CS3500 Lab 3- Memory Management Solution

• The first task is to implement a function that prints the contents of a page table.
• Define the function in kernel/vm.c having following prototype: void vmprint(pagetable_t).
• Insert a call to vmprint in exec.c to print the page table for the first user process; its output should be as below.
• Insert the function prototype in defs.h

• Sample Output
page table 0x0000000087f6e000
..0: pte 0x0000000021fda801 pa 0x0000000087f6a000
.. ..0: pte 0x0000000021fda401 pa 0x0000000087f69000
.. .. ..0: pte 0x0000000021fdac1f pa 0x0000000087f6b000
.. .. ..1: pte 0x0000000021fda00f pa 0x0000000087f68000
.. .. ..2: pte 0x0000000021fd9c1f pa 0x0000000087f67000
..255: pte 0x0000000021fdb401 pa 0x0000000087f6d000
.. ..511: pte 0x0000000021fdb001 pa 0x0000000087f6c000
.. .. ..510: pte 0x0000000021fdd807 pa 0x0000000087f76000
.. .. ..511: pte 0x000000002000200b pa 0x0000000080008000

Note:
• The first line prints the address of the argument of vmprint.
• Each PTE line shows the PTE index in its page directory, the pte, the physical address for the PTE.
• The output should also indicate the level of the page directory:
the top-level entries are preceeded by ”..”, the next level down with another ”..”, and so on. You should not print entries that are not mapped.
• You can use the idea from freewalk function in vm.c



• Your next task is to delete page allocation from the sbrk(n) system call implementation, which is the function sys_sbrk() in sysproc.c.
• The sbrk(n) system call grows the process’s memory size by n bytes, and then returns the start of the newly allocated region (i.e., the old size).
• Your new sbrk(n) should just increment the process’s size (myproc()−>sz) by n and return the old size (note. No memory is allocated)

• Remember to delete the memory allocation but you still need to increase the process size
• Sample Output
Make this modification, boot xv6, and type echohi to the shell. You should see something like this:
init: starting sh $ echo hi
usertrap(): unexpected scause 0x000000000000000f pid=3 sepc=0x0000000000001258 stval=0x0000000000004008 va=0x0000000000004000 pte=0x0000000000000000 panic: uvmunmap: not mapped

Submission
The assignment should be done individually.
The following artifacts need to submitted:
(a) Video: demonstrating that it works on your system. At the start of the video display the ifconfig of your system, by running the command $ifconfig and then, demonstrate the program functionality.
(b) Code files: Run make clean and zip the entire xv6-riscv repo, containing your solution.
(c) Report: Explain the steps you followed to solve each problem.

References
RISCV Privileged Instruction Set Manual














• Modify the code in trap.c to respond to a page fault from user space by mapping a newly-allocated page of physical memory at the faulting address, and then returning back to user space to let the process continue executing.

• You should add your code just before the printf call that produced the ”usertrap() : ...” message so that when you call echo hi it should be able to allocate new page.

• Sample Output $ echo hi
statement should work correctly.

Note: you will find some additional problems that have to be solved to make it work correctly
• You can check whether a fault is a page fault by seeing if r_scause() is 13 or 15 in usertrap().
• Look at the arguments to the printf() in usertrap() that reports the page fault, in order to see how to find the virtual address that caused the page fault.
• Steal code from uvmalloc() in vm.c, which is what sbrk()calls(via growproc()). You’ll need to call kalloc() and mappages().
• Use PGROUNDDOWN(va) to round the faulting virtual address down to a page boundary.
• uvmunmap() will panic; modify it to not panic if some pages aren’t mapped.