fixed typo in mmap example

[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.  Your
OS can properly handle multiple threads of execution with proper
synchronization, and can load multiple user programs at once.  However,
the number and size of programs that can run is limited by the machine's
main memory size.  In this assignment, you will remove that limitation.

You will build this assignment on top of 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.  Test programs from the previous
project should also work with this project.

You will continue to handle Pintos disks and file systems the same way
you did in the previous assignment (@pxref{Using the File System}).

@menu
* Project 3 Background::        
* Project 3 Requirements::      
* Project 3 FAQ::               
@end menu

@node Project 3 Background
@section Background

@menu
* Project 3 Source Files::      
* Page Faults::                 
* Disk as Backing Store::       
* Memory Mapped Files Background::  
@end menu

@node Project 3 Source Files
@subsection Source Files

You will work in the @file{vm} directory for this project.  The
@file{vm} directory contains only @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 be in new
files or in files introduced in earlier projects.

You will probably be encountering just a few files 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.  You will use this interface to access the swap disk.
@end table

@menu
* Page Faults::
* Disk as Backing Store::
* Memory Mapped Files::
@end menu

@node Page Faults
@subsection Page Faults

For the last assignment, whenever a context switch occurred, the new
process installed its own page table into the machine.  The new 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 owned, 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.

@menu
* Page Table Structure::        
* Working with Virtual Addresses::  
* Accessed and Dirty Bits::     
@end menu

@node Page Table Structure
@subsubsection Page Table Structure

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 provide an
abstract interface to all the page table functionality that you should 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 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 page called a
``page table'' (PT) arranged, similarly, as an array of 1,024
32-bit page table entries (PTEs), each of which translates a single 4
kB virtual page to a physical page.

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 occurs 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.  Thus, you
can effectively ignore this CPU feature.}

@enumerate 1
@item
The most-significant 10 bits of the virtual address (bits 22@dots{}32)
index 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@dots{}21) index
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 least-significant 12 bits of the virtual address (bits 0@dots{}11)
are added to the data page's physical base address, yielding the final
physical address.
@end enumerate

@example
@group
 31                  22 21                  12 11                   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

@node Working with Virtual Addresses
@subsubsection Working with Virtual Addresses

Header @file{threads/mmu.h} has useful functions for various
operations on virtual addresses.  You should look over the header
yourself.  The most important functions are described below.

@deftypefun uintptr_t pd_no (const void *@var{va})
Returns the page directory index for virtual address @var{va}.
@end deftypefun

@deftypefun uintptr_t pt_no (const void *@var{va})
Returns the page table index for virtual address @var{va}.
@end deftypefun

@deftypefun unsigned pg_ofs (const void *@var{va})
Returns the page offset of virtual address @var{va}.
@end deftypefun

@deftypefun {void *} pg_round_down (const void *@var{va})
Returns @var{va} rounded down to the nearest page boundary, that is,
@var{va} with its page offset set to 0.
@end deftypefun

@deftypefun {void *} pg_round_up (const void *@var{va})
Returns @var{va} rounded up to the nearest page boundary.
@end deftypefun

@node Accessed and Dirty Bits
@subsubsection Accessed and Dirty Bits

Most of the page table is under the control of the operating system, but
two bits in each page table entry are also manipulated by the CPU.  On
any read or write to the page referenced by a PTE, the CPU sets the
PTE's @dfn{accessed bit} to 1; on any write, the CPU sets the @dfn{dirty
bit} to 1.  The CPU never resets these bits to 0, but the OS may do so.

You will need to use the accessed and dirty bits in your submission to
choose which pages to evict from memory and to decide whether evicted
pages need to be written to disk.  The page table code in
@file{userprog/pagedir.c} provides functions for checking and setting
these bits.  These functions are described at the end of this section.

You need to watch out for @dfn{aliases}, that is, two (or more)
different virtual pages that refer to the same physical page frame.
When an aliased page is accessed, the accessed and dirty bits are
updated in only one page table entry (the one for the virtual address
used to access the page).  The accessed and dirty bits for the other
aliased virtual addresses are not updated.

In Pintos, every user virtual page is aliased to its kernel virtual
address.  You must manage these aliases somehow.  For example, your code
could check and update the accessed and dirty bits for both addresses.
Alternatively, the kernel could avoid the problem by only accessing user
data through the user virtual address.

Other aliases should only arise if you implement sharing, as extra
credit (@pxref{VM Extra Credit}), or as bugs elsewhere in your code.

