Merge branch 'riscv' of g.csail.mit.edu:xv6-dev into riscv
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Robert Morris
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<html>
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<head>
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<title>Lab: mmap</title>
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<link rel="stylesheet" href="homework.css" type="text/css" />
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</head>
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<body>
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<h1>Lab: mmap</h1>
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<p>In this lab you will use </tt>mmap</tt> on Linux to demand-page a
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very large table and add memory-mapped files to xv6.
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<h2>Using mmap on Linux</h2>
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<p>This assignment will make you more familiar with how to manage virtual memory
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in user programs using the Unix system call interface. You can do this
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assignment on any operating system that supports the Unix API (a Linux Athena
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machine, your laptop with Linux or MacOS, etc.).
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<p>Download the <a href="mmap.c">mmap homework assignment</a> and look
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it over. The program maintains a very large table of square root
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values in virtual memory. However, the table is too large to fit in
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physical RAM. Instead, the square root values should be computed on
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demand in response to page faults that occur in the table's address
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range. Your job is to implement the demand faulting mechanism using a
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signal handler and UNIX memory mapping system calls. To stay within
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the physical RAM limit, we suggest using the simple strategy of
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unmapping the last page whenever a new page is faulted in.
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<p>To compile <tt>mmap.c</tt>, you need a C compiler, such as gcc. On Athena,
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you can type:
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<pre>
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$ add gnu
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</pre>
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Once you have gcc, you can compile mmap.c as follows:
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<pre>
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$ gcc mmap.c -lm -o mmap
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</pre>
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Which produces a <tt>mmap</tt> file, which you can run:
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<pre>
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$ ./mmap
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page_size is 4096
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Validating square root table contents...
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oops got SIGSEGV at 0x7f6bf7fd7f18
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</pre>
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<p>When the process accesses the square root table, the mapping does not exist
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and the kernel passes control to the signal handler code in
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<tt>handle_sigsegv()</tt>. Modify the code in <tt>handle_sigsegv()</tt> to map
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in a page at the faulting address, unmap a previous page to stay within the
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physical memory limit, and initialize the new page with the correct square root
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values. Use the function <tt>calculate_sqrts()</tt> to compute the values.
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The program includes test logic that verifies if the contents of the
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square root table are correct. When you have completed your task
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successfully, the process will print “All tests passed!”.
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<p>You may find that the man pages for mmap() and munmap() are helpful references.
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<pre>
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$ man mmap
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$ man munmap
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</pre>
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<h2>Implement memory-mapped files in xv6</h2>
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<p>In this assignment you will implement memory-mapped files in xv6.
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The test program <tt>mmaptest</tt> tells you what should work.
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<p>Here are some hints about how you might go about this assignment:
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<ul>
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<li>Start with adding the two systems calls to the kernel, as you
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done for other systems calls (e.g., <tt>sigalarm</tt>), but
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don't implement them yet; just return an
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error. run <tt>mmaptest</tt> to observe the error.
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<li>Keep track for each process what <tt>mmap</tt> has mapped.
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You will need to allocate a <tt>struct vma</tt> to record the
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address, length, permissions, etc. for each virtual memory area
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(VMA) that maps a file. Since the xv6 kernel doesn't have a
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memory allocator in the kernel, you can use the same approach has
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for <tt>struct file</tt>: have a global array of <tt>struct
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vma</tt>s and have for each process a fixed-sized array of VMAs
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(like the file descriptor array).
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<li>Implement <tt>mmap</tt>: allocate a VMA, add it to the process's
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table of VMAs, fill in the VMA, and find a hole in the process's
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address space where you will map the file. You can assume that no
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file will be bigger than 1GB. The VMA will contain a pointer to
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a <tt>struct file</tt> for the file being mapped; you will need to
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increase the file's reference count so that the structure doesn't
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disappear when the file is closed (hint:
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see <tt>filedup</tt>). You don't have worry about overlapping
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VMAs. Run <tt>mmaptest</tt>: the first <tt>mmap</tt> should
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succeed, but the first access to the mmaped- memory will fail,
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because you haven't updated the page fault handler.
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<li>Modify the page-fault handler from the lazy-allocation and COW
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labs to call a VMA function that handles page faults in VMAs.
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This function allocates a page, reads a 4KB from the mmap-ed
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file into the page, and maps the page into the address space of
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the process. To read the page, you can use <tt>readi</tt>,
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which allows you to specify an offset from where to read in the
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file (but you will have to lock/unlock the inode passed
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to <tt>readi</tt>). Don't forget to set the permissions correctly
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on the page. Run <tt>mmaptest</tt>; you should get to the
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first <tt>munmap</tt>.
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<li>Implement <tt>munmap</tt>: find the <tt>struct vma</tt> for
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the address and unmap the specified pages (hint:
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use <tt>uvmunmap</tt>). If <tt>munmap</tt> removes all pages
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from a VMA, you will have to free the VMA (don't forget to
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decrement the reference count of the VMA's <tt>struct
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file</tt>); otherwise, you may have to shrink the VMA. You can
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assume that <tt>munmap</tt> will not split a VMA into two VMAs;
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that is, we don't unmap a few pages in the middle of a VMA. If
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an unmapped page has been modified and the file is
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mapped <tt>MAP_SHARED</tt>, you will have to write the page back
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to the file. RISC-V has a dirty bit (<tt>D</tt>) in a PTE to
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record whether a page has ever been written too; add the
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declaration to kernel/riscv.h and use it. Modify <tt>exit</tt>
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to call <tt>munmap</tt> for the process's open VMAs.
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Run <tt>mmaptest</tt>; you should <tt>mmaptest</tt>, but
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probably not <tt>forktest</tt>.
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<li>Modify <tt>fork</tt> to copy VMAs from parent to child. Don't
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forget to increment reference count for a VMA's <tt>struct
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file</tt>. In the page fault handler of the child, it is OK to
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allocate a new page instead of sharing the page with the
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parent. The latter would be cooler, but it would require more
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implementation work. Run <tt>mmaptest</tt>; make sure you pass
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both <tt>mmaptest</tt> and <tt>forktest</tt>.
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</ul>
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<p>Run usertests to make sure you didn't break anything.
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<p>Optional challenges:
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<ul>
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<li>If two processes have the same file mmap-ed (as
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in <tt>forktest</tt>), share their physical pages. You will need
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reference counts on physical pages.
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<li>The solution above allocates a new physical page for each page
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read from the mmap-ed file, even though the data is also in kernel
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memory in the buffer cache. Modify your implementation to mmap
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that memory, instead of allocating a new page. This requires that
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file blocks be the same size as pages (set <tt>BSIZE</tt> to
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4096). You will need to pin mmap-ed blocks into the buffer cache.
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You will need worry about reference counts.
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<li>Remove redundancy between your implementation for lazy
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allocation and your implementation of mmapp-ed files. (Hint:
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create an VMA for the lazy allocation area.)
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<li>Modify <tt>exec</tt> to use a VMA for different sections of
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the binary so that you get on-demand-paged executables. This will
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make starting programs faster, because <tt>exec</tt> will not have
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to read any data from the file system.
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<li>Implement on-demand paging: don't keep a process in memory,
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but let the kernel move some parts of processes to disk when
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physical memory is low. Then, page in the paged-out memory when
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the process references it. Port your linux program from the first
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assignment to xv6 and run it.
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</ul>
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</body>
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</html>
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