cs61-f20-psets.git

Then run

\$ git pull; git pull handout main

This will merge our Problem Set 3 code with your previous work. If you have any “conflicts” from Problem Set 1, resolve them before continuing further. Run git push to save your work back to your personal repository.

You may also create a new cs61-psets repository for this assignment.

## Initial state

For this assignment, there is no handy make check functionality. Instead, you should run your instance of WeensyOS and visually compare it to the images you see below in the assignment.

Run make run in your pset3 directory. You should see something like this, which shows four versions of the p-allocator process running in parallel:

This image loops forever; in an actual run, the bars will move to the right and stay there. Don’t worry if your image has different numbers of K’s or otherwise has different details.

If your bars run painfully slowly, edit the p-allocator.cc file and reduce the ALLOC_SLOWDOWN constant. Stop now to read and understand p-allocator.cc.

Here’s what’s going on in the physical memory display.

• WeensyOS displays the current state of physical and virtual memory. Each character represents 4 KiB of memory: a single page. There are 2 MiB of physical memory in total. (How many pages is this?)
• WeensyOS runs four processes, 1 through 4. Each process is compiled from the same source code (p-allocator.cc), but linked to use a different region of memory.
• Each process asks the kernel for more heap space, one page at a time, until it runs out of room. As usual, each process’s heap begins just above its code and global data, and ends just below its stack. The processes allocate space at different rates: compared to Process 1, Process 2 allocates space twice as quickly, Process 3 goes three times faster, and Process 4 goes four times faster. (A random number generator is used, so the exact rates may vary.) The marching rows of numbers show how quickly the heap spaces for processes 1, 2, 3, and 4 are allocated.

Here are two labeled memory diagrams, showing what the characters mean and how memory is arranged.

The virtual memory display is similar.

• The virtual memory display cycles between the four processes' address spaces. However, all the address spaces are the same for now.
• Blank spaces in the virtual memory display correspond to unmapped addresses. If a process (or the kernel) tries to access such an address, the processor will page fault.
• The character shown at address X in the virtual memory display identifies the owner of the corresponding physical page.
• In the virtual memory display, a character is reverse video if an application process is allowed to access the corresponding address. Initially, any process can modify all of physical memory, including the kernel. Memory is not properly isolated.

## Goal

You will implement complete and correct memory isolation for WeensyOS processes. Then you'll implement full virtual memory, which will improve utilization. You'll implement fork—creating new processes at runtime—and exit—destroying processes at runtime.

We need to provide a lot of support code for this assignment, but the code you write will all go in kernel.cc.

## Notes

### Running WeensyOS

If QEMU’s default display causes accessibility problems, you will want to run make run-console.

There are several ways to debug WeensyOS. We recommend:

• Add log_printf statements to your code (the output of log_printf is written to the file log.txt).

• Use assertions to catch problems early (for instance, call check_page_table to test a page table for obvious issues, or add your own).

• Sometimes a mistake will cause the OS to crash hard and reboot. Use make D=1 run 2>debuglog.txt to get additional, painfully-verbose debugging output. Search through debuglog.txt for check_exception lines to see where the exceptions occur.

A powerful, yet simple, technique for debugging extreme crashes is to narrow down where they occur using infinite loops. Add an infinite loop (while (true) {}) to your kernel. If the resulting kernel crashes, then the infinite loop is after the crash point; if it infinite loops, then the infinite loop is before the crash point.

• Printouts such as assertions and fault reports include the virtual address of the faulting instruction, but they do not always include symbol information. Use files obj/kernel.asm (for the kernel) and obj/p-PROCESSNAME.asm (for processes) to map instruction addresses to instructions.

### Memory system layout

WeensyOS memory system layout is described by several constants.

KERNEL_STACK_TOP Top of kernel stack. The kernel stack is one page long.
PROC_START_ADDR Start of application code. Applications should not be able to access memory below PROC_START_ADDR, except for the single page at console.
MEMSIZE_PHYSICAL Size of physical memory in bytes. WeensyOS does not support physical addresses ≥ MEMSIZE_PHYSICAL. Equals 0x200000 (2MB).
MEMSIZE_VIRTUAL Size of virtual memory. WeensyOS does not support virtual addresses ≥ MEMSIZE_VIRTUAL. Equals 0x300000 (3MB).
PAGESIZE Size of a memory page. Equals 4096 (or, equivalently, 1 << 12).
PAGEOFFMASK Mask for the offset portion of an address. Equals 4095 (PAGESIZE - 1). If (a & PAGEOFFMASK) == 0, then a is page-aligned.

### Kernel and process address spaces

WeensyOS begins with the kernel and all processes sharing a single address space. This is defined by the kernel_pagetable page table. kernel_pagetable is initialized to the identity mapping: virtual address X maps to physical address X.

As you work through the pset, you will shift processes to use independent address spaces, where each process can access only a subset of physical memory.

