Comment.

[pintos-anon] / doc / vm.texi

@node Project 3--Virtual Memory, Project 4--File Systems, Project 2--User Programs, Top
@chapter Project 3: Virtual Memory

By now you should be familiar with the inner workings of Pintos.
You've already come a long way: your OS can properly handle multiple
threads of execution with proper synchronization, and can load
multiple user programs at once.  However, when loading user programs,
your OS is limited by how much main memory the simulated machine has.
In this assignment, you will remove that limitation.

You will be using the @file{vm} directory for this project.  The
@file{vm} directory contains only the @file{Makefile}s.  The only
change from @file{userprog} is that this new @file{Makefile} turns on
the setting @option{-DVM}.  All code you write will either be newly
generated files (e.g.@: if you choose to implement your paging code in
their own source files), or will be modifications to pre-existing code
(e.g.@: you will change the behavior of @file{process.c}
significantly).

There are only a couple of source files you will probably be
encountering for the first time:

@table @file
@item devices/disk.h
@itemx devices/disk.c
Provides access to the physical disk, abstracting away the rather
awful IDE interface.
@end table

You will be building this assignment on the last one.  It will benefit
you to get your project 2 in good working order before this assignment
so those bugs don't keep haunting you.

All the test programs from the previous project should also work with
this project.  You should also write programs to test the new features
introduced in this project.

Your submission should define @code{THREAD_JOIN_IMPLEMENTED} in
@file{constants.h} (@pxref{Conditional Compilation}).

@menu
* VM Design::                   
* Page Faults::                 
* Disk as Backing Store::       
* Memory Mapped Files::         
* Stack::                       
* Problem 3-1 Page Table Management::  
* Problem 3-2 Paging To and From Disk::  
* Problem 3-3 Memory Mapped Files::  
* Virtual Memory FAQ::          
@end menu

@node VM Design
@section A Word about Design

It is important for you to note that in addition to getting virtual
memory working, this assignment is also meant to be an open-ended
design problem.  We will expect you to come up with a design that
makes sense.  You will have the freedom to choose how to handle page
faults, how to organize the swap disk, how to implement paging, etc.
In each case, we will expect you to provide a defensible justification
in your design documentation as to why your choices are reasonable.
You should evaluate your design on all the available criteria: speed
of handling a page fault, space overhead in memory, minimizing the
number of page faults, simplicity, etc.

In keeping with this, you will find that we are going to say as little
as possible about how to do things.  Instead we will focus on what end
functionality we require your OS to support.

@node Page Faults
@section Page Faults

For the last assignment, whenever a context switch occurred, the new
process would install its own page table into the machine.  The page
table contained all the virtual-to-physical translations for the
process.  Whenever the processor needed to look up a translation, it
consulted the page table.  As long as the process only accessed
memory that it didn't own, all was well.  If the process accessed
memory it didn't own, it ``page faulted'' and @func{page_fault}
terminated the process.

When we implement virtual memory, the rules have to change.  A page
fault is no longer necessarily an error, since it might only indicate
that the page must be brought in from a disk file or from swap.  You
will have to implement a more sophisticated page fault handler to
handle these cases.

On the 80@var{x}86, the page table format is fixed by hardware.  We
have provided code for managing page tables for you to use in
@file{userprog/pagedir.c}.  The functions in there should provide an
abstract interface to all the page table functionality that you need
to complete the project.  However, you may still find it worthwhile to
understand a little about the hardware page table format, so we'll go
into a little of detail about that in this section.

The top-level paging data structure is a 4 kB page called the ``page
directory'' (PD) arranged as an array of 1,024 32-bit page directory
entries (PDEs), each of which represents 4 MB of virtual memory.  Each
PDE may point to the physical address of another 4 kB page called a
``page table'' (PT) arranged in the same fashion as an array of 1,024
32-bit page table entries (PTEs), each of which translates a single 4
kB virtual page into physical memory.

Thus, translation of a virtual address into a physical address follows
the three-step process illustrated in the diagram
below:@footnote{Actually, virtual to physical translation on the
80@var{x}86 architecture happens via an intermediate ``linear
address,'' but Pintos (and most other 80@var{x}86 OSes) set up the CPU
so that linear and virtual addresses are one and the same, so that you
can effectively ignore this CPU feature.}

@enumerate 1
@item
The top 10 bits of the virtual address (bits 22:32) are used to index
into the page directory.  If the PDE is marked ``present,'' the
physical address of a page table is read from the PDE thus obtained.
If the PDE is marked ``not present'' then a page fault occurs.

