Project 3: Demand Paging

Spring 2023

Due: Sat, June 10th at 11:59pm

In project 2, each process had a page table that was initialized with physical pages and their contents when the process was created. In project 3, you will be implementing a more sophisticated memory management system where physical pages are allocated on demand and pages that cannot fit in physical memory will be stored on disk.


You will implement and debug virtual memory in two steps. First, you will implement demand paging using page faults to dynamically initialize process virtual pages on demand, rather than initializing page frames for each process in advance at exec time as you did in project 2. Next, you will implement page replacement, enabling your kernel to evict a virtual page from memory to free up a physical page frame to satisfy a page fault. Demand paging and page replacement together allow your kernel to "overbook" memory by executing more processes than would fit in machine memory at any one time, using page faults to multiplex the available physical page frames among the larger number of process virtual pages. When implemented correctly, virtual memory is undetectable to user programs unless they monitor their own performance.

You project will implement the following functionality:

  1. Demand Paging. Pages will be in physical memory only as needed. When no physical pages are free, it is necessary to free pages, possibly evicting pages to swap.
  2. Lazy Loading. To fulfill the spirit of demand paging, processes should load no pages when started, and depend on demand paging to provide even the first instruction they execute. When you are done, loadSections will not allocate even a single page.
  3. Page Pinning. At times it will be necessary to "pin" a page in memory, making it temporarily impossible to evict.

The changes you make to Nachos will be in these two files in the vm directory:

You will notice that these classes inherit from UserKernel and UserProcess. Try to depend on the implementation of your base classes as much as possible. Note that, with readVirtualMemory and writeVirtualMemory, very little code needs to know the details of virtual addressing. For example, you should not have to change any of the primary code which serviced the read syscall. That said, you should not change the base classes in any way that makes them dependent on project 3. It should still be possible to run nachos from the proj2 subdirectory to run user-level programs.

You will compile and run the project in the proj3 directory. Unlike the first two projects, you will not need to learn any new Nachos modules and will continue to use functionality that you became familiar with in project 2. Before starting your implementation, also see the Tips section below.

Design Aspects

Central to this project are the following design aspects:

  1. TranslationEntry bits. You will extend your kernel's handling of the page tables to use three special bits in each TranslationEntry (TE):

  2. Swap File. To manage swapped out pages on disk, use the StubFileSystem (via ThreadedKernel.fileSystem) as in project 2. There are many design choices, but we suggest using a single, global swap file across all processes. This file should last the lifetime of your kernel, and be properly deleted upon Nachos termination. Be sure to choose a reasonably unique file name.

    When designing the swap file, keep in mind that the units of swap are pages. Thus you should try to conserve disk space using the same techniques applied in virtual memory: if you end up having gaps in your swap space (which will necessarily occur with a global swap file upon program termination), try to fill them. As with physical memory in project 2, a global free list works well. You can assume that the swap file can grow arbitrarily, and that there should not be any read/write errors. Assert if there are.

  3. Global Memory Accounting. In addition to tracking free pages (which may be managed as in project 2), there are now two additional pieces of memory information of relevance to all processes: which pages are pinned, and which process owns which pages. The former is necessary to prevent the eviction of certain "sensitive" pages. The latter is necessary to managing eviction of pages. There are many approaches to solving this problem, but we suggest using a global inverted page table (see the tips below).

  4. Page Pinning. When your code is using a physical page for system calls (e.g., in readVirtualMemory or writeVirtualMemory) or I/O (e.g., reading from the COFF file or the swap file), you will need to "pin" the physical page while you are using it. Consider the following actions:

    1. Process A is executing the program at user-level and invokes the read system call.
    2. Process A enters the kernel, and is part way through writing to user memory.
    3. A timer interrupt triggers a context switch, entering process B.
    4. Process B immediately generates numerous page faults, which in turn cause pages to be evicted from other processes, including some used by process A.
    5. Eventually, process A is scheduled to run again, and continues handling the read syscall as before.

    In this example, the page to which A is writing should be pinned in memory so that it is not chosen for page eviction. Otherwise, if process B evicted the page, then when process A was rescheduled it would accidentally write over the page B loaded. Just use another data structure to keep track of which pages are pinned.


