Invert the priority scheme, so that PRI_MIN is now the lowest priority

[pintos-anon] / doc / vm.texi

diff --git a/doc/vm.texi b/doc/vm.texi
index 544701a5b90844f24588e3d5d7dadc81d0244ec9..e6ff141067f58ce2b80cbb2f2a807a018267519d 100644 (file)
--- a/doc/vm.texi
+++ b/doc/vm.texi
@@ -316,11 +316,12 @@ Some way of translating in software from virtual page frames to
 physical page frames.  Pintos provides a hash table that you may find
 useful for this purpose (@pxref{Hash Table}).
 
 physical page frames.  Pintos provides a hash table that you may find
 useful for this purpose (@pxref{Hash Table}).
 

-It is possible to do this translation without adding a new data
-structure, by modifying the code in @file{userprog/pagedir.c}.  However,
-if you do that you'll need to carefully study and understand section 3.7
-in @bibref{IA32-v3}, and in practice it is probably easier to add a new
-data structure.
+You don't strictly need a new data structure for this.  You could
+instead modify the code in @file{userprog/pagedir.c}.  If you do that
+you'll need to thoroughly understand how 80@var{x}86 page tables work
+by, e.g.,@: studying section 3.7, ``Page Translation Using 32-Bit
+Physical Addressing,'' in @bibref{IA32-v3a}.  In practice, most groups
+use a separate data structure.

 
 @item
 Some way of finding a page on disk (in a file or in swap) if it is not
 
 @item
 Some way of finding a page on disk (in a file or in swap) if it is not


@@ -419,11 +420,10 @@ Bits}) to implement an approximation to LRU.  Your algorithm should
 perform at least as well as the ``second chance'' or ``clock''
 algorithm.
 
 perform at least as well as the ``second chance'' or ``clock''
 algorithm.
 

-Your design should allow for parallelism.  Multiple processes should
-be able to process page faults at once.  If one page fault require
+Your design should allow for parallelism.  If one page fault requires

 I/O, in the meantime processes that do not fault should continue
 I/O, in the meantime processes that do not fault should continue

-executing and other page faults that do not require I/O should be able to
-complete.  These criteria require some synchronization effort.
+executing and other page faults that do not require I/O should be able
+to complete.  These criteria require some synchronization effort.

 
 @node Lazy Loading
 @subsection Lazy Loading
 
 @node Lazy Loading
 @subsection Lazy Loading


@@ -440,25 +440,25 @@ be written to swap.
 
 The core of the program loader is the loop in @func{load_segment} in
 @file{userprog/process.c}.
 
 The core of the program loader is the loop in @func{load_segment} in
 @file{userprog/process.c}.

-Each time around the loop, @code{read_bytes} receives the number of
-bytes to read from the executable file and @code{zero_bytes} receives
+Each time around the loop, @code{page_read_bytes} receives the number of
+bytes to read from the executable file and @code{page_zero_bytes} receives

 the number of bytes to initialize to zero following the bytes read.  The
 two always sum to @code{PGSIZE} (4,096).  The handling of a page depends
 on these variables' values:
 
 @itemize @bullet
 @item
 the number of bytes to initialize to zero following