The kernel, though, still needs the ability to access all of physical memory. Therefore, all kernel functions run using the kernel_pagetable page table. Thus, in kernel functions, each virtual address maps to the physical address with the same number. The exception_entry and syscall_entry assembly codes explicitly install kernel_pagetable when they begin, and exception_return and the syscall return path install the process’s page table as they exit.

Each process page table must contain kernel mappings for the kernel stack and for the exception_entry and syscall_entry code paths.

## Step 1: Kernel isolation

WeensyOS processes could stomp all over the kernel’s memory if they wanted. Better stop that. Change kernel_start, the kernel initialization function, so that kernel memory is inaccessible to applications—except for the memory holding the CGA console (the single page at CONSOLE_ADDR == 0xB8000).

When you are done, WeensyOS should look like this. In the virtual map, kernel memory is no longer reverse-video, since the user can’t access it. Note the lonely CGA console memory block.

Use vmiter to create memory mappings. Start from the vmiter loop in the kernel_start function.

In addition, make sure that your sys_page_alloc system call preserves kernel isolation: Applications shouldn’t be able to use sys_page_alloc to screw up the kernel. This requires changes to the SYSCALL_PAGE_ALLOC case in syscall. Read the description of sys_page_alloc in u-lib.hh to get a feeling for the possible errors.

## Step 2: Isolated address spaces

Implement process isolation by giving each process its own independent page table. Your OS should look like this:

Each process only has permission to access its own pages, which you can tell because only its own pages are shown in reverse video.

How to implement per-process page tables in process_setup:

• Allocate a new, initially-empty page table for the process by calling kalloc_pagetable.

• Copy the mappings from kernel_pagetable into this new page table using vmiter::map. This ensures that the required kernel mappings are present in the new page table. You can do this using a loop with two vmiters, or you can set the mappings yourself (they are identity mappings).

Note: vmiter::map will allocate page table pages as needed.

• Then you will need to make sure that any page that belongs to the process is mapped as user-accessible. These are the pages the process needs to access, including its code, data, stack, and heap segments. There are several places you’ll need to change.

Note the diagram now has four pages for each process in the kernel area, starting at 0x1000. These are the four-level page tables for each process. (The colored background indicates that these pages contain kernel-private page table data, even though the pages “belong” to the process.) The first page was allocated explicitly in process_setup; the other pages were allocated by vmiter::map as the page table was initialized.

One common solution, shown above, leaves addresses above PROC_START_ADDR totally unmapped by default, but other designs work too. As long as a virtual address mapping has no PTE_U bit, its process isolation properties are unchanged. For instance, this solution, in which all mappings are present but accessible only to the kernel, also implements process isolation correctly:

If you create an incorrect page table, WeensyOS might crazily reboot. Don’t panic; see the debugging hints above.

## Step 3: Virtual page allocation

So far, WeensyOS processes use physical page allocation for process memory: Process code, data, stack, and heap pages with virtual address X always use the physical pages with physical address X. This is inflexible and limits utilization.

Change your operating system to allocate all process data, including its code, globals, stack, and heap, using kalloc instead of direct access to the pages array.

Here’s how our OS looks after this step.

Virtual page allocation will complicate the code that initializes process code in process_setup. You’ll need to figure out why (hint: which page table is in force in process_setup?) and find a way around it (hint: vmiter or set_pagetable).

## Step 4: Overlapping address spaces

Now the processes are isolated, which is awesome, but they’re still not taking full advantage of virtual memory. Isolated address spaces can use the same virtual addresses for different physical memory. There’s no need to keep the four process address spaces disjoint.

In this step, change each process’s stack to grow down starting at address 0x300000 == MEMSIZE_VIRTUAL. Now the processes have enough heap room to use up all of physical memory!

If there’s no physical memory available, sys_page_alloc should return an error code to the calling process, such as -1. Do not kill the calling process! Lack of memory is a potentially recoverable problem.

## Step 5: Fork

The fork system call starts a new process as a copy of an existing process. The process that calls fork is called the parent process, and the newly created copy is called the child process. The system call appears to return twice: it returns 0 to the child, and returns the child’s process ID to the parent.

Run WeensyOS with make run or make run-console. At any time, press the ‘f’ key. This will soft-reboot WeensyOS and ask it to run a single p-fork process, rather than the gang of allocators. You should see something like this:

This is because you haven’t implemented fork yet.

Implement fork.

• When a process calls fork, look for a free process slot in the ptable[] array for the child. Don’t use slot 0. If no slot exists, return -1 to the caller.

• If a free slot is found, copy the parent’s page table for the child. For addresses below PROC_START_ADDR (0x100000), the parent and child page tables will have identical mappings (same physical addresses, same permissions). But for addresses at or above PROC_START_ADDR, the child’s page table must map different pages that contain copies of any user-accessible, writable memory. This ensures process isolation: two processes should not share any writable memory except the console.

