Make more useful.

[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.  There is
no new code to get acquainted with for this assignment.  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).

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 do software
translation on TLB misses, how to represent the swap partition, 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 @code{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.  The
top-level 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 22:31) 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 12:21) 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 0:11) are added to the
data page's physical base address, producing the final physical
address.
@end enumerate

@example
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 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 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 for problem 3 of this assignment, but you should
design your solutions to problems 1 and 2 to account for 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, in which case some sort of fault should occur.

@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---note
that we provide one in @file{lib/kernel}).

@item
Some way of translating from physical page frames back to virtual
page frames, so that when you replace a page, 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 part 2, but planning ahead is a good
idea.
@end itemize

You need to do the roughly the following to handle a page fault:

@enumerate 1
@item
Determine the location of the physical page backing the virtual
address that faulted.  It might be in the file system, in swap,
already be in physical memory and just not set up in the page table,
or it might be an invalid virtual address.

If the virtual address is invalid, that is, if there's no physical
page backing it, 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 physical page is not in physical memory, bring it into memory.
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
Each user process's @code{struct thread} has a @samp{pagedir} member
that points to its own per-process page directory.  Read the PDE for
the faulting virtual address.

@item 
If the PDE is marked ``not present'' then allocate a new page table
page and initialize the PDE to point to the new page table.  As when
you allocated a data page, you might have to first evict some other
page from memory.

@item
Follow the PDE to the page table.  Point the PTE for the faulting
virtual address to the physical page found in step 2.
@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.

There are many possible ways to implement virtual memory.  The above
is simply an outline of our suggested implementation.  You may choose
any implementation you like, as long as it accomplishes the goal.

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

Implement paging to and from disk.

You will need routines to move a page from memory to disk and from
disk to memory.  You may use the Pintos file system for swap space, or
you may use the disk on interface @code{hd1:1}, which is otherwise
unused.  A swap disk can theoretically be faster than using the file
system, because it avoid file system overhead and because the swap
disk and file system disk will be on separate hard disk controllers.
You will definitely need to be able to retrieve pages from files in
any case, so to avoid special cases it may be easier to use a file for
swap.  You will still be using the basic file system provided with
Pintos.  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 store on disk should not be constrained to be in sequential
order, and consequently your swap file (or swap disk) should not
require unused empty space.  You will also need a way to track all of
the physical memory pages, in order 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 in part 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.  Therefore, 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
@code{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, which you should handle by
reading the partial page from disk at executable load time and zeroing
the rest of the page.  It is the only case in which loading should not
be ``lazy''; even 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.

You may optionally 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 part 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 @asis
@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.  

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

Unmaps the segment specified by id.  This cannot be used to unmap
segments mapped by the executable loader.  Returns 0 on success, -1 on
failure.  When a file is unmapped, all outstanding changes are written
to the file, and the segment's pages are removed from the process's
list of used virtual pages.
@end table

Calls to @code{mmap} must fail if the address is not page-aligned, if
the length is not positive and a multiple of @var{PGSIZE}.  You also
must error check to make sure that the new segment does not overlap
already existing segments, and fail if it isn't.  If the length passed
to @code{mmap} is less than the file's length, you should only map the
first part of the file.  If the length passed to @code{mmap} is longer
than the file, the file should grow to the requested length.  Similar
to the code segment, your VM system should be able to use the
@code{mmap}'d file itself as backing store for the mmap segment, since
the changes to the @code{mmap} segment will eventually be written to
the file.  (In fact, you may choose to implement executable mappings
as a special case of file mappings.)

@node Virtual Memory FAQ
@section FAQ

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

Yes.

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

FIXME

There are two things you need to use this hashtable:

1. You need to decide on a key type. The key should be something
that is unique for each object as inserting two objects with
the same key will cause the second to overwrite the first.
(The keys are compared with ==, so you should stick to
integers and pointers unless you know how to do operator
overloading.) You also need to write a hash function that
converts key values to integers, which you will pass into the
hash table constructor.

2. Your key needs to be a field of your object type, and you
will need to supply a 'get' function that given an object
returns the key.

Here's a quick example of how to construct a hash table. In
this table the keys are Thread pointers and the objects are
integers (you will be using different key/value pairs I'm
sure). In addition, this hash function is pretty puny. You
should probably use a better one.

@example
FIXME
@end example

and to construct the hash table:

HashTable<Thread *, HashObject *> *htable;

htable = new HashTable<Thread *, HashObject *>(ExtractKeyFromHashObject,
                                            MyKeyToHashValue);

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{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 like you want it to.

@item
@b{Is the data segment page-aligned?}

No.

@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. The
test/script and testvm/script files are the linker scripts for the
multiprogramming and virtual memory assignments respectively. You can
learn more about linker scripts by reading the ``Scripts'' chapter in
the linker manual, accessible via @samp{info ld}.

@item Page Table Management FAQs
@enumerate 1
@item
@b{How do we manage allocation of pages used for page tables?}

You can use any reasonable algorithm to do so.  However, you should
make sure that memory used for page tables doesn't grow so much that
it encroaches deeply on the memory used for data pages.

Here is one reasonable algorithm.  At OS boot time, reserve some fixed
number of pages for page tables.  Then, each time a new page table
page is needed, select one of these pages in ``round robin'' fashion.
If the page in use, clean up any pointers to it.  Then use it for the
new page table page.

@item
@b{Our code handles the PageFault exceptions. However, the number of
page faults handled does not show up in the final stats output. Is
there a counter that we must increment to correct this problem?}

FIXME 

Yes, you'll need to update kernel->stats->numPageFaults when
you handle a page fault in your code.
@end enumerate

@item Paging FAQs

@enumerate 1
@item
@b{Can we assume (and enforce) that the user's stack will
never increase beyond one page?}

No.  This value was useful for project 2, but for this assignment, you
need to implement an extensible stack segment.

@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).
Implementing @code{sbrk()} has been an extra-credit assignment in
previous years, but adds little additional complexity to a
well-designed system.

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

Yes. If a page fault appears just below the last stack segment page,
you must add a new page to the bottom of the stack. 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.

@item
@b{But what do you mean by ``appear'' to be stack accesses? How big can a
stack growth be?  Under what circumstances do we grow the stack?}

If it looks like a stack request, then you grow the stack. Yes, that's
ambiguous. You need to make a reasonable decision about what looks
like a stack request. For example, you could decide a page, or two
pages, or ten pages, or more@enddots{}  Or, you could use some other
heuristic to figure this out.

Make a reasonable decision and document it in your code and in
your design document.  Please make sure to justify your decision.

@item
@b{How big should the file(s) we're using as a backing store for memory
be?}

These files will need to be able to grow based on the number of pages
you're committed to storing on behalf of the processes currently in
memory.  They should be able to grow to the full size of the disk.
@end enumerate

@item Memory Mapped File FAQs

@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 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);
@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?}

@item
You should follow the Unix convention and the mapping should still be
valid.  This is similar to the question in the User Programs FAQ about
a process with a file descriptor to a file that has been removed.

@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} extends the file to the requested length,
and Pintos provides no way to shorten a file.  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 mmap'd files?}

When a process finishes each of its @code{mmap}'d 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 mmaped file, should be automatically unmap it
for them?}

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