2019-07-24 12:37:26 +00:00
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<title>Lab: xv6 lazy page allocation</title>
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<link rel="stylesheet" href="homework.css" type="text/css" />
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<body>
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<h1>Lab: xv6 lazy page allocation</h1>
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<p>
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One of the many neat tricks an O/S can play with page table hardware
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is lazy allocation of heap memory. Xv6 applications ask the kernel for
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heap memory using the sbrk() system call. In the kernel we've given
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you, sbrk() allocates physical memory and maps it into the process's
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virtual address space. There are programs that allocate memory but
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never use it, for example to implement large sparse arrays.
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Sophisticated kernels delay allocation of each page of memory until
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the application tries to use that page -- as signaled by a page fault.
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You'll add this lazy allocation feature to xv6 in this lab.
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<h2>Part One: Eliminate allocation from sbrk()</h2>
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Your first task is to delete page allocation from the sbrk(n) system
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call implementation, which is the function sys_sbrk() in sysproc.c. The
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sbrk(n) system call grows the process's memory size by n bytes, and
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then returns the start of the newly allocated region (i.e., the old
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size). Your new sbrk(n) should just increment the process's size
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(myproc()->sz) by n and return the old size. It should not allocate memory
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-- so you should delete the call to growproc() (but you still need to
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increase the process's size!).
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<p>
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Try to guess what the result of this modification will be: what will
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break?
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<p>
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Make this modification, boot xv6, and type <tt>echo hi</tt> to the shell.
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You should see something like this:
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<pre>
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init: starting sh
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$ echo hi
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usertrap(): unexpected scause 0x000000000000000f pid=3
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sepc=0x00000000000011dc stval=0x0000000000004008
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va=0x0000000000004000 pte=0x0000000000000000
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panic: unmappages: not mapped
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</pre>
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The "usertrap(): ..." message is from the user trap handler in trap.c;
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it has caught an exception that it does not know how to handle. Make
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sure you understand why this page fault occurs. The "stval=0x0..04008"
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indicates that the virtual address that caused the page fault is
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0x4008.
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<h2>Part Two: Lazy allocation</h2>
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Modify the code in trap.c to respond to a page fault from user space
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by mapping a newly-allocated page of physical memory at the faulting
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address, and then returning back to user space to let the process
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continue executing. You should add your code just before
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the <tt>printf</tt> call that produced the "usertrap(): ..."
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message.
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<p>
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Hint: look at the printf arguments to see how to find the virtual
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address that caused the page fault.
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<p>
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Hint: steal code from allocuvm() in vm.c, which is what sbrk()
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calls (via growproc()).
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<p>
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Hint: use PGROUNDDOWN(va) to round the faulting virtual address
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down to a page boundary.
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<p>
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Hint: <tt>usertrapret()</tt> in order to avoid
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the <tt>printf</tt> and the <tt>myproc()->killed = 1</tt>.
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<p>
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Hint: you'll need to call mappages().
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<p>Hint: you can check whether a fault is a page fault by r_scause()
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is 13 or 15 in trap().
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<p>Hint: modify unmappages() to not free pages that aren't mapped.
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<p>Hint: if the kernel crashes, look up sepc in kernel/kernel.asm
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<p>Hint: if you see the error "imcomplete type proc", include "proc.h"
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(and "spinlock.h").
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<p>Hint: the first test in sbrk() allocates something large, this
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should succeed now.
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<p>
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If all goes well, your lazy allocation code should result in <tt>echo
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hi</tt> working. You should get at least one page fault (and thus lazy
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allocation) in the shell, and perhaps two.
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<p>If you have the basics working, now turn your implementation into
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2019-08-01 20:52:38 +00:00
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one that handles the corner cases too:
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2019-07-24 12:37:26 +00:00
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<ul>
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<li> Handle negative sbrk() arguments. sbrktest() in usertests will
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tests this.
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<li> Handle fork correctly. sbrktst() will test this.
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<li> Make sure that kernel use of not-yet-allocated user addresses
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works; for example, if a program passes an sbrk()-allocated
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address to write(). sbrktest() will test this.
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<li> Handle out of memory correctly. sbrktst() will test this.
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<li> Handle faults on the invalid page below the stack. stacktest()
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in usertests will tests this.
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</ul>
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<p>Run all tests in usertests() to make sure your solution doesn't
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break other tests.
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<p>
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<div class="question">
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<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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</div>
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</body>
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</html>
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