1. (30%) Implement demand paging. In this first part, you will continue to preallocate a physical page frame for each virtual page of each newly created process at exec time, just as in project 2. And as before, for now continue to return an error from the exec system call if there are not enough free page frames to hold the process' new address space. You will not yet need to implement the swap file, page replacement, page pinning, an inverted page table, etc. Instead, you just need to make the following changes:

  1. In VMProcess.loadSections, initialize all of the TranslationEntries as invalid. This will cause the machine to trigger a page fault exception when the process accesses a page. Also do not initialize the page by, e.g., loading from the COFF file. Instead, you will do this on demand when the process causes a page fault. As a result, loadSections will continue to allocate physical page frames in the page table for each virtual page, but delay loading the frames with content until they are actually referenced by the process.

  2. Handle page fault exceptions in VMProcess.handleException. When the process references an invalid page, the machine will raise a page fault exception (if a page is marked valid, no fault is generated). Modify your exception handler to catch this exception and handle it by preparing the requested page on demand.

  3. Add a method to prepare the requested page on demand. Note that faults on different pages are handled in different ways. A fault on a code page should read the corresponding code page from the COFF file, a fault on a data page should read the corresponding data page from the COFF file, and a fault on a stack page or arguments page should zero-fill the frame.

    For this step, for reference look at the COFF file loading code from UserProcess.loadSections from project 2. If the process faults on page 0, for example, then load the first page of code from the executable file into it. More generally, when you handle a page fault you will use the value of the faulting address to determine how to initialize that page: if the faulting address is in the code segment, then you will be loading a code page; if the address is in the data segment, then load the appropriate data page; if it is any other page, zero-fill it. It is fine to loop through the sections of the COFF file until you find the approprate section and page to use (assuming it is in the COFF file).

    Once you have paged in the faulted page, mark the TranslationEntry as valid. Then let the machine restart execution of the user program at the faulting instruction: return from the exception, but do not increment the PC (as is done when handling a system call) so that the machine will re-execute the faulting instruction. If you set up the page (by initializing it) and page table (by setting the valid bit) correctly, then the instruction will execute correctly and the process will continue on its way, none the wiser.

  4. Update readVirtualMemory and writeVirtualMemory to handle invalid pages and page faults. Both methods directly access physical memory to read/write data between user-level virtual address spaces and the Nachos kernel. These methods will now need to check to see if the virtual page is valid. If it is valid, it can use the physical page as before. If the page is not valid, then it will need to fault the page in as with any other page fault.

Testing: As long as there is enough physical memory to fully load a program, then you should be able to use test programs from project 2 to test this part of project 3. See the tips in the Testing section below for how you can control (increase or decrease) the number of physical pages (e.g., write10 is going to need more than the default of 16 pages). If you give Nachos enough physical pages, you can even run the swap4 and swap5 tests (and these tests do not use any system calls other than exit).

2. (70%) Now implement demand paged virtual memory with page replacement. In this second part, not only do you delay initializing pages, but now you delay the allocation of physical page frames until a process actually references a virtual page that is not already loaded in memory.

  1. In part one for VMProcess.loadSections, you allocated physical pages for each virtual page, but you marked them as invalid so that they would be initialized on a page fault. Now change VMProcess.loadSections so that it does not even allocate a physical page. Instead, merely mark all the TranslationEntries as invalid.
  2. Extend your page fault exception handler to allocate a page frame on-the-fly when a page fault occurs. In part one, you just initialized the contents of the virtual page when a page fault occurred. In this part, now allocate a physical page for the virtual page and use your code from part 1 above to initialize it, mark the TranslationEntry as valid, and return from the exception.

You can get the above two changes working without having page replacement implemented for the case where you run a single program that does not consume all of physical memory. Before moving on, be sure that the two changes above work for a single program that fits into memory.

Now implement page replacement to free up a physical page frame to handle page faults:

  1. Extend your page fault exception handler to evict pages once physical memory becomes full. First, you will need to select a victim page to evict from memory. Your page eviction strategy should be the clock algorithm. Then mark the TranslationEntry for that page as invalid.

  2. Evict the victim page. If the page is clean (i.e., not dirty), then the page can be used immediately; you can always recover the contents of the page from disk. If the page is dirty, though, the kernel must save the page contents in the swap file on disk.

  3. Read in the contents of the faulted page either from the executable file or from swap (see below).

  4. Implement the swap file for storing pages evicted from physical memory. You will want to implement methods to create a swap file, write pages from memory to swap (for page out), read from swap to memory (for page in), etc.

As you implement the above operations, keep the following points in mind:

Finally, you should only do as many page reads and writes as necessary to execute the program, and as dictated by the page replacement algorithm. You will soon discover that the first page fault is different than subsequent ones on a particular page. As described above, on the first fault on a page you need to read from the executable file, and on the second you may need to read from swap. Your implementation needs to be able to handle this situation. In short: