Draft mount/umount lab as an alternative lab for fs.html
fs.html should perhaps be split in small homeworks as in previous years in preparation for lectures and/or as demos during lecture.
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Frans Kaashoek
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labs/fs1.html
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labs/fs1.html
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<html>
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<head>
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<title>Lab: mount/umount</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: mount/umount</h1>
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<p>In this lab you will add support for mounting/unmounting of file
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systems to xv6. This lab will expose you to many parts of the xv6 file
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system, including pathname lookup, inodes, logging, disk driver,
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concurrency, etc.
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<p>Your job is modify xv6 so that your modified kernel passes the
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tests in mounttest. You will have to implement two system
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calls: <tt>mount(char *source, char *target)</tt>
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and <tt>umount(char *target)</tt>. Mount attaches the device
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referenced by <tt>source</tt> (e.g., <tt>/disk1</tt>) at the
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location specified by <tt>target</tt>. For
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example, <tt>mount("/disk1", "/m")</tt> will attach <tt>disk1</tt>
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at the directory <tt>/m</tt>. After this mount call, users can use
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pathnames such as <tt>/m/README</tt> to read the
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file <tt>README</tt> stored in the root directory
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on <tt>disk1</tt>. <tt>Umount</tt> removes the attachment. For
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example, <tt>umount("/m")</tt> unmounts disk1 from <tt>/m</tt>.
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<p>There are several major challenges in implementing the mount system
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calls:
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<ul>
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<li>Adding the actual system calls so that user programs can call
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them. This is similar to previous labs in which you added
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systems calls xv6.
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<li>Supporting several disks. You will have generalize to
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virtio_disk.c to support at least two disks.
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<li>Logging file system modifications to the right disk. xv6
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assumes there is only disk and file system calls typically start
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with <tt>begin_op</tt> and end with <tt>end_op</tt>, logging all
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modifications between these two calls to the log on the one
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disk. With mount, modifications to the file system on the
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second disk must be logged to the second disk.
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<li>Modifying pathname lookup (<tt>namex</tt>) so that when a
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lookup cross a mount point, it continues at the root inode of
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the attached disk.
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</ul>
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<p>The rest of this assignment provides some hints how you might go
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about the above challenges.
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<h2>Adding system calls</h2>
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<p>Add the stubs for the two systems calls to xv6 so that you can
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compile mounttest and add two empty functions for the two system calls
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to sysfile.c. Run mounttest and it will fail on the first call
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to <tt>mount</tt>.
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<h2>Adding a second disk</h2>
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<p>To be able to mount another disk, you need to extend xv6 to support
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at least two disks. Modify virtio_disk.c to support an array of two
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disks instead of a single disk. The address of the second disk
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is <tt>0x10002000</tt>; modify the macro <tt>R</tt> to take a disk
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number (0, 1,..) and read/write to the memory address for that disk.
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<p>All functions in <tt>virtio_disk.c</tt> need to take the disk
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number as an argument to update the state of the disk that is
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read/written to or to receive an interrupt from the disk.
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Modify <tt>virtio_disk_init</tt> to take a disk number as an argument
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and update is to that it initializes that disk. Similar, go through
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the other functions; make these changes should be most mechanical
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(i.e., text substitutions).
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<p>The second disk interrupts at IRQ 2; modify trap.c to receive that
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interrupt and <tt>virtio_disk_intr</tt> with the number of the disk
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that generated the interrupt.
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<p>Modify the file Makefile to tell qemu to provide a second
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disk. Define the variable <tt>QEMUEXTRA = -drive
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file=fs1.img,if=none,format=raw,id=x1 -device
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virtio-blk-device,drive=x1,bus=virtio-mmio-bus.1</tt> and
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add <tt>$(QEMUEXTRA)</tt> to the end of <tt>QEMUOPTS</tt>.
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<p>Create a second disk image <tt>fs1.img</tt>. Easiest thing to do
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is just copy the file <tt>fs.img</tt>. You might want to add rules
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to the Makefile to make this image and remove it on <tt>make
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clean</tt>.
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<p>Add to the user program init a call to create a device for the new
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disk. For example, add the line <tt>mknod("disk1", DISK, 1);</tt> to
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init.c. This will create an inode of type device in the root
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directory with major number <tt>DISK</tt> and minor number 1.
