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diff --git a/labs/fs1.html b/labs/fs1.html new file mode 100644 index 0000000..cf5bc8e --- /dev/null +++ b/labs/fs1.html @@ -0,0 +1,198 @@ +<html> +<head> +<title>Lab: mount/umount</title> +<link rel="stylesheet" href="homework.css" type="text/css" /> +</head> +<body> + +<h1>Lab: mount/umount</h1> + +<p>In this lab you will add support for mounting/unmounting of file +systems to xv6. This lab will expose you to many parts of the xv6 file +system, including pathname lookup, inodes, logging, disk driver, +concurrency, etc. + +<p>Your job is modify xv6 so that your modified kernel passes the + tests in mounttest. You will have to implement two system + calls: <tt>mount(char *source, char *target)</tt> + and <tt>umount(char *target)</tt>. Mount attaches the device + referenced by <tt>source</tt> (e.g., <tt>/disk1</tt>) at the + location specified by <tt>target</tt>. For + example, <tt>mount("/disk1", "/m")</tt> will attach <tt>disk1</tt> + at the directory <tt>/m</tt>. After this mount call, users can use + pathnames such as <tt>/m/README</tt> to read the + file <tt>README</tt> stored in the root directory + on <tt>disk1</tt>. <tt>Umount</tt> removes the attachment. For + example, <tt>umount("/m")</tt> unmounts disk1 from <tt>/m</tt>. + +<p>There are several major challenges in implementing the mount system +calls: + + <ul> + + <li>Adding the actual system calls so that user programs can call + them. This is similar to previous labs in which you added + systems calls xv6. + + <li>Supporting several disks. You will have generalize to + virtio_disk.c to support at least two disks. + + <li>Logging file system modifications to the right disk. xv6 + assumes there is only disk and file system calls typically start + with <tt>begin_op</tt> and end with <tt>end_op</tt>, logging all + modifications between these two calls to the log on the one + disk. With mount, modifications to the file system on the + second disk must be logged to the second disk. + + <li>Modifying pathname lookup (<tt>namex</tt>) so that when a + lookup cross a mount point, it continues at the root inode of + the attached disk. + + </ul> + +<p>The rest of this assignment provides some hints how you might go +about the above challenges. + +<h2>Adding system calls</h2> + +<p>Add the stubs for the two systems calls to xv6 so that you can +compile mounttest and add two empty functions for the two system calls +to sysfile.c. Run mounttest and it will fail on the first call +to <tt>mount</tt>. + + +<h2>Adding a second disk</h2> + +<p>To be able to mount another disk, you need to extend xv6 to support +at least two disks. Modify virtio_disk.c to support an array of two +disks instead of a single disk. The address of the second disk +is <tt>0x10002000</tt>; modify the macro <tt>R</tt> to take a disk +number (0, 1,..) and read/write to the memory address for that disk. + +<p>All functions in <tt>virtio_disk.c</tt> need to take the disk +number as an argument to update the state of the disk that is +read/written to or to receive an interrupt from the disk. +Modify <tt>virtio_disk_init</tt> to take a disk number as an argument +and update is to that it initializes that disk. Similar, go through +the other functions; make these changes should be most mechanical +(i.e., text substitutions). + +<p>The second disk interrupts at IRQ 2; modify trap.c to receive that +interrupt and <tt>virtio_disk_intr</tt> with the number of the disk +that generated the interrupt. + +<p>Modify the file Makefile to tell qemu to provide a second +disk. Define the variable <tt>QEMUEXTRA = -drive +file=fs1.img,if=none,format=raw,id=x1 -device +virtio-blk-device,drive=x1,bus=virtio-mmio-bus.1</tt> and +add <tt>$(QEMUEXTRA)</tt> to the end of <tt>QEMUOPTS</tt>. + +<p>Create a second disk image <tt>fs1.img</tt>. Easiest thing to do + is just copy the file <tt>fs.img</tt>. You might want to add rules + to the Makefile to make this image and remove it on <tt>make + clean</tt>. + +<p>Add to the user program init a call to create a device for the new + disk. For example, add the line <tt>mknod("disk1", DISK, 1);</tt> to + init.c. This will create an inode of type device in the root + directory with major number <tt>DISK</tt> and minor number 1. + +<p>The first argument of the <tt>mount</tt> system call ("disk1") will + refer to the device you created using <tt>mknod</tt> above. In your + implementation of the mount system call, + call <tt>virtio_disk_init</tt> with the minor number as the argument + to initialize the second