xv6-65oo2/labs/lazy.html
2019-07-24 08:37:43 -04:00

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<title>Lab: xv6 lazy page allocation</title>
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<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.
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