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diff --git a/labs/mmap.html b/labs/mmap.html deleted file mode 100644 index 6f779c4..0000000 --- a/labs/mmap.html +++ /dev/null @@ -1,171 +0,0 @@ -<html> -<head> -<title>Lab: mmap</title> -<link rel="stylesheet" href="homework.css" type="text/css" /> -</head> -<body> - -<h1>Lab: mmap</h1> - -<p>In this lab you will use </tt>mmap</tt> on Linux to demand-page a -very large table and add memory-mapped files to xv6. - -<h2>Using mmap on Linux</h2> - -<p>This assignment will make you more familiar with how to manage virtual memory -in user programs using the Unix system call interface. You can do this -assignment on any operating system that supports the Unix API (a Linux Athena -machine, your laptop with Linux or MacOS, etc.). - -<p>Download the <a href="mmap.c">mmap homework assignment</a> and look -it over. The program maintains a very large table of square root -values in virtual memory. However, the table is too large to fit in -physical RAM. Instead, the square root values should be computed on -demand in response to page faults that occur in the table's address -range. Your job is to implement the demand faulting mechanism using a -signal handler and UNIX memory mapping system calls. To stay within -the physical RAM limit, we suggest using the simple strategy of -unmapping the last page whenever a new page is faulted in. - -<p>To compile <tt>mmap.c</tt>, you need a C compiler, such as gcc. On Athena, -you can type: -<pre> -$ add gnu -</pre> -Once you have gcc, you can compile mmap.c as follows: -<pre> -$ gcc mmap.c -lm -o mmap -</pre> -Which produces a <tt>mmap</tt> file, which you can run: -<pre> -$ ./mmap -page_size is 4096 -Validating square root table contents... -oops got SIGSEGV at 0x7f6bf7fd7f18 -</pre> - -<p>When the process accesses the square root table, the mapping does not exist -and the kernel passes control to the signal handler code in -<tt>handle_sigsegv()</tt>. Modify the code in <tt>handle_sigsegv()</tt> to map -in a page at the faulting address, unmap a previous page to stay within the -physical memory limit, and initialize the new page with the correct square root -values. Use the function <tt>calculate_sqrts()</tt> to compute the values. -The program includes test logic that verifies if the contents of the -square root table are correct. When you have completed your task -successfully, the process will print “All tests passed!”. - -<p>You may find that the man pages for mmap() and munmap() are helpful references. -<pre> -$ man mmap -$ man munmap -</pre> - - -<h2>Implement memory-mapped files in xv6</h2> - -<p>In this assignment you will implement memory-mapped files in xv6. - 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. Port your linux program from the first - assignment to xv6 and run it. - - </ul> - -</body> -</html> |