Lab 連結：Lab: mmap

Lab: mmap (hard)

The mmap and munmap system calls allow UNIX programs to exert detailed control over their address spaces. They can be used to share memory among processes, to map files into process address spaces, and as part of user-level page fault schemes such as the garbage-collection algorithms discussed in lecture. In this lab you’ll add mmap and munmap to xv6, focusing on memory-mapped files.

目前的理解會像是把 memory address map 到一個 file 中，可以有多個 process share 同一份資料的好處

You should implement enough mmap and munmap functionality to make the mmaptest test program work. If mmaptest doesn’t use a mmap feature, you don’t need to implement that feature.

這一題要我們實做出 mmap

mmap can be called in many ways, but this lab requires only a subset of its features relevant to memory-mapping a file. You can assume that addr will always be zero, meaning that the kernel should decide the virtual address at which to map the file. mmap returns that address, or 0xffffffffffffffff if it fails. length is the number of bytes to map; it might not be the same as the file’s length. prot indicates whether the memory should be mapped readable, writeable, and/or executable; you can assume that prot is PROT_READ or PROT_WRITE or both. flags will be either MAP_SHARED, meaning that modifications to the mapped memory should be written back to the file, or MAP_PRIVATE, meaning that they should not. You don’t have to implement any other bits in flags. fd is the open file descriptor of the file to map. You can assume offset is zero (it’s the starting point in the file at which to map).

  • Start by adding _mmaptest to UPROGS, and mmap and munmap system calls, in order to get user/mmaptest.c to compile. For now, just return errors from mmap and munmap. We defined PROT_READ etc for you in kernel/fcntl.h. Run mmaptest, which will fail at the first mmap call.
  • Fill in the page table lazily, in response to page faults. That is, mmap should not allocate physical memory or read the file. Instead, do that in page fault handling code in (or called by) usertrap, as in the lazy page allocation lab. The reason to be lazy is to ensure that mmap of a large file is fast, and that mmap of a file larger than physical memory is possible.

這裡會用到 lazy allocation 的技巧

  • Keep track of what mmap has mapped for each process. Define a structure corresponding to the VMA (virtual memory area) described in Lecture 15, recording the address, length, permissions, file, etc. for a virtual memory range created by mmap. Since the xv6 kernel doesn’t have a memory allocator in the kernel, it’s OK to declare a fixed-size array of VMAs and allocate from that array as needed. A size of 16 should be sufficient.
  • Implement mmap: find an unused region in the process’s address space in which to map the file, and add a VMA to the process’s table of mapped regions. The VMA should contain a pointer to a struct file for the file being mapped; mmap should increase the file’s reference count so that the structure doesn’t disappear when the file is closed (hint: see filedup). Run mmaptest: the first mmap should succeed, but the first access to the mmap-ed memory will cause a page fault and kill mmaptest.

這裡在說需要在 process 的 address space 中找一塊沒有用的來當作 map to the file 的區塊

  • Add code to cause a page-fault in a mmap-ed region to allocate a page of physical memory, read 4096 bytes of the relevant file into that page, and map it into the user address space. Read the file with readi, which takes an offset argument at which to read in the file (but you will have to lock/unlock the inode passed to readi). Don’t forget to set the permissions correctly on the page. Run mmaptest; it should get to the first munmap.

使用 page-fault 的策略

  • Implement munmap: find the VMA for the address range and unmap the specified pages (hint: use uvmunmap). If munmap removes all pages of a previous mmap, it should decrement the reference count of the corresponding struct file. If an unmapped page has been modified and the file is mapped MAP_SHARED, write the page back to the file. Look at filewrite for inspiration.
  • Ideally your implementation would only write back MAP_SHARED pages that the program actually modified. The dirty bit (D) in the RISC-V PTE indicates whether a page has been written. However, mmaptest does not check that non-dirty pages are not written back; thus you can get away with writing pages back without looking at D bits.
  • Modify exit to unmap the process’s mapped regions as if munmap had been called. Run mmaptest; mmap_test should pass, but probably not fork_test.
  • Modify fork to ensure that the child has the same mapped regions as the parent. Don’t forget to increment the reference count for a VMA’s struct file. In the page fault handler of the child, it is OK to allocate a new physical page instead of sharing a page with the parent. The latter would be cooler, but it would require more implementation work. Run mmaptest; it should pass both mmap_test and fork_test.

需要先了解 filedup()

// Increment ref count for file f.
struct file*
filedup(struct file *f)
{
  acquire(&ftable.lock);
  if(f->ref < 1)
    panic("filedup");
  f->ref++;
  release(&ftable.lock);
  return f;
}

程式實做

  • kernel/fcntl.h
#define O_RDONLY  0x000
#define O_WRONLY  0x001
#define O_RDWR    0x002
#define O_CREATE  0x200
#define O_TRUNC   0x400

#ifdef LAB_MMAP
#define PROT_NONE       0x0
#define PROT_READ       0x1
#define PROT_WRITE      0x2
#define PROT_EXEC       0x4

#define MAP_SHARED      0x01
#define MAP_PRIVATE     0x02
#endif