You will continue to handle Pintos disks and file systems the same way
you did in the previous assignment (@pxref{Using the File System}).
-The tests for project 3 (and later projects) will run much faster if
-you use the qemu emulator, e.g.@: via @code{make check
-PINTOSOPTS='--qemu'}.
-
@menu
* Project 3 Background::
* Project 3 Suggested Order of Implementation::
@node Pages
@subsubsection Pages
-A @dfn{page}, sometimes called a @dfn{virtual page}, is a contiguous
+A @dfn{page}, sometimes called a @dfn{virtual page}, is a continuous
region of virtual memory 4,096 bytes (the @dfn{page size}) in length. A
page must be @dfn{page-aligned}, that is, start on a virtual address
evenly divisible by the page size. Thus, a 32-bit virtual address can
@subsubsection Frames
A @dfn{frame}, sometimes called a @dfn{physical frame} or a @dfn{page
-frame}, is a contiguous region of physical memory. Like pages, frames
+frame}, is a continuous region of physical memory. Like pages, frames
must be page-size and page-aligned. Thus, a 32-bit physical address can
be divided into a 20-bit @dfn{frame number} and a 12-bit @dfn{frame
offset} (or just @dfn{offset}), like this:
@node Swap Slots
@subsubsection Swap Slots
-A @dfn{swap slot} is a contiguous, page-size region of disk space on the
+A @dfn{swap slot} is a continuous, page-size region of disk space on the
swap disk. 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:
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.
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
projects / pintos-anon / blobdiff