CS333 – Lectures on Operating Systems (Mythili Vutukuru, IIT Bombay) Solved

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Lab: Memory management xv6
The goal of this lab is to understand memory management in xv6.
Before you begin
• Download, install, and run the original xv6 OS code. You can use your regular desktop/laptop to run xv6; it runs on an x86 emulator called QEMU that emulates x86 hardware on your local machine. In the xv6 folder, run make, followed by make qemu or make-qemu-nox, to boot xv6 and open a shell.
• We have modified some xv6 files for this lab, and these patched files are provided as part of this lab’s code. Before you begin the lab, copy the patched files into the main xv6 code directory.
• For this lab, you will need to understand the following files: syscall.c, syscall.h, sysproc.c, user.h, usys.S, vm.c, proc.c, trap.c, defs.h, mmu.h.
– The files sysproc.c, syscall.c, syscall.h, user.h, usys.S link user system calls to system call implementation code in the kernel.
– mmu.h and defs.h are header files with various useful definitions.
– The file vm.c contains most of the logic for memory management in the xv6 kernel, and proc.c contains process-related system call implementations.
– The file trap.c contains trap handling code for all traps including page faults.
Part A: Displaying memory information
You will first implement the following new system calls in xv6.
• numvp() should return the number of virtual/logical pages in the user part of the address space of the process, up to the program size stored in struct proc. You must count the stack guard page as well in your calculations.
Because xv6 does not use demand paging, you can expect the number of virtual and physical pages to be the same initially. However, the next part of the lab will change this property.
Part B: Memory mapping
Physical memory for a memory-mapped virtual page should be allocated on demand, only when the page is accessed by the user. When the user accesses a memory-mapped page, a page fault will occur, and physical memory should only be allocated as part of the page fault handling. Further, if you memory mapped more than one page, physical memory should only be allocated for those pages that are accessed, and not for all pages in the memory-mapped region. Once again, use the virtual/physical page counts to verify that physical pages are allocated only on demand.
To solve this lab, you must handle the page fault trap in trap.c and add code to allocate memory on demand when a page fault occurs. You can check whether a trap is a page fault by checking if tf->trapno is equal to T PGFLT. Look at the arguments to the cprintf statements in trap.c to figure out how one can find the virtual address that caused the page fault. Use PGROUNDDOWN(va) to round the faulting virtual address down to the start of a page boundary. Once you correctly handle the page fault, do break or return in order to avoid the processing of other traps. Remember, it is important to call switchuvm to update the CR3 register and TLB every time you change the page table of the process. This update to the page table will enable the process to resume execution when you handle the page fault correctly. One final hint: if you think you need to call mappages() from trap.c, you will need to delete the static keyword in the declaration of mappages() in vm.c, and you will need to declare mappages() in trap.c. An easier option would be to write a new function to handle page faults within vm.c itself, and call this new function from the page fault handling code in trap.c.
We have provied a simple test program to test your implementation. This program invokes mmap multiple times, and accesses the memory-mapped pages. It prints out virtual and physical page counts periodically, to let you check whether the page counts are being updated correctly. Please write more such test cases to thoroughly test your implementation.
• Place all the files you modified in a directory, with the directory name being your roll number (say, 12345678).
• Tar and gzip the directory using the command tar -zcvf 12345678.tar.gz 12345678 to produce a single compressed file of your submission directory. Submit this tar gzipped file on Moodle.


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