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leyi-123 / xv6-labs-2020

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xv6 is a re-implementation of Dennis Ritchie's and Ken Thompson's Unix Version 6 (v6). xv6 loosely follows the structure and style of v6, but is implemented for a modern RISC-V multiprocessor using ANSI C.

This repo implements an enhanced version of xv6 by following the labs of 6.S081.

Building and Running xv6

You will need a RISC-V "newlib" tool chain from https://github.com/riscv/riscv-gnu-toolchain, and qemu compiled for riscv64-softmmu. Once they are installed, and in your shell search path, you can run "make qemu".

Lab Notes

Unix utilities

Did some warmup exercises for getting used to C and unix syscalls.

Generally easy to complete. But pipes were quite interesting to play with :)

System calls

Implemented two simple syscalls.

Still fairly easy to complete.

Page tables

Implemented a per-process kernel page table which also includes user memory mapping.

This lab was a lot of harder than the previous two. Page table looked easy, yet, it's powerful and not straightforward to use. I was glad I learned a lot more about it after this lab.

Notes:

  • MMU gets the physical addr of the page table and uses it to translate virtual address to physical address.
  • The page table in xv6 is a tree structure with page-sized nodes.
  • Each non-leaf node contains 512 PTEs. Each PTE contains the physical addr of another PTE or leaf node and permission.
  • Each leaf node is a physical memory page.
  • Kernel page table is directly mapped to physical memory, so we won't have this "how to get physical addr of page table before setting up MMU" problem.
  • I did not pass the "execout" test for the kernel page table user mapping :(

Traps

Implemented backtrace by walking up the stack, and periodic callback system calls.

This lab requires a good understanding about what's happending during the trap. I was surprised how easy to implement callback is the os level with a bit of hacking in trap functions.

Note:

  • In xv6, each process has exactly one kernel stack page and user stack page, which store all the function calls states in kernel space and user space respectively.
  • All necessary trap handling data is stored in the trapframe. which includes the registers and a few kernel states.

Lazy

Implemented lazy allocation for sbrk.

No new idea in this lab, but lazy allocation is a good demonstration of how flexible the os can be by combining page table and trap(page fault).

COW

Implemented copy-on-write fork.

This is another great lab (besides of the page table one). Specifically I learned a lot from the instructer's solution which was a good desmonstration on taking baby-steps and verify each step.

My commit history describes my strategy for tackling this lab. Each commit represents a baby-step for implementing the solution.

Wed Dec 23 12:19:56 cow: Modified fork so parent and child share the same physical pages and got store page fault as expected
Wed Dec 23 12:21:35 cow: Modified traps so the store page fault on copy-on-write page get handled, and met the problem that freeproc freed pages what are still used by parent process
Wed Dec 23 12:52:29 cow: Added reference count so we don't delete page that is sill being used, got OOM because COW does not free page
Wed Dec 23 12:53:23 cow: free page in OOM and passed three test, but failed at file test
Wed Dec 23 13:18:53 cow: file test failed because all processes shared the same copy-on-write page; Fixed it by properly handling COW in copyout; Passed cowtest but usertests failed with panic walk.
Wed Dec 23 13:24:46 cow: Fixed panic walk by checking input address in copytout
Wed Dec 23 14:00:53 cow: got a zero pointer bug in memmove, and found and fixed the bug with gdb

Thread

Implemented a simple user-level threading library and some threading programs.

This is a fairly straightforward lab, yet I was still amazed how simple threading can be implemented (by restoring and recovering a few registers).

Lock

Improved parallelism of memory allocator and block cache by reducing lock contention.

This is quite fun lab. The way to reduce lock contention is usually using more fine-grained locks, but that ofen introduces deadlock issues, which is the main challenge for this lab.

Note:

  • In the per-cpu memory allocator implementation, it's possible to at most acquire a freelist lock at one time if every time only one page is stolen by a cpu.
  • In the block cache implementation, it is unavoidable to acquire multiple locks when moving a buffer from one bucket to another bucket, and it has to be in a critical section. I managed to minimize the critical section to only cover the buffer evication part, so there wasn't many lock conflits caused in the tests.

File system

Improved the filesystem to support large files and symbolic links.

This labs is mainly used to enhance your understanding of inode and pathname implementation of xv6 filesystem.

Note:

  • in-memory inodes is managed by a reference count mechanism. For example, open a file will increase the reference count for the underlying inode.

Mmap

Implemented mmap and munmap system calls.

This labs is not difficult to complete as long as you carefully take small steps.

Note:

  • in-memory inodes is managed by a reference count mechanism. For example, open a file will increase the reference count for the underlying inode.

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