Wording improvements, from "Valentin I. Spitkovsky"

[pintos-anon] / doc / vm.texi

diff --git a/doc/vm.texi b/doc/vm.texi
index cf4f7640864c438323487b9fd0452952d90223f5..fdbc5c68c492b2900745d29ad6d40cc9ab211080 100644 (file)
--- a/doc/vm.texi
+++ b/doc/vm.texi
@@ -260,7 +260,7 @@ page fault might only indicate that the page must be brought in from a
 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
@@ -576,28 +576,28 @@ 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
+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.
@@ -613,7 +613,7 @@ space.  The entire file is mapped into consecutive virtual pages
 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.
 
@@ -714,6 +714,12 @@ process.  If you carefully designed your data structures,
 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