Due: Thursday 12/01 11:59 pm EST
Note: Lab 3b is optional for 318 section students, but is required for
418/618 section students.. Students in 318 section who choose to complete
this lab may receive extra credits of max 10% of the project grade.
In this assignment, you will complete the second part of Pintos virtual memory enhancements. You will continue working in the vm
directory for this project.
You need to hand in your code for this lab in a branch called lab3b-handin. Create this branch with git checkout -b lab3b-handin lab3a-handin.
Before you turn in your project, you must copy the
project 3b design document template into your source tree under the name
pintos/src/vm/PART2_DESIGNDOC
and fill it in. We recommend that you
read the design document template before you start working on the
project. See section D. Project Documentation, for a sample design document
that goes along with a fictitious project.
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.(3)
However, the 80x86 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, PUSH would
not be restartable in a straightforward fashion.) Similarly, the
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 the esp member of the
struct intr_frame passed to syscall_handler() or
page_fault(), respectively. If you verify user pointers before
accessing them (see section 3 Accessing User Memory in project 2), 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. Since the processor only
saves the stack pointer when an exception causes a switch from user
to kernel mode, reading esp out of the struct intr_frame
passed to page_fault() would yield an undefined value, not the
user stack pointer. You will need to arrange another way, such as
saving esp into struct thread on the initial transition
from user to kernel mode.
You should impose some absolute limit on stack size, as do most OSes.
Some OSes make the limit user-adjustable, e.g. with the
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 allocate and initialize it with the command line arguments at load time, with no need to wait for it to be faulted in.
All stack pages should be candidates for eviction. An evicted stack page should be written to swap.
The file system is most commonly accessed with read and
write system calls. A secondary interface is to "map" the file
into virtual pages, using the mmap system call. The program can
then use memory instructions directly on the file data.
Suppose file foo
is 0x1000 bytes (4 kB, or one page) long.
If foo
is mapped into memory starting at address 0x5000, then
any memory accesses to locations 0x5000...0x5fff will access
the corresponding bytes of foo
.
Here's a program that uses mmap to print a file to the console.
It opens the file specified on the command line, maps it at virtual
address 0x10000000, writes the mapped data to the console (fd 1),
and unmaps the file.
#include <stdio.h>
#include <syscall.h>
int main (int argc UNUSED, char *argv[])
{
void *data = (void *) 0x10000000; /* Address at which to map. */
int fd = open (argv[1]); /* Open file. */
mapid_t map = mmap (fd, data); /* Map file. */
write (1, data, filesize (fd)); /* Write file to console. */
munmap (map); /* Unmap file (optional). */
return 0;
}
|
A similar program with full error handling is included as mcat.c
in the examples
directory, which also contains mcp.c
as a
second example of mmap.
Your submission must be able to track what memory is used by memory mapped files. You need a table of file mappings to track which files are mapped into which pages. This is necessary to properly handle page faults in the mapped regions and to ensure that mapped files do not overlap any other segments within the process.
Your VM system must lazily load pages in mmap regions and use the
mmaped file itself as backing store for the mapping. That is,
evicting a page mapped by mmap writes it back to the file it was
mapped from.
If the file's length is not a multiple of 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 the
file system,
and discard them when the page is written back to disk.
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 mmap may fail if the file open as fd has a
length of zero bytes. It must fail if 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 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.
mmap by the same
process that has not yet been unmapped.
All mappings are implicitly unmapped when a process exits, whether via
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 munmap is called or the process
exits, following the Unix convention. See Removing an Open File, for
more information. You should use the file_reopen function to
obtain a separate and independent reference to the file for each of
its mappings.
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 mmap system call also has
an argument allowing the client to specify whether the page is shared or
private (i.e. copy-on-write).
git push
to GitHub, especially in the last few minutes! Your
submission must reside in a branch called lab3b-handin.
If you are developing in other branches, in the end, don't forget to merge changes
from that branch to the lab3b-handin branch.
Here's a summary of our reference solution, produced by the
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. See section 4.4 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.
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(-) |
Yes.
You might notice that, in the stack growth tests, the user program faults
on an address that is above the user program's current stack pointer,
even though the PUSH and PUSHA instructions would cause
faults 4 and 32 bytes below the current stack pointer.
This is not unusual. The PUSH and PUSHA instructions are
not the only instructions that can trigger user stack growth.
For instance, a user program may allocate stack space by decrementing the
stack pointer using a SUB $n, %esp instruction, and then use a
MOV ..., m(%esp) instruction to write to a stack location within
the allocated space that is m bytes above the current stack pointer.
Such accesses are perfectly valid, and your kernel must grow the
user program's stack to allow those accesses to succeed.
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.