@item
The next 10 bits of the virtual address (bits 12:22) are used to index
into the page table.  If the PTE is marked ``present,'' the physical
address of a data page is read from the PTE thus obtained.  If the PTE
is marked ``not present'' then a page fault occurs.


@item
The bottom 12 bits of the virtual address (bits 0:12) are added to the
data page's physical base address, producing the final physical
address.
@end enumerate

@example
@group
32                    22                     12                      0
+--------------------------------------------------------------------+
| Page Directory Index |   Page Table Index   |    Page Offset       |
+--------------------------------------------------------------------+
             |                    |                     |
     _______/             _______/                _____/
    /                    /                       /
   /    Page Directory  /      Page Table       /    Data Page
  /     .____________. /     .____________.    /   .____________.
  |1,023|____________| |1,023|____________|    |   |____________|
  |1,022|____________| |1,022|____________|    |   |____________|
  |1,021|____________| |1,021|____________|    \__\|____________|
  |1,020|____________| |1,020|____________|       /|____________|
  |     |            | |     |            |        |            |
  |     |            | \____\|            |_       |            |
  |     |      .     |      /|      .     | \      |      .     |
  \____\|      .     |_      |      .     |  |     |      .     |
       /|      .     | \     |      .     |  |     |      .     |
        |      .     |  |    |      .     |  |     |      .     |
        |            |  |    |            |  |     |            |
        |____________|  |    |____________|  |     |____________|
       4|____________|  |   4|____________|  |     |____________|
       3|____________|  |   3|____________|  |     |____________|
       2|____________|  |   2|____________|  |     |____________|
       1|____________|  |   1|____________|  |     |____________|
       0|____________|  \__\0|____________|  \____\|____________|
                           /                      /
@end group
@end example

Header @file{threads/mmu.h} has useful functions for various
operations on virtual addresses.  You should look over the header
yourself, but its most important functions include these:

@table @code
@item pd_no(@var{va})
Returns the page directory index in virtual address @var{va}.

@item pt_no(@var{va})
Returns the page table index in virtual address @var{va}.

@item pg_ofs(@var{va})
Returns the page offset in virtual address @var{va}.

@item pg_round_down(@var{va})
Returns @var{va} rounded down to the nearest page boundary, that is,
@var{va} but with its page offset set to 0.

@item pg_round_up(@var{va})
Returns @var{va} rounded up to the nearest page boundary.
@end table

@node Disk as Backing Store
@section Disk as Backing Store

In VM systems, since memory is less plentiful than disk, you will
effectively use memory as a cache for disk.  Looking at it from
another angle, you will use disk as a backing store for memory.  This
provides the abstraction of an (almost) unlimited virtual memory size.
Part of your task in this project is to do this, with the additional
constraint that your performance should be close to that provided by
physical memory.  You will use the page tables' ``dirty'' bits to
denote whether pages need to be written back to disk when they're
evicted from main memory and the ``accessed'' bit for page replacement
algorithms.  Whenever the hardware writes memory, it sets the dirty
bit, and if it reads or writes to the page, it sets the accessed bit.

As with any caching system, performance depends on the policy used to
decide which things are kept in memory and which are only stored on
disk.  On a page fault, the kernel must decide which page to replace.
Ideally, it will throw out a page that will not be referenced for a
long time, keeping in memory those pages that are soon to be
referenced.  Another consideration is that if the replaced page has
been modified, the page must be first saved to disk before the needed
page can be brought in.  Many virtual memory systems avoid this extra
overhead by writing modified pages to disk in advance, so that later
page faults can be completed more quickly.

@node Memory Mapped Files
@section Memory Mapped Files

The traditional way to access the file system is via @code{read} and
@code{write} system calls, but that requires an extra level of copying
between the kernel and the user level.  A secondary interface is
simply to ``map'' the file into the virtual address space.  The
program can then use load and store instructions directly on the file
data.  (An alternative way of viewing the file system is as ``durable
memory.''  Files just store data structures.  If you access data
structures in memory using load and store instructions, why not access
data structures in files the same way?)

Memory mapped files are typically implemented using system calls.  One
system call maps the file to a particular part of the address space.
For example, one might conceptually map the file @file{foo}, which is
1000 bytes
long, starting at address 5000.  Assuming that nothing else is already
at virtual addresses 5000@dots{}6000, any memory accesses to these
locations will access the corresponding bytes of @file{foo}.

