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diff --git a/labs/fs.html b/labs/fs.html index c201d45..34f64e0 100644 --- a/labs/fs.html +++ b/labs/fs.html @@ -1,4 +1,4 @@ -<html> +q<html> <head> <title>Lab: file system</title> <link rel="stylesheet" href="homework.css" type="text/css" /> @@ -140,6 +140,110 @@ blocks only as needed, like the original <tt>bmap()</tt>. <h2>Memory-mapped files</h2> +<p>In this assignment you will implement the core of the systems + calls <tt>mmap</tt> and <tt>munmap</tt>; see the man pages for an + explanation what they do (run <tt>man 2 mmap</tt> in your terminal). + The test program <tt>mmaptest</tt> tells you what should work. + +<p>Here are some hints about how you might go about this assignment: + + <ul> + <li>Start with adding the two systems calls to the kernel, as you + done for other systems calls (e.g., <tt>sigalarm</tt>), but + don't implement them yet; just return an + error. run <tt>mmaptest</tt> to observe the error. + + <li>Keep track for each process what <tt>mmap</tt> has mapped. + You will need to allocate a <tt>struct vma</tt> to record the + address, length, permissions, etc. for each virtual memory area + (VMA) that maps a file. Since the xv6 kernel doesn't have a + memory allocator in the kernel, you can use the same approach has + for <tt>struct file</tt>: have a global array of <tt>struct + vma</tt>s and have for each process a fixed-sized array of VMAs + (like the file descriptor array). + + <li>Implement <tt>mmap</tt>: allocate a VMA, add it to the process's + table of VMAs, fill in the VMA, and find a hole in the process's + address space where you will map the file. You can assume that no + file will be bigger than 1GB. The VMA will contain a pointer to + a <tt>struct file</tt> for the file being mapped; you will need to + increase the file's reference count so that the structure doesn't + disappear when the file is closed (hint: + see <tt>filedup</tt>). You don't have worry about overlapping + VMAs. Run <tt>mmaptest</tt>: the first <tt>mmap</tt> should + succeed, but the first access to the mmaped- memory will fail, + because you haven't updated the page fault handler. + + <li>Modify the page-fault handler from the lazy-allocation and COW + labs to call a VMA function that handles page faults in VMAs. + This function allocates a page, reads a 4KB from the mmap-ed + file into the page, and maps the page into the address space of + the process. To read the page, you can use <tt>readi</tt>, + which allows you to specify an offset from where to read in the + file (but you will have to lock/unlock the inode passed + to <tt>readi</tt>). Don't forget to set the permissions correctly + on the page. Run <tt>mmaptest</tt>; you should get to the + first <tt>munmap</tt>. + + <li>Implement <tt>munmap</tt>: find the <tt>struct vma</tt> for + the address and unmap the specified pages (hint: + use <tt>uvmunmap</tt>). If <tt>munmap</tt> removes all pages + from a VMA, you will have to free the VMA (don't forget to + decrement the reference count of the VMA's <tt>struct + file</tt>); otherwise, you may have to shrink the VMA. You can + assume that <tt>munmap</tt> will not split a VMA into two VMAs; + that is, we don't unmap a few pages in the middle of a VMA. If + an unmapped page has been modified and the file is + mapped <tt>MAP_SHARED</tt>, you will have to write the page back + to the file. RISC-V has a dirty bit (<tt>D</tt>) in a PTE to + record whether a page has ever been written too; add the + declaration to kernel/riscv.h and use it. Modify <tt>exit</tt> + to call <tt>munmap</tt> for the process's open VMAs. + Run <tt>mmaptest</tt>; you should <tt>mmaptest</tt>, but + probably not <tt>forktest</tt>. + + <li>Modify <tt>fork</tt> to copy VMAs from parent to child. Don't + forget to increment reference count for a VMA's <tt>struct + file</tt>. In the page fault handler of the child, it is OK to + allocate a new page instead of sharing the page with the + parent. The latter would be cooler, but it would require more + implementation work. Run <tt>mmaptest</tt>; make sure you pass + both <tt>mmaptest</tt> and <tt>forktest</tt>. + + </ul> + +<p>Run usertests to make sure you didn't break anything. + +<p>Optional challenges: + <ul> + + <li>If two processes have the same file mmap-ed (as + in <tt>forktest</tt>), share their physical pages. You will need + reference counts on physical pages. + + <li>The solution above allocates a new physical page for each page + read from the mmap-ed file, even though the data is also in kernel + memory in the buffer cache. Modify your implementation to mmap + that memory, instead of allocating a new page. This requires that + file blocks be the same size as pages (set <tt>BSIZE</tt> to + 4096). You will need to pin mmap-ed blocks into the buffer cache. + You will need worry about reference counts. + + <li>Remove redundancy between your implementation for lazy + allocation and your implementation of mmapp-ed files. (Hint: + create an VMA for the lazy allocation area.) + + <li>Modify <tt>exec</tt> to use a VMA for different sections of + the binary so that you get on-demand-paged executables. This will + make starting programs faster, because <tt>exec</tt> will not have + to read any data from the file system. + + <li>Implement on-demand paging: don't keep a process in memory, + but let the kernel move some parts of processes to disk when + physical memory is low. Then, page in the paged-out memory when + the process references it. + + </ul> </body> </html> |