diff options
Diffstat (limited to 'labs')
| -rw-r--r-- | labs/fs.html | 249 |
1 files changed, 249 insertions, 0 deletions
diff --git a/labs/fs.html b/labs/fs.html new file mode 100644 index 0000000..34f64e0 --- /dev/null +++ b/labs/fs.html @@ -0,0 +1,249 @@ +q<html> +<head> +<title>Lab: file system</title> +<link rel="stylesheet" href="homework.css" type="text/css" /> +</head> +<body> + +<h1>Lab: file system</h1> + +<p>In this lab you will add large files and <tt>mmap</tt> to the xv6 file system. + +<h2>Large files</h2> + +<p>In this assignment you'll increase the maximum size of an xv6 +file. Currently xv6 files are limited to 268 blocks, or 268*BSIZE +bytes (BSIZE is 1024 in xv6). This limit comes from the fact that an +xv6 inode contains 12 "direct" block numbers and one "singly-indirect" +block number, which refers to a block that holds up to 256 more block +numbers, for a total of 12+256=268. You'll change the xv6 file system +code to support a "doubly-indirect" block in each inode, containing +256 addresses of singly-indirect blocks, each of which can contain up +to 256 addresses of data blocks. The result will be that a file will +be able to consist of up to 256*256+256+11 blocks (11 instead of 12, +because we will sacrifice one of the direct block numbers for the +double-indirect block). + +<h3>Preliminaries</h3> + +<p>Modify your Makefile's <tt>CPUS</tt> definition so that it reads: +<pre> +CPUS := 1 +</pre> + +<b>XXX doesn't seem to speedup things</b> +<p>Add +<pre> +QEMUEXTRA = -snapshot +</pre> +right before +<tt>QEMUOPTS</tt> +<p> +The above two steps speed up qemu tremendously when xv6 +creates large files. + +<p><tt>mkfs</tt> initializes the file system to have fewer +than 1000 free data blocks, too few to show off the changes +you'll make. Modify <tt>param.h</tt> to +set <tt>FSSIZE</tt> to: +<pre> + #define FSSIZE 20000 // size of file system in blocks +</pre> + +<p>Download <a href="big.c">big.c</a> into your xv6 directory, +add it to the UPROGS list, start up xv6, and run <tt>big</tt>. +It creates as big a file as xv6 will let +it, and reports the resulting size. It should say 140 sectors. + +<h3>What to Look At</h3> + +The format of an on-disk inode is defined by <tt>struct dinode</tt> +in <tt>fs.h</tt>. You're particularly interested in <tt>NDIRECT</tt>, +<tt>NINDIRECT</tt>, <tt>MAXFILE</tt>, and the <tt>addrs[]</tt> element +of <tt>struct dinode</tt>. Look Figure 7.3 in the xv6 text for a +diagram of the standard xv6 inode. + +<p> +The code that finds a file's data on disk is in <tt>bmap()</tt> +in <tt>fs.c</tt>. Have a look at it and make sure you understand +what it's doing. <tt>bmap()</tt> is called both when reading and +writing a file. When writing, <tt>bmap()</tt> allocates new +blocks as needed to hold file content, as well as allocating +an indirect block if needed to hold block addresses. + +<p> +<tt>bmap()</tt> deals with two kinds of block numbers. The <tt>bn</tt> +argument is a "logical block" -- a block number relative to the start +of the file. The block numbers in <tt>ip->addrs[]</tt>, and the +argument to <tt>bread()</tt>, are disk block numbers. +You can view <tt>bmap()</tt> as mapping a file's logical +block numbers into disk block numbers. + +<h3>Your Job</h3> + +Modify <tt>bmap()</tt> so that it implements a doubly-indirect +block, in addition to direct blocks and a singly-indirect block. +You'll have to have only 11 direct blocks, rather than 12, +to make room for your new doubly-indirect block; you're +not allowed to change the size of an on-disk inode. +The first 11 elements of <tt>ip->addrs[]</tt> should be +direct blocks; the 12th should be a singly-indirect block +(just like the current one); the 13th should be your new +doubly-indirect block. + +<p> +You don't have to modify xv6 to handle deletion of files with +doubly-indirect blocks. + +<p> +If all goes well, <tt>big</tt> will now report that it +can write sectors. It will take <tt>big</tt> minutes +to finish. + +<b>XXX this runs for a while!</b> + +<h3>Hints</h3> + +<p> +Make sure you understand <tt>bmap()</tt>. Write out a diagram of the +relationships between <tt>ip->addrs[]</tt>, the indirect block, the +doubly-indirect block and the singly-indirect blocks it points to, and +data blocks. Make sure you understand why adding a doubly-indirect +block increases the maximum file size by 256*256 blocks (really -1), +since you have to decrease the number of direct blocks by one). + +<p> +Think about how you'll index the doubly-indirect block, and +the indirect blocks it points to, with the logical block +number. + +<p>If you change the definition of <tt>NDIRECT</tt>, you'll +probably have to change the size of <tt>addrs[]</tt> +in <tt>struct inode</tt> in <tt>file.h</tt>. Make sure that +<tt>struct inode</tt> and <tt>struct dinode</tt> have the +same number of elements in their <tt>addrs[]</tt> arrays. + +<p>If you change the definition of <tt>NDIRECT</tt>, make sure to create a +new <tt>fs.img</tt>, since <tt>mkfs</tt> uses <tt>NDIRECT</tt> too to build the +initial file systems. If you delete <tt>fs.img</tt>, <tt>make</tt> on Unix (not +xv6) will build a new one for you. + +<p>If your file system gets into a bad state, perhaps by crashing, +delete <tt>fs.img</tt> (do this from Unix, not xv6). <tt>make</tt> will build a +new clean file system image for you. + +<p>Don't forget to <tt>brelse()</tt> each block that you +<tt>bread()</tt>. + +<p>You should allocate indirect blocks and doubly-indirect +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> |