A consequence of memory mapped files is that address spaces are
sparsely populated with lots of segments, one for each memory mapped
file (plus one each for code, data, and stack).  You will implement
memory mapped files in problem 3-3.  You should
design your solutions to problems 3-1 and 3-2 to anticipate this.

@node Stack
@section Stack

In project 2, the stack was a single page at the top of the user
virtual address space.  The stack's location does not change in this
project, but your kernel should allocate additional pages to the stack
on demand.  That is, if the stack grows past its current bottom, the
system should allocate additional pages for the stack as necessary
(unless those pages are unavailable because they are in use by another
segment).

It is impossible to predict how large the stack will grow at compile
time, so we must allocate pages as necessary.  You should only
allocate additional pages if they ``appear'' to be stack accesses.
You must devise a heuristic that attempts to distinguish stack
accesses from other accesses.  Document and explain the heuristic in
your design documentation.

The first stack page need not be loaded lazily.  You can initialize it
with the command line at load time, with no need to wait for it to be
faulted in.  Even if you did wait, the very first instruction in the
user program is likely to be one that faults in the page.

@node Problem 3-1 Page Table Management
@section Problem 3-1: Page Table Management

Implement page directory and page table management to support virtual
memory.  You will need data structures to accomplish the following
tasks:

@itemize @bullet
@item
Some way of translating in software from virtual page frames to
physical page frames.  Consider using a hash table (@pxref{Hash
Table}).

@item
Some way of translating from physical page frames back to virtual page
frames, so that when you evict a physical page from its frame, you can
invalidate its translation(s).

@item
Some way of finding a page on disk if it is not in memory.  You won't
need this data structure until problem 3-2, but planning ahead is a
good idea.
@end itemize

The page fault handler, @func{page_fault} in
@file{threads/exception.c}, needs to do roughly the following:

@enumerate 1
@item
Locate the page backing the virtual
address that faulted.  It might be in the file system, in swap,
or it might be an invalid virtual address.
If you implement sharing, it might even
already be in physical memory and just not set up in the page table,

If the virtual address is invalid, that is, if there's nothing
assigned to go there, or if the virtual address is above
@code{PHYS_BASE}, meaning that it belongs to the kernel instead of the
user, then the process's memory access must be disallowed.  You should
terminate the process at this point, being sure to free all of its
resources.

@item
If the page is not in physical memory, fetch it by appropriate means.
If necessary to make room, first evict some other page from memory.
(When you do that you need to first remove references to the page from
any page table that refers to it.)

@item
Point the page table entry for the faulting virtual address to the
physical page.  You can use the functions in @file{userprog/pagedir.c}.
@end enumerate

You'll need to modify the ELF loader in @file{userprog/process.c} to
do page table management according to your new design.  As supplied,
it reads all the process's pages from disk and initializes the page
tables for them at the same time.  For testing purposes, you'll
probably want to leave the code that reads the pages from disk, but
use your new page table management code to construct the page tables
only as page faults occur for them.

You should use the @func{palloc_get_page} function to get the page
frames that you use for storing user virtual pages.  Be sure to pass
the @code{PAL_USER} flag to this function when you do so, because that
allocates pages from a ``user pool'' separate from the ``kernel pool''
that other calls to @func{palloc_get_page} make.

There are many possible ways to implement virtual memory.  The above
is simply an outline of our suggested implementation.

@node Problem 3-2 Paging To and From Disk
@section Problem 3-2: Paging To and From Disk

Implement paging to and from files and the swap disk.  You may use the
disk on interface @code{hd1:1} as the swap disk, using the disk
interface prototyped in @code{devices/disk.h}.

You will need routines to move a page from memory to disk and from
disk to memory, where ``disk'' is either a file or the swap disk.  If
you do everything correctly, your VM should still work when you
implement your own file system for the next assignment.

You will need a way to track pages which are used by a process but
which are not in physical memory, to fully handle page faults.  Pages
that you write to swap should not be constrained to be in sequential
order.  You will also need a way to track all of the physical memory
pages, to find an unused one when needed, or to evict a page
when memory is needed but no empty pages are available.  The data
structures that you designed for problem 3-1 should do most of the work for
you.

