X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?a=blobdiff_plain;f=doc%2Fvm.texi;h=28754c76da50086d0dfa96ad63b71e1ab67e7df9;hb=081c3133d9fc92b8f5203259bd09492916c23c60;hp=6b07490b56fa8412e0113deac5a47226f7b7c331;hpb=cd769b966b7c76725aa12fd1904e6724b9baad94;p=pintos-anon

diff --git a/doc/vm.texi b/doc/vm.texi
index 6b07490..28754c7 100644
--- a/doc/vm.texi
+++ b/doc/vm.texi
@@ -213,6 +213,9 @@ could check and update the accessed and dirty bits for both addresses.
 Alternatively, the kernel could avoid the problem by only accessing user
 data through the user virtual address.
 
+Other aliases should only arise if you implement sharing, as extra
+credit (@pxref{VM Extra Credit}), or as bugs elsewhere in your code.
+
 @deftypefun bool pagedir_is_dirty (uint32_t *@var{pd}, const void *@var{vpage})
 @deftypefunx bool pagedir_is_accessed (uint32_t *@var{pd}, const void *@var{vpage})
 Returns true if page directory @var{pd} contains a page table entry for
@@ -476,12 +479,11 @@ script, so the code you turn in must properly handle all cases.
 Implement stack growth.  In project 2, the stack was a single page at
 the top of the user virtual address space, and programs were limited to
 that much stack.  Now, if the stack grows past its current size,
-allocate additional page as necessary.
+allocate additional pages as necessary.
 
 Allocate additional pages only if they ``appear'' to be stack accesses.
 Devise a heuristic that attempts to distinguish stack accesses from
-other accesses.  You can retrieve the user program's current stack
-pointer from the @struct{intr_frame}'s @code{esp} member.
+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
@@ -497,9 +499,24 @@ not be restartable in a straightforward fashion.)  Similarly, the
 @code{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 @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.
-(Some OSes make the limit user-adjustable, e.g.@: with the
-@command{ulimit} command on many Unix systems.)
+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
@@ -577,9 +594,15 @@ Here's a summary of our reference solution, produced by the
 @command{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 we started from
-the reference solution to project 2.  @xref{Project 2 FAQ}, for the
-summary of project 2.
+This summary is relative to the Pintos base code, but the reference
+solution for project 3 starts from the reference solution to project 2.
+@xref{Project 2 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.
 
 @verbatim
  Makefile.build       |    4 
@@ -607,6 +630,7 @@ summary of project 2.
 Yes.
 
 @item What extra credit is available?
+@anchor{VM Extra Credit}
 
 You may implement sharing: when multiple processes are created that use
 the same executable file, share read-only pages among those processes
@@ -648,6 +672,12 @@ you like, but it should be the underlying mechanism.
 Also, you can use the @option{-u} option to @command{pintos} to limit
 the size of the user pool, which makes it easy to test your VM
 implementation with various user memory sizes.
+
+@item Data pages might need swap space.  Can I swap them out at process load?
+
+No.  Reading data pages from the executable and writing them to swap
+immediately at program startup is not demand paging.  You need to demand
+page everything (except partial pages).
 @end table
 
 @node Memory Mapped File FAQ