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<p>The first argument of the <tt>mount</tt> system call ("disk1") will
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refer to the device you created using <tt>mknod</tt> above. In your
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implementation of the mount system call,
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call <tt>virtio_disk_init</tt> with the minor number as the argument
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to initialize the second disk. (We reserve minor number 0 for the
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first disk.)
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<p>Boot xv6, run mounttest, and make sure <tt>virtio_disk_init</tt>
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gets called (e.g., add print statement). You won't know if your
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changes are correct, but your code should compile and invoke the
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driver for the second disk.
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<h2>Modify the logging system</h2>
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<p>After calling <tt>virtio_disk_init</tt>, you need to also
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call <tt>loginit</tt> to initialize the logging system for the
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second disk (and restore the second disk if a power failure happened
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while modifying the second disk). Generalize the logging system to
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support to two logs, one on disk 0 and one disk 1. These changes
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are mostly mechanical (e.g., <tt>log.</tt> changes
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to <tt>log[n].</tt>), similar to generalizing the disk driver to
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support two disks.
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<p>To make xv6 compile, you need to provide a disk number
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to <tt>begin_op</tt> and <tt>end_op</tt>. It will be a challenge to
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figure out what the right value is; for now just specify the first
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disk (i.e., 0). This isn't correct, since modifications to the
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second disk should be logged on the second disk, but we have no way
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yet to read/write the second disk. Come back to this later when you
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have a better idea how things will fit together, but make sure that
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xv6 compiles and still runs.
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<h2>Pathname lookup</h2>
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<p>Modify <tt>namex</tt> to traverse mount points: when <tt>namex</tt>
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sees an inode to which a file system is attached, it should traverse
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to the root inode of that file system. Hint: modify the in-memory
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inode in file.h to keep some additional state, and initialize that
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state in the mount system call. Note that the inode already has a
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field for disk number (i.e., <tt>dev</tt>), which is initialized and
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passed to reads and writes to the driver. <tt>dev</tt> corresponds
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to the minor number for disk devices.
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<p>Your modified xv6 should be able to pass the first tests in
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mounttest (i.e., <tt>stat</tt>). This is likely to be challenging,
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however, because now your kernel will be reading from the second
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disk for the first time, and you may run into many issues.
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<p>Even though <tt>stat</tt> may return correctly, your code is likely
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to be incorrect, because in <tt>namex</tt>
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because <tt>iunlockput</tt> may modify the second disk (e.g., if
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another process removes the file or directory) and those
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modifications must be written to the second disk. Your job is to
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fix the calls to <tt>begin_op</tt> and <tt>end_op</tt> to take the
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right device. One challenge is that <tt>begin_op</tt> is called at
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the beginning of a system call but then you don't know the device
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that will be involved; you will have to postpone this call until you
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know which inode is involved (which tells you will which device is
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involved). Another challenge is that you cannot postpone
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calling <tt>begin_op</tt> passed <tt>ilock</tt> because that
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violates lock ordering in xv6; you should not be
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calling <tt>begin_op</tt> while holding locks on inodes. (The log
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system allows a few systems calls to run; if a system call that
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holds an inode lock isn't admitted and one of the admitted system
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calls needs that inode to complete, then xv6 will deadlock.)
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<p>Once you have implemented a plan for <tt>begin_op</tt>
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and <tt>end_op</tt>, see if your kernel can pass <tt>test0</tt>. It
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is likely that you will have to modify your implementation of the
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mount system call to handle several corner cases. See the tests
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in <tt>test0</tt>.
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<p>Run usertests to see if you didn't break anything else. Since you
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modified <tt>namex</tt> and <tt>begin/end_op</tt>, which are at the
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core of the xv6 file system, you might have introduced bugs, perhaps
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including deadlocks. Deadlocks manifest themselves as no output
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being produced because all processes are sleeping (hit ctrl-p a few
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times). Your kernel might also suffer kernel panics, because your
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changes violate invariants. You may have to iterate a few times to
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get a good design and implementation.
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<h2>umount</h2>
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<p>Once your kernel passes usertests and test0 of mounttest, implement
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umount. Make sure your kernel can pass test1 of mounttest.
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<p>Test2 of mounttest stresses <namex> more; if you have done
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everything right above, your kernel may be able to pass it.
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</body>
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</html>
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<h2>Optional challenges</h2>
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<p>Modify xv6 so that init mounts the first disk on the root inode.
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This will allow you to remove some code specific for the first disk
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from the kernel.
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<p>Support mounts on top of mounts.
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