@deftypefun bool pagedir_is_dirty (uint32_t *@var{pd}, const void *@var{vpage})
@deftypefunx bool pagedir_is_accessed (uint32_t *@var{pd}, const void *@var{vpage})
Returns true if page directory @var{pd} contains a page table entry for
virtual page @var{vpage} that is marked dirty (or accessed).  Otherwise,
returns false.
@end deftypefun

@deftypefun void pagedir_set_dirty (uint32_t *@var{pd}, const void *@var{vpage}, bool @var{value})
@deftypefunx void pagedir_set_accessed (uint32_t *@var{pd}, const void *@var{vpage}, bool @var{value})
If page directory @var{pd} has a page table entry for @var{vpage}, then
its dirty (or accessed) bit is set to @var{value}.
@end deftypefun

@node Disk as Backing Store
@subsection Disk as Backing Store

VM systems effectively use memory as a cache for disk.  From another
perspective, disk is a ``backing store'' for memory.  This provides the
abstraction of an (almost) unlimited virtual memory size.  You must
implement such a system, with the additional constraint that performance
should be close to that provided by physical memory.  You can use dirty
bits to tell whether pages need to be written back to disk when they're
evicted from main memory, and the accessed bits for page replacement
algorithms (@pxref{Accessed and Dirty Bits}).

As with any caching system, performance depends on the policy used to
decide what to keep in the cache and what to evict.  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 written 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 (but you do not have to implement this
optimization).

@node Memory Mapped Files Background
@subsection Memory Mapped Files

The file system is most commonly accessed with @code{read} and
@code{write} system calls.  A secondary interface is 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 view
is to see the file system is as ``durable memory'': files just store
data structures, so if you access ordinary data structures using normal
program instructions, why not access durable data structures the same
way?

Suppose file @file{foo} is @t{0x1000} bytes (4 kB, or one page) long.
If @file{foo} is mapped into memory starting at address @t{0x5000}, then
any memory accesses to locations @t{0x5000}@dots{}@t{0x5fff} 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).

@node Project 3 Requirements
@section Requirements

This assignment is an open-ended design problem.  We are going to say as
little as possible about how to do things.  Instead we will focus on
what functionality we require your OS to support.  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.

@menu
* Project 3 Design Document::   
* Page Table Management::       
* Paging To and From Disk::     
* Lazy Loading::                
* Stack Growth::                
* Memory Mapped Files::         
@end menu

@node Project 3 Design Document
@subsection Design Document

Before you turn in your project, you must copy @uref{vm.tmpl, , the
project 3 design document template} into your source tree under the name
@file{pintos/src/vm/DESIGNDOC} and fill it in.  We recommend that you
read the design document template before you start working on the
project.  @xref{Project Documentation}, for a sample design document
that goes along with a fictitious project.

@node Page Table Management
@subsection 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.  Pintos provides a hash table that you may find
useful for this purpose (@pxref{Hash Table}).

It is possible to do this translation without adding a new data
structure, by modifying the code in @file{userprog/pagedir.c}.  However,
if you do that you'll need to carefully study and understand section
3.7, ``Page Translation Using 32-Bit Physical Addressing,'' in
@bibref{IA32-v3a}, and in practice it is probably easier to add a new
data structure.

@item
Some way of finding a page on disk (in a file or in swap) if it is not
in memory.

You can generalize the virtual-to-physical page table, so that it allows
you to locate a page wherever it is in physical memory or on disk, or
you can make this a separate 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 page table entry (or entries).
@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, but not 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.  
In this case, terminate the process and 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.  As a first step, you might
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 (@pxref{Why PAL_USER?}).

You might find the Pintos bitmap code, in @file{lib/kernel/bitmap.c} and
@file{lib/kernel/bitmap.h}, useful for tracking pages.  A bitmap is an
array of bits, each of which can be true or false.  Bitmaps are
typically used to track usage in a set of (identical) resources: if
resource @var{n} is in use, then bit @var{n} of the bitmap is true.

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

@node Paging To and From Disk
@subsection 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}.  From the @file{vm/build}
directory, use the command @code{pintos-mkdisk swap.dsk @var{n}} to
create an @var{n} MB swap disk named @file{swap.dsk}.  Afterward,
@file{swap.dsk} will automatically be attached as @code{hd1:1} when you run
@command{pintos}.  Alternatively, you can tell @command{pintos} to
use a temporary @var{n}-MB swap disk for a single run with
@option{--swap-disk=@var{n}}.

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 a good job, your VM should still work when you
implement your own file system for the next assignment.

