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.
+@item devices/block.h
+@itemx devices/block.c
+Provides sector-based read and write access to block device. You will
+use this interface to access the swap partition as a block device.
@end table
@node Memory Terminology
@node Swap Slots
@subsubsection Swap Slots
-A @dfn{swap slot} is a continuous, page-size region of disk space on the
-swap disk. Although hardware limitations dictating the placement of
+A @dfn{swap slot} is a continuous, page-size region of disk space in the
+swap partition. Although hardware limitations dictating the placement of
slots are looser than for pages and frames, swap slots should be
page-aligned because there is no downside in doing so.
file or swap. You will have to implement a more sophisticated page
fault handler to handle these cases. Your page fault handler, which you
should implement by modifying @func{page_fault} in
-@file{threads/exception.c}, needs to do roughly the following:
+@file{userprog/exception.c}, needs to do roughly the following:
@enumerate 1
@item
The most important operation on the frame table is obtaining an unused
frame. This is easy when a frame is free. When none is free, a frame
-must be made free by evicting some page from its frame. If no frame can
-be evicted without allocating a swap slot, but swap is full, some
-process must be killed to free memory (the choice of process to kill is
-up to you).
+must be made free by evicting some page from its frame.
+
+If no frame can be evicted without allocating a swap slot, but swap is
+full, panic the kernel. Real OSes apply a wide range of policies to
+recover from or prevent such situations, but these policies are beyond
+the scope of this project.
The process of eviction comprises roughly the following steps:
The swap table tracks in-use and free swap slots. It should allow
picking an unused swap slot for evicting a page from its frame to the
-swap disk. It should allow freeing a swap slot when its page is read
+swap partition. It should allow freeing a swap slot when its page is read
back or the process whose page was swapped is terminated.
-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
+You may use the @code{BLOCK_SWAP} block device for swapping, obtaining
+the @struct{block} that represents it by calling @func{block_get_role}.
+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
+--swap-size=@var{n}} to create an disk named @file{swap.dsk} that
+contains a @var{n}-MB swap partition.
+Afterward, @file{swap.dsk} will automatically be attached as an extra disk
+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}}.
+run with @option{--swap-size=@var{n}}.
Swap slots should be allocated lazily, that is, only when they are
actually required by eviction. Reading data pages from the executable
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.
+partition, how to implement paging, etc.
@menu
* Project 3 Design Document::
@itemize @bullet
@item
If @code{page_read_bytes} equals @code{PGSIZE}, the page should be demand
-paged from disk on its first access.
+paged from the underlying file on its first access.
@item
If @code{page_zero_bytes} equals @code{PGSIZE}, the page does not need to
@item
Otherwise, neither @code{page_read_bytes} nor @code{page_zero_bytes}
equals @code{PGSIZE}. In this case, an initial part of the page is to
-be read from disk and the remainder zeroed.
+be read from the underlying file and the remainder zeroed.
@end itemize
@node Stack Growth
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
+by a user program, you can retrieve it from the @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.
+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 @code{esp} out of the @struct{intr_frame}
+passed to @func{page_fault} would yield an undefined value, not the
+user stack pointer. You will need to arrange another way, such as
+saving @code{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
@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.)
+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.
starting at @var{addr}.
Your VM system must lazily load pages in @code{mmap} regions and use the
-@code{mmap}'d file itself as backing store for the mapping. That is,
+@code{mmap}ed file itself as backing store for the mapping. That is,
evicting a page mapped by @code{mmap} writes it back to the file it was
mapped from.
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,
+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
sharing of read-only pages should not make this part significantly
harder.
+@item How do we resume a process after we have handled a page fault?
+
+Returning from @func{page_fault} resumes the current user process
+(@pxref{Internal Interrupt Handling}).
+It will then retry the instruction to which the instruction pointer points.
+
@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
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
+Also, you can use the @option{-ul} kernel command-line option to limit
the size of the user pool, which makes it easy to test your VM
implementation with various user memory sizes.
@end table
projects / pintos-anon / blobdiff