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Frans Kaashoek 2019-08-01 16:52:38 -04:00
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20
labs/cow.html
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@ -1,11 +1,18 @@
<h2>Programming Assignment: Copy-on-Write Fork for xv6</h2> <html>
<head>
<title>Lab: Copy-on-Write Fork for xv6</title>
<link rel="stylesheet" href="homework.css" type="text/css" />
</head>
<body>
<h1>Lab: Copy-on-Write Fork for xv6</h2>
<p> <p>
Your task is implement copy-on-write fork in the xv6 kernel. You are Your task is implement copy-on-write fork in the xv6 kernel. You are
done if your modified kernel executes both the cow and usertests done if your modified kernel executes both the cow and usertests
programs successfully. programs successfully.
<h3>The problem</h3> <h2>The problem</h2>
The fork() system call in xv6 copies all of the parent process's The fork() system call in xv6 copies all of the parent process's
user-space memory into the child. If the parent is large, copying can user-space memory into the child. If the parent is large, copying can
@ -17,7 +24,7 @@ the copied pages are thrown away without ever being used. Of course,
sometimes both child and parent modify memory at the same virtual sometimes both child and parent modify memory at the same virtual
address after a fork(), so for some pages the copying is truly needed. address after a fork(), so for some pages the copying is truly needed.
<h3>The solution</h3> <h2>The solution</h2>
The goal of copy-on-write (COW) fork() is to defer allocating and The goal of copy-on-write (COW) fork() is to defer allocating and
copying physical memory pages for the child until they are actually copying physical memory pages for the child until they are actually
@ -41,7 +48,7 @@ memory a little trickier. A given physical page may be referred to by
multiple processes' page tables, and should be freed when the last multiple processes' page tables, and should be freed when the last
reference disappears. reference disappears.
<h3>The cow test program</h3> <h2>The cow test program</h2>
To help you test your implementation, we've provided an xv6 program To help you test your implementation, we've provided an xv6 program
called cow (source in user/cow.c). cow runs various tests, but called cow (source in user/cow.c). cow runs various tests, but
@ -80,7 +87,7 @@ ALL TESTS PASSED
$ $
</pre> </pre>
<h3>Hints</h3> <h2>Hints</h2>
Here's one reasonable plan of attack. Modify uvmcopy() to map the Here's one reasonable plan of attack. Modify uvmcopy() to map the
parent's physical pages into the child, instead of allocating new parent's physical pages into the child, instead of allocating new
@ -97,3 +104,6 @@ same scheme as page faults when it encounters a COW page.
It may be useful to have a way to record, for each PTE, whether it is It may be useful to have a way to record, for each PTE, whether it is
a COW mapping. You can use the RSW (reserved for software) bits in a COW mapping. You can use the RSW (reserved for software) bits in
the RISC-V PTE for this. the RISC-V PTE for this.
</body>
</html>

2
labs/lazy.html
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@ -99,7 +99,7 @@ hi</tt> working. You should get at least one page fault (and thus lazy
allocation) in the shell, and perhaps two. allocation) in the shell, and perhaps two.
<p>If you have the basics working, now turn your implementation into <p>If you have the basics working, now turn your implementation into
one that handles the corner cases too. one that handles the corner cases too:
<ul> <ul>