disk. (We reserve minor number 0 for the + first disk.) + +<p>Boot xv6, run mounttest, and make sure <tt>virtio_disk_init</tt> + gets called (e.g., add print statement). You won't know if your + changes are correct, but your code should compile and invoke the + driver for the second disk. + +<h2>Modify the logging system</h2> + +<p>After calling <tt>virtio_disk_init</tt>, you need to also + call <tt>loginit</tt> to initialize the logging system for the + second disk (and restore the second disk if a power failure happened + while modifying the second disk). Generalize the logging system to + support to two logs, one on disk 0 and one disk 1. These changes + are mostly mechanical (e.g., <tt>log.</tt> changes + to <tt>log[n].</tt>), similar to generalizing the disk driver to + support two disks. + +<p>To make xv6 compile, you need to provide a disk number + to <tt>begin_op</tt> and <tt>end_op</tt>. It will be a challenge to + figure out what the right value is; for now just specify the first + disk (i.e., 0). This isn't correct, since modifications to the + second disk should be logged on the second disk, but we have no way + yet to read/write the second disk. Come back to this later when you + have a better idea how things will fit together, but make sure that + xv6 compiles and still runs. + +<h2>Pathname lookup</h2> + +<p>Modify <tt>namex</tt> to traverse mount points: when <tt>namex</tt> + sees an inode to which a file system is attached, it should traverse + to the root inode of that file system. Hint: modify the in-memory + inode in file.h to keep some additional state, and initialize that + state in the mount system call. Note that the inode already has a + field for disk number (i.e., <tt>dev</tt>), which is initialized and + passed to reads and writes to the driver. <tt>dev</tt> corresponds + to the minor number for disk devices. + +<p>Your modified xv6 should be able to pass the first tests in + mounttest (i.e., <tt>stat</tt>). This is likely to be challenging, + however, because now your kernel will be reading from the second + disk for the first time, and you may run into many issues. + +<p>Even though <tt>stat</tt> may return correctly, your code is likely + to be incorrect, because in <tt>namex</tt> + because <tt>iunlockput</tt> may modify the second disk (e.g., if + another process removes the file or directory) and those + modifications must be written to the second disk. Your job is to + fix the calls to <tt>begin_op</tt> and <tt>end_op</tt> to take the + right device. One challenge is that <tt>begin_op</tt> is called at + the beginning of a system call but then you don't know the device + that will be involved; you will have to postpone this call until you + know which inode is involved (which tells you will which device is + involved). Another challenge is that you cannot postpone + calling <tt>begin_op</tt> passed <tt>ilock</tt> because that + violates lock ordering in xv6; you should not be + calling <tt>begin_op</tt> while holding locks on inodes. (The log + system allows a few systems calls to run; if a system call that + holds an inode lock isn't admitted and one of the admitted system + calls needs that inode to complete, then xv6 will deadlock.) + +<p>Once you have implemented a plan for <tt>begin_op</tt> + and <tt>end_op</tt>, see if your kernel can pass <tt>test0</tt>. It + is likely that you will have to modify your implementation of the + mount system call to handle several corner cases. See the tests + in <tt>test0</tt>. + +<p>Run usertests to see if you didn't break anything else. Since you + modified <tt>namex</tt> and <tt>begin/end_op</tt>, which are at the + core of the xv6 file system, you might have introduced bugs, perhaps + including deadlocks. Deadlocks manifest themselves as no output + being produced because all processes are sleeping (hit ctrl-p a few + times). Your kernel might also suffer kernel panics, because your + changes violate invariants. You may have to iterate a few times to + get a good design and implementation. + +<h2>umount</h2> + +<p>Once your kernel passes usertests and test0 of mounttest, implement + umount. Make sure your kernel can pass test1 of mounttest. + +<p>Test2 of mounttest stresses <namex> more; if you have done + everything right above, your kernel may be able to pass it. + +</body> +</html> + +<h2>Optional challenges</h2> + +<p>Modify xv6 so that init mounts the first disk on the root inode. + This will allow you to remove some code specific for the first disk + from the kernel. + +<p>Support mounts on top of mounts. |