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-<html>
-<head>
-<title>Lab: xv6 lazy page allocation</title>
-<link rel="stylesheet" href="homework.css" type="text/css" />
-</head>
-<body>
-
-<h1>Lab: xv6 lazy page allocation</h1>
-
-<p>
-One of the many neat tricks an O/S can play with page table hardware
-is lazy allocation of heap memory. Xv6 applications ask the kernel for
-heap memory using the sbrk() system call. In the kernel we've given
-you, sbrk() allocates physical memory and maps it into the process's
-virtual address space. There are programs that allocate memory but
-never use it, for example to implement large sparse arrays.
-Sophisticated kernels delay allocation of each page of memory until
-the application tries to use that page -- as signaled by a page fault.
-You'll add this lazy allocation feature to xv6 in this lab.
-
-<h2>Part One: Eliminate allocation from sbrk()</h2>
-
-Your first task is to delete page allocation from the sbrk(n) system
-call implementation, which is the function sys_sbrk() in sysproc.c. The
-sbrk(n) system call grows the process's memory size by n bytes, and
-then returns the start of the newly allocated region (i.e., the old
-size). Your new sbrk(n) should just increment the process's size
-(myproc()->sz) by n and return the old size. It should not allocate memory
--- so you should delete the call to growproc() (but you still need to
-increase the process's size!).
-
-<p>
-Try to guess what the result of this modification will be: what will
-break?
-
-<p>
-Make this modification, boot xv6, and type <tt>echo hi</tt> to the shell.
-You should see something like this:
-
-<pre>
-init: starting sh
-$ echo hi
-usertrap(): unexpected scause 0x000000000000000f pid=3
-            sepc=0x00000000000011dc stval=0x0000000000004008
-va=0x0000000000004000 pte=0x0000000000000000
-panic: unmappages: not mapped
-</pre>
-
-The "usertrap(): ..." message is from the user trap handler in trap.c;
-it has caught an exception that it does not know how to handle. Make
-sure you understand why this page fault occurs. The "stval=0x0..04008"
-indicates that the virtual address that caused the page fault is
-0x4008.
-
-<h2>Part Two: Lazy allocation</h2>
-
-Modify the code in trap.c to respond to a page fault from user space
-by mapping a newly-allocated page of physical memory at the faulting
-address, and then returning back to user space to let the process
-continue executing. You should add your code just before
-the <tt>printf</tt> call that produced the "usertrap(): ..."
-message.
-
-<p>
-Hint: look at the printf arguments to see how to find the virtual
-address that caused the page fault.
-
-<p>
-Hint: steal code from allocuvm() in vm.c, which is what sbrk()
-calls (via growproc()).
-
-<p>
-Hint: use PGROUNDDOWN(va) to round the faulting virtual address
-down to a page boundary.
-
-<p>
-Hint: <tt>usertrapret()</tt> in order to avoid
-the <tt>printf</tt> and the <tt>myproc()->killed&nbsp;=&nbsp;1</tt>.
-
-<p>
-Hint: you'll need to call mappages().
-  
-<p>Hint: you can check whether a fault is a page fault by r_scause()
-  is 13 or 15 in trap().
-
-<p>Hint: modify unmappages() to not free pages that aren't mapped.
-
-<p>Hint: if the kernel crashes, look up sepc in kernel/kernel.asm
-
-<p>Hint: if you see the error "imcomplete type proc", include "proc.h"
-  (and "spinlock.h").
-
-<p>Hint: the first test in sbrk() allocates something large, this
-  should succeed now.
-
-<p>
-If all goes well, your lazy allocation code should result in <tt>echo
-hi</tt> working. You should get at least one page fault (and thus lazy
-allocation) in the shell, and perhaps two.
-
-<p>If you have the basics working, now turn your implementation into
-  one that handles the corner cases too:
-
-<ul>
-
-  <li> Handle negative sbrk() arguments.  sbrktest() in usertests will
-  tests this.
-
-  <li> Handle fork correctly. sbrktst() will test this.
-
-  <li> Make sure that kernel use of not-yet-allocated user addresses
-     works; for example, if a program passes an sbrk()-allocated
-    address to write().  sbrktest() will test this.
-
-  <li> Handle out of memory correctly.  sbrktst() will test this.
-
-  <li> Handle faults on the invalid page below the stack.  stacktest()
-  in usertests will tests this.
-
-</ul>
-  
-<p>Run all tests in usertests() to make sure your solution doesn't
-break other tests.
-
-<p>
-<div class="question">
-<p><b>Submit</b>: The code that you added to trap.c in a file named <em>hwN.c</em> where <em>N</em> is the homework number as listed on the schedule.
-</div>
-
-
-</body>
-</html>