To fully handle page faults, you will need a way to track pages that
are used by a process but which are not in physical memory.  Pages in
swap should not be constrained to any particular ordering.  You will
also need a way to track physical page frames, to find an unused one
when needed, or to evict a page when memory is needed but no empty pages
are available.  The page table data structure that you designed should
do most of the work for you.

Implement a global page replacement algorithm.  You should be able to
use the ``accessed'' and ``dirty'' bits (@pxref{Accessed and Dirty
Bits}) to implement an approximation to LRU.  Your algorithm should
perform at least as well as the ``second chance'' or ``clock''
algorithm.

Your design should allow for parallelism.  Multiple processes should
be able to process page faults at once.  If one page fault require
I/O, in the meantime processes that do not fault should continue
executing and other page faults that do not require I/O should be able to
complete.  These criteria require some synchronization effort.

@node Lazy Loading
@subsection Lazy Loading

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 need all of its resources immediately, so it doesn't make
sense to load all its code, data, and stack into memory when the process
is created.  Instead, bring pages in from
the executable only as needed.  Use the
executable file itself as backing store for read-only segments, since
these segments won't change.  This means that read-only pages should not
be written to swap.

The core of the program loader is the loop in @func{load_segment} in
@file{userprog/process.c}.
Each time around the loop, @code{read_bytes} receives the number of
bytes to read from the executable file and @code{zero_bytes} receives
the number of bytes to initialize to zero following the bytes read.  The
two always sum to @code{PGSIZE} (4,096).  The handling of a page 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{Makefile.userprog} for
details.  We will not test your submission with this special linker
script, so the code you turn in must properly handle all cases.

@node Stack Growth
@subsection Stack Growth

Implement stack growth.  In project 2, the stack was a single page at
the top of the user virtual address space, and programs were limited to
that much stack.  Now, if the stack grows past its current size,
allocate additional pages as necessary.

Allocate additional pages only if they ``appear'' to be stack accesses.
Devise a heuristic that attempts to distinguish stack accesses from
other accesses.

User programs are buggy if they write to the stack below the stack
pointer, because typical real OSes may interrupt a process at any time
to deliver a ``signal,'' which pushes data on the stack.@footnote{This rule is
common but not universal.  One modern exception is the
@uref{http://www.x86-64.org/documentation/abi.pdf, @var{x}86-64 System V
ABI}, which designates 128 bytes below the stack pointer as a ``red
zone'' that may not be modified by signal or interrupt handlers.}
However, the 80@var{x}86 @code{PUSH} instruction checks access
permissions before it adjusts the stack pointer, so it may cause a page
fault 4 bytes below the stack pointer.  (Otherwise, @code{PUSH} would
not be restartable in a straightforward fashion.)  Similarly, the
@code{PUSHA} instruction pushes 32 bytes at once, so it can fault 32
bytes below the stack pointer.

You will need to be able to obtain the current value of the user
program's stack pointer.  Within a system call or a page fault generated
by a user program, you can retrieve it from @code{esp} member of the
@struct{intr_frame} passed to @func{syscall_handler} or
@func{page_fault}, respectively.  If you verify user pointers before
accessing them (@pxref{Accessing User Memory}), these are the only cases
you need to handle.  On the other hand, if you depend on page faults to
detect invalid memory access, you will need to handle another case,
where a page fault occurs in the kernel.  Reading @code{esp} out of the
@struct{intr_frame} passed to @func{page_fault} in that case will obtain
the kernel stack pointer, not the user stack pointer.  You will need to
arrange another way, e.g.@: by saving @code{esp} into @struct{thread} on
the initial transition from user to kernel mode.

You may impose some absolute limit on stack size, as do most OSes.
Some OSes make the limit user-adjustable, e.g.@: with the
@command{ulimit} command on many Unix systems.  On many GNU/Linux systems,
the default limit is 8 MB.

The first stack page need not be allocated lazily.  You can initialize
it with the command line arguments 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 Memory Mapped Files
@subsection Memory Mapped Files

Implement memory mapped files, including the following system calls.

@deftypefn {System Call} mapid_t mmap (int @var{fd}, void *@var{addr})
Maps the file open as @var{fd} into the process's virtual address
space.  The entire file is mapped into consecutive virtual pages
starting at @var{addr}.

If the file's length is not a multiple of @code{PGSIZE}, 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.
A partial page need not be lazily loaded, as in the case of a partial
page in an executable (@pxref{Lazy Loading}).

If successful, this function returns a ``mapping ID'' that
uniquely identifies the mapping within the process.  On failure,
it must return -1, which otherwise should not be a valid mapping id,
and the process's mappings must be unchanged.