You will need a page replacement algorithm.  The hardware sets the
accessed and dirty bits when it accesses memory.  You can gain access
to this information using the functions prototyped in
@file{userprog/pagedir.h}.  You should be able to take advantage of
this information to implement some algorithm which attempts to achieve
LRU-type behavior.  We expect that your algorithm perform at least as
well as a reasonable implementation of the second-chance (clock)
algorithm.  You will need to show in your test cases the value of your
page replacement algorithm by demonstrating for some workload that it
pages less frequently using your algorithm than using some inferior
page replacement policy.  The canonical example of a poor page
replacement policy is random replacement.

Since you will already be paging from disk, you should implement a
``lazy'' loading scheme for new processes.  When a process is created,
it will not run immediately.  Therefore, it doesn't make sense to load
all its code, data, and stack into memory when the process is created,
since it might incur additional disk accesses to do so (if it gets
paged out before it runs).  When loading a new process, you should
leave most pages on disk, and bring them in as demanded when the
program begins running.  Your VM system should also use the executable
file itself as backing store for read-only segments, since these
segments won't change.

There are a few special cases.  Look at the loop in
@func{load_segment} in @file{userprog/process.c}.  Each time
around the loop, @code{read_bytes} represents the number of bytes to
read from the executable file and @code{zero_bytes} represents the number
of bytes to initialize to zero following the bytes read.  The two
always sum to @code{PGSIZE}.  The page handling depends on these
variables' values:

@itemize @bullet
@item
If @code{read_bytes} equals @code{PGSIZE}, the page should be demand
paged from disk on its first access.

@item 
If @code{zero_bytes} equals @code{PGSIZE}, the page does not need to
be read from disk at all because it is all zeroes.  You should handle
such pages by creating a new page consisting of all zeroes at the
first page fault.

@item
If neither @code{read_bytes} nor @code{zero_bytes} equals
@code{PGSIZE}, then part of the page is to be read from disk and the
remainder zeroed.  This is a special case.  You are allowed to handle
it by reading the partial page from disk at executable load time and
zeroing the rest of the page.  This is the only case in which we will
allow you to load a page in a non-``lazy'' fashion.  Many real OSes
such as Linux do not load partial pages lazily.
@end itemize

Incidentally, if you have trouble handling the third case above, you
can eliminate it temporarily by linking the test programs with a
special ``linker script.''  Read @file{tests/userprog/Makefile} for
details.  We will not test your submission with this special linker
script, so the code you turn in must properly handle all cases.

For extra credit, you may implement sharing: when multiple processes
are created that use the same executable file, share read-only pages
among those processes instead of creating separate copies of read-only
segments for each process.  If you carefully designed your data
structures in problem 3-1, sharing of read-only pages should not make this
part significantly harder.

@node Problem 3-3 Memory Mapped Files
@section Problem 3-3: Memory Mapped Files

Implement memory mapped files.

You will need to implement the following system calls:

@table @code
@item SYS_mmap
@itemx bool mmap (int @var{fd}, void *@var{addr}, unsigned @var{length})

Maps the file open as @var{fd} into the process's address space
starting at @var{addr} for @var{length} bytes.  Returns true if
successful, false on failure.  Failure cases include the following:

@itemize @bullet
@item
@var{addr} is not page-aligned.

@item
@var{length} is not positive.

@item
The range of pages mapped overlaps any existing set of mapped pages,
including the stack or pages mapped at executable load time.
@end itemize

@var{length} is treated as if it were rounded up to the nearest
multiple of the page size, that is, as if the first statement in the
system call's implementation were
@example
length = ROUND_UP (length, PGSIZE);
@end example
(The @code{ROUND_UP} macro is defined in @file{<round.h>}.)
The remainder of this description assumes that this has been done.

If @var{length} is less than @var{fd}'s length, you should only map
the first @var{length} bytes of the file.  If @var{length} is greater
than @var{fd}'s length, when the file's length is also rounded up to a
page multiple, the call should fail.  Ideally it would extend the
file, but our file system does not yet support growing files.

If @var{length} is greater than @var{fd}'s (unrounded) length, then some
bytes in the final mapped page ``stick out'' beyond the end of the
file.  Set these bytes to zero when the page is faulted in from
disk, and discard them when the page is written back to disk.

Your VM system should use the @code{mmap}'d file itself as
backing store for the mapped segment.  That is, to evict a page mapped by
@code{mmap} must be evicted, write it to the file it was mapped from.
(In fact, you may choose to implement executable mappings as a special
case of file mappings.)

