Project 3b: xv6 VM Layout
There are two objectives to this assignment:
Read these updates to keep up with any small fixes in the specification.
In this project, you'll be changing xv6 to support a few features virtually every modern OS does. The first is causing an exception to occur when your program dereferences a null pointer; the second is rearranging the address space so as to place the stack at the high end. Sounds simple? Well, it mostly is. But there are a few details.
Part A: Null-pointer Dereference
In xv6, the VM system uses a simple two-level page table as discussed in class. As it currently is structured, user code is loaded into the very first part of the address space. Thus, if you dereference a null pointer, you will not see an exception (as you might expect); rather, you will see whatever code is the first bit of code in the program that is running. Try it and see!
Thus, the first thing you might do is create a program that dereferences a null pointer. It is simple! See if you can do it. Then run it on Linux as well as xv6, to see the difference.
Your job here will be to figure out how xv6 sets up a page table, and then
change it to leave the first two pages (0x0 - 0x2000) unmapped. The code segment
should be starting at 0x2000. Thus,
once again, this project is mostly about understanding the code, and not
writing very much. Look at how
You should also look at
Remember that the first process is a little special and is constructed by
the kernel directly? Take a look at
The rest of your task will be completed by looking through the code to figure out where there are checks or assumptions made about the address space. Think about what happens when you pass a parameter into the kernel, for example; if passing a pointer, the kernel needs to be very careful with it, to ensure you haven't passed it a bad pointer. How does it do this now? Does this code need to change in order to work in your new version of xv6?
One last hint: you'll have to look at the xv6 makefile as well. In there user programs are compiled so as to set their entry point (where the first instruction is) to 0. If you change xv6 to make the first two pages invalid, clearly the entry point will have to be the new beginning of the code segment. Thus, something in the makefile will need to change to reflect this as well.
You should be able to demonstrate what happens when user code tries to access a null pointer. If you do this part correctly, xv6 should trap and kill the process without too much trouble on your part.
Part B: Stack Rearrangement
The xv6 address space (after modifications from Part A) is set up like this:
USERTOP = 640KB (free) heap (grows towards the high-end of the address space) stack (fixed-sized, one page) code (2 unmapped pages) ADDR = 0x0
In this part of the xv6 project, you'll rearrange the address space to look more like what we've discussed in class:
USERTOP = 640KB stack (at end of address space; grows backwards) ... (gap >= 5 pages) heap (grows towards the high-end of the address space) code (2 unmapped pages) ADDR = 0x0
This will take a little work on your part. First, you'll have to figure out where xv6 allocates and initializes the code, heap, and user stack; then, you'll have to figure out how to change the allocation to be in new places. You'll need to place the stack at the high-end of the xv6 user address space, instead of between the code and heap.
Some tricky parts: one thing you'll have to be very careful with is how xv6
currently tracks the size of a process's address space (currently with the
You should also be wary of growing your heap and overwriting your stack. In this project, you should always leave 5 unallocated (invalid) pages between the stack and heap. These are reserved for supporting memory mapping segment in the future.
The high end of the xv6 user address space is 640KB (see the USERTOP value defined in the xv6 code). Thus your first stack page should live at 636KB-640KB.
One final part of this project, which is challenging: automatically growing the stack backwards when needed. Doing so would require you to see if a fault occurred on the page above the stack and then, instead of killing the offending process, allocating a new page, mapping it into the address space, and continuing to run.
The source code for xv6 can be found in ~cs537-1/ta/xv6/ . Everything you need to build and run and even debug the kernel is in there. Start with a fresh kernel, instead of using your old one with the MLFQ scheduler.
You may also find the following readings about xv6 useful: xv6 book.
Particularly useful for this project: Anything about fork() and exec(), as well as virtual memory (chapter 2).
Test Along The Way
It is a good habit to get basic functionalities working before moving to advanced features. You can run an individual test with the following command:
shell% ~cs537-1/ta/tests/3b/runtests testname