A call to @code{mmap} may fail if the file open as @var{fd} has a
length of zero bytes.  It must fail if @var{addr} is not page-aligned
or if the range of pages mapped overlaps any existing set of mapped
pages, including the stack or pages mapped at executable load time.
It must also fail if @var{addr} is 0, because some Pintos code assumes
virtual page 0 is not mapped.  Finally, file descriptors 0 and 1,
representing console input and output, are not mappable.

Your VM system should use the @code{mmap}'d file itself as backing
store for the mapping.  That is, to evict a page mapped by
@code{mmap}, write it to the file it was mapped from.  (In fact, you
may choose to implement executable mappings as special, copy-on-write
file mappings.)
@end deftypefn

@deftypefn {System Call} void munmap (mapid_t @var{mapping})
Unmaps the mapping designated by @var{mapping}, which must be a
mapping ID returned by a previous call to @code{mmap} by the same
process that has not yet been unmapped.
@end deftypefn

All mappings are implicitly unmapped when a process exits, whether via
@code{exit} or by any other means.  When a mapping is unmapped, whether
implicitly or explicitly, all pages written to by the process are
written back to the file, and pages not written must not be.  The pages
are then removed from the process's list of virtual pages.

Closing or removing a file does not unmap any of its mappings.  Once
created, a mapping is valid until @code{munmap} is called or the process
exits, following the Unix convention.  @xref{Removing an Open File}, for
more information.

If two or more processes map the same file, there is no requirement that
they see consistent data.  Unix handles this by making the two mappings
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).

@node Project 3 FAQ
@section FAQ

@table @b
@item How much code will I need to write?

Here's a summary of our reference solution, produced by the
@command{diffstat} program.  The final row gives total lines inserted
and deleted; a changed line counts as both an insertion and a deletion.

This summary is relative to the Pintos base code, but the reference
solution for project 3 starts from the reference solution to project 2.
@xref{Project 2 FAQ}, for the summary of project 2.

The reference solution represents just one possible solution.  Many
other solutions are also possible and many of those differ greatly from
the reference solution.  Some excellent solutions may not modify all the
files modified by the reference solution, and some may modify files not
modified by the reference solution.

@verbatim
 Makefile.build       |    4 
 devices/timer.c      |   42 ++
 threads/init.c       |    5 
 threads/interrupt.c  |    2 
 threads/thread.c     |   31 +
 threads/thread.h     |   37 +-
 userprog/exception.c |   12 
 userprog/pagedir.c   |   10 
 userprog/process.c   |  319 +++++++++++++-----
 userprog/syscall.c   |  545 ++++++++++++++++++++++++++++++-
 userprog/syscall.h   |    1 
 vm/frame.c           |  162 +++++++++
 vm/frame.h           |   23 +
 vm/page.c            |  297 ++++++++++++++++
 vm/page.h            |   50 ++
 vm/swap.c            |   85 ++++
 vm/swap.h            |   11 
 17 files changed, 1532 insertions(+), 104 deletions(-)
@end verbatim

@item Do we need a working Project 2 to implement Project 3?

Yes.

@item What extra credit is available?
@anchor{VM 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 page table data structures,
sharing of read-only pages should not make this part significantly
harder.

@end table

@menu
* Page Table and Paging FAQ::   
* Memory Mapped File FAQ::      
@end menu

@node Page Table and Paging FAQ
@subsection Page Table and Paging FAQ

@table @b
@item Does the virtual memory system need to support data segment growth?

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).  Supporting
data segment growth should add little additional complexity to a
well-designed system.

@item Why should I use @code{PAL_USER} for allocating page frames?
@anchor{Why PAL_USER?}

Passing @code{PAL_USER} to @func{palloc_get_page} causes it to allocate
memory from the user pool, instead of the main kernel pool.  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 many failures because so many
kernel functions need to obtain memory.
You can layer some other allocator on top of @func{palloc_get_page} if
you like, but it should be the underlying mechanism.

Also, 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.

@item Data pages might need swap space.  Can I swap them out at process load?

No.  Reading data pages from the executable and writing them to swap
immediately at program startup is not demand paging.  You need to demand
page everything (except partial pages).
@end table

@node Memory Mapped File FAQ
@subsection Memory Mapped File FAQ

@table @b
@item 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 then use @code{mmap}:

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

int main (void)
@{
    void *addr = (void *) 0x10000000;
    int fd = open ("foo");
    mapid_t map = mmap (fd, addr);
    if (map != -1)
        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 (STDOUT_FILENO, addr, 64);
@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 (map);
@end example

The @command{mcp} program in @file{src/examples} shows how to copy a
file using memory-mapped I/O.
@end table