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<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>
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