@item SYS_munmap
@itemx bool munmap (void *addr, unsigned length)

Unmaps @var{length} bytes starting at @var{addr}.  Returns true on
success, false on failure.  Failure cases include the following:

@itemize @bullet
@item
@var{addr} is not page-aligned.

@item
@var{length} is not positive.

@item
One or more pages within the range to be unmapped were not mapped
using the @code{mmap} system call.
@end itemize

As with @code{mmap}, @var{length} is treated as if it were rounded up
to the nearest multiple of the page size.

It is valid to unmap only some of the pages that were mapped in a
previous system call.
@end table

All mappings are implicitly unmapped when a process exits, whether via
@code{exit} or by any other means.  When a file is unmapped, whether
implicitly or explicitly, all outstanding changes are written to the
file, and the pages are removed from the process's list of used
virtual pages.

@node Virtual Memory FAQ
@section FAQ

@enumerate 1
@item
@b{Do we need a working HW 2 to implement HW 3?}

Yes.

@item
@anchor{Hash Table}
@b{How do I use the hash table provided in @file{lib/kernel/hash.c}?}

First, you need to embed a @code{hash_elem} object as a member of the
object that the hash table will contain.  Each @code{hash_elem} allows
the object to a member of at most one hash table at a given time.  All
the hash table functions that deal with hash table items actually use
the address of a @code{hash_elem}.  You can convert a pointer to a
@code{hash_elem} member into a pointer to the structure in which
member is embedded using the @code{hash_entry} macro.

Second, you need to decide on a key type.  The key should be something
that is unique for each object, because a given hash table may not
contain two objects with equal keys.  Then you need to write two
functions.  The first is a @dfn{hash function} that converts a key
into an integer.  Some sample hash functions that you can use or just
examine are given in @file{lib/kernel/hash.c}.  The second function
needed is a @dfn{comparison function} that compares a pair and returns
true if the first is less than the second.  These two functions have
to be compatible with the prototypes for @code{hash_hash_func} and
@code{hash_less_func} in @file{lib/kernel/hash.h}.

Here's a quick example.  Suppose you want to put @struct{thread}s
in a hash table.  First, add a @code{hash_elem} to the thread
structure by adding a line to its definition:

@example
hash_elem h_elem;               /* Hash table element. */
@end example

We'll choose the @code{tid} member in @struct{thread} as the key,
and write a hash function and a comparison function:

@example
/* Returns a hash for E. */
unsigned
thread_hash (const hash_elem *e, void *aux UNUSED)
@{
  struct thread *t = hash_entry (e, struct thread, h_elem);
  return hash_int (t->tid);
@}

/* Returns true if A's tid is less than B's tid. */
bool
thread_less (const hash_elem *a_, const hash_elem *b_, 
             void *aux UNUSED)
@{
  struct thread *a = hash_entry (a_, struct thread, h_elem);
  struct thread *b = hash_entry (b_, struct thread, h_elem);
  return a->tid < b->tid;
@}
@end example

Then we can create a hash table like this:

@example
struct hash threads;

hash_init (&threads, thread_hash, thread_less, NULL);
@end example

Finally, if @code{@var{t}} is a pointer to a @struct{thread},
then we can insert it into the hash table with:

@example
hash_insert (&threads, &@var{t}->h_elem);
@end example

If you have any other questions about hash tables, the CS109
and CS161 textbooks have good chapters on them, or you can come
to any of the TA's office hours for further clarification.

@item
@b{What are the @var{aux} parameters to the hash table functions good
for?}

In simple cases you won't have any need for the @var{aux} parameters.
In these cases you can just pass a null pointer to @func{hash_init}
for @var{aux} and ignore the values passed to the hash function and
comparison functions.  (You'll get a compiler warning if you don't use
the @var{aux} parameter, but you can turn that off with the
@code{UNUSED} macro, as shown above, or you can just ignore it.)

@var{aux} is useful when you have some property of the data in the
hash table that's both constant and needed for hashing or comparisons,
but which is not stored in the data items themselves.  For example, if
the items in a hash table contain fixed-length strings, but the items
themselves don't indicate what that fixed length is, you could pass
the length as an @var{aux} parameter.

@item
@b{The current implementation of the hash table does not do something
that we need it to do. What gives?}

You are welcome to modify it.  It is not used by any of the code we
provided, so modifying it won't affect any code but yours.  Do
whatever it takes to make it work the way you want.