So fork must examine every virtual address in the parent page table. Whenever the parent process has an application-writable, non-console page at virtual address V, then fork must allocate a new physical page P; copy the data from the parent’s page into P, using memcpy; and finally map page P at address V in the child page table, using the permissions from the parent page table.

• The child process’s registers are initialized as a copy of the parent process’s registers, except for reg_rax.

When you’re done, you should see something like this after pressing ‘f’.

An image like this means you forgot to copy the data for some pages, so the processes are actually sharing stack and/or data pages:

## Step 6: Freeing memory

So far none of your test programs have ever freed memory or exited. Memory allocation’s pretty easy until you add free! So let’s add free by allowing applications to exit.

In this exercise you’ll implement the sys_exit system call, which exits the current process. This exercise is a capstone since freeing memory will tend to expose weaknesses and problems in your other code.

To test your work, use make run and then type ‘e’. This reboots WeensyOS to run the p-forkexit program. (Initially it’ll crash because sys_exit() isn’t implemented yet.) p-forkexit processes all alternate among forking new children, allocating memory, and exiting. The result is that once your code is correct, p-forkexit makes crazy patterns forever, like this:

2020 note. Your picture will not have S characters.

A fully correct OS can run p-forkexit indefinitely. An OS with a memory leak will display a persistent blank spot in the physical memory map—the leaked page—and if run long enough, blank spots will take over the screen. This OS has a pretty bad leak; within 10 seconds it has run out of memory:

This OS’s leak is slower, but if you look at the bottom row of the physical memory map, you should see a persistently unused pages just above and to the left of the “V” in “VIRTUAL”. Persistently unused pages are a hallmark of leaks.

Reducing ALLOC_SLOWDOWN in p-forkexit may encourage errors to manifest, but you may need to be patient.

1. Complete kfree so that kfree frees memory. Make sure that kalloc can re-use freed memory.

2. sys_exit should mark a process as free and free all of its memory. This includes the process’s code, data, heap, and stack pages, as well as the pages used for its page directory and page table pages. The memory should become available again for future allocations.

Use vmiter and ptiter (see below) to enumerate the relevant pages. But be careful not to free the console.

3. You will also need to change fork and perhaps other places. Think about all the places that allocate or share memory: what information must they maintain? In p-forkexit, unlike in previous parts of the pset, sys_fork and sys_page_alloc can run when there isn’t quite enough memory to create a new process or allocate or map a page. Your code should handle this cleanly, without leaking memory in any situation.

• If there isn’t enough free memory to successfully complete sys_page_alloc, the system call should return -1 to the caller.

• If there isn’t enough free memory to successfully complete sys_fork, the system call should clean up (i.e., free any memory that was allocated for the new process before memory ran out), mark the possibly-partially-initialized child process as free, and then return -1 to the parent.

• Both sys_page_alloc and sys_fork may need to allocate multiple pages of memory. If any of these allocations fails, then the whole function must fail, leaving the state of allocated memory unchanged. When this happens, your code must clean up any previous “successful” allocations before returning or there will be a memory leak. Be careful.

• Note that allocation failures should not exit the calling process!

There should be no memory leaks!

## Step 7: Shared read-only memory

It’s wasteful for fork() to copy all of a process’s memory. For example, most processes, including p-fork, never change their code. So what if we shared the memory containing the code? That’d be fine for process isolation, as long as neither process could write the code.

Change process_setup to map read-only program segments using read-only memory. Use the seg.writable() function to detect whether a program segment can use read-only memory. Then, in fork, share read-only pages between processes rather than copying them; and, in exit, ensure that shared pages are correctly accounted for when a process exits.

When you’re done, running p-fork should look like this:

Each process’s virtual address space begins with an “S”, indicating that the corresponding physical page is Shared by multiple processes.

## Extra credit

If you are finished and can't wait to do more of this type of work, try:

• Copy-on-write page allocation!

• Write more system calls and test programs! Some ideas:

• sys_page_alloc is like a restricted mmap system call: the user always defines where the page lives (so addr == nullptr is not supported), the system call allocates exactly one page (so sz == PAGESIZE always), and the map is copied when a process forks (so the flags are like MAP_ANON | MAP_PRIVATE and the protection is PROT_READ | PROT_WRITE | PROT_EXEC). Eliminate some of these restrictions by passing more arguments to the kernel and extending the SYSCALL_PAGE_ALLOC implementation.

• Implement sys_page_free/munmap.

• Implement a system call that puts the calling process to sleep until a given amount of time has elapsed, as measured by ticks (which counts timer interrupts).

• Implement a kill system call that lets one process kill others!

You will need to write a new test program to test this functionality.

## Turnin

You will turn in your code by pushing your git repository to github.com/cs61/YOUR-PSET_REPO.git and updating the grading server.