@item
@b{What controls the layout of user programs?}

The linker is responsible for the layout of a user program in
memory. The linker is directed by a ``linker script'' which tells it
the names and locations of the various program segments.  You can
learn more about linker scripts by reading the ``Scripts'' chapter in
the linker manual, accessible via @samp{info ld}.
@end enumerate

@menu
* Problem 3-1 and 3-2 FAQ::    
* Problem 3-3 Memory Mapped File FAQ::  
@end menu

@node Problem 3-1 and 3-2 FAQ
@subsection Problem 3-1 and 3-2 FAQ

@enumerate 1
@item
@b{Does the virtual memory system need to support growth of the data
segment?}

No.  The size of the data segment is determined by the linker.  We
still have no dynamic allocation in Pintos (although it is possible to
``fake'' it at the user level by using memory-mapped files).  However,
implementing it would add little additional complexity to a
well-designed system.

@item
@b{Why do I need to pass @code{PAL_USER} to @func{palloc_get_page}
when I allocate physical page frames?}@anchor{Why PAL_USER?}

You can layer some other allocator on top of @func{palloc_get_page}
if you like, but it should be the underlying mechanism, directly or
indirectly, for two reasons.  First, running out of pages in the user
pool just causes user programs to page, but running out of pages in
the kernel pool will cause all kinds of problems, because many kernel
functions depend on being able to allocate memory.  Second, you can
use the @option{-ul} option to @command{pintos} to limit the size of
the user pool, which makes it easy to test your VM implementation with
various user memory sizes.
@end enumerate

@node Problem 3-3 Memory Mapped File FAQ
@subsection Problem 3-3: Memory Mapped File FAQ

@enumerate 1
@item
@b{How do we interact with memory-mapped files?}

Let's say you want to map a file called @file{foo} into your address
space at address @t{0x10000000}. You open the file, determine its
length, and then use @code{mmap}:

@example
#include <stdio.h>
#include <syscall.h>

int main (void)
@{
    void *addr = (void *) 0x10000000;
    int fd = open ("foo");
    int length = filesize (fd);
    if (mmap (fd, addr, length))
        printf ("success!\n");
@}
@end example

Suppose @file{foo} is a text file and you want to print the first 64
bytes on the screen (assuming, of course, that the length of the file
is at least 64).  Without @code{mmap}, you'd need to allocate a
buffer, use @code{read} to get the data from the file into the buffer,
and finally use @code{write} to put the buffer out to the display. But
with the file mapped into your address space, you can directly address
it like so:

@example
write (addr, 64, STDOUT_FILENO);
@end example

Similarly, if you wanted to replace the first byte of the file,
all you need to do is:

@example
addr[0] = 'b';
@end example

When you're done using the memory-mapped file, you simply unmap
it:

@example
munmap (addr, length);
@end example

@item
@b{What if two processes memory-map the same file?}

There is no requirement in Pintos that the two processes see
consistent data.  Unix handles this by making the processes share the
same physical page, but the @code{mmap} system call also has an
argument allowing the client to specify whether the page is shared or
private (i.e.@: copy-on-write).

@item
@b{What happens if a user removes a @code{mmap}'d file?}

You should follow the Unix convention and the mapping should still be
valid.  @xref{Removing an Open File}, for more information.

@item
@b{What if a process writes to a page that is memory-mapped, but the
location written to in the memory-mapped page is past the end
of the memory-mapped file?}

Can't happen.  @code{mmap} checks that the mapped region is within the
file's length and Pintos provides no way to shorten a file.  (Until
project 4, there's no way to extend a file either.)  You can remove a
file, but the mapping remains valid (see the previous question).

@item
@b{Do we have to handle memory mapping @code{stdin} or @code{stdout}?}

No.  Memory mapping implies that a file has a length and that a user
can seek to any location in the file.  Since the console device has
neither of these properties, @code{mmap} should return false when the
user attempts to memory map a file descriptor for the console device.

@item
@b{What happens when a process exits with mapped files?}

When a process finishes, each of its mapped files is implicitly
unmapped.  When a process @code{mmap}s a file and then writes into the
area for the file it is making the assumption the changes will be
written to the file.

@item
@b{If a user closes a mapped file, should it be automatically
unmapped?}

No, once created the mapping is valid until @code{munmap} is called
or the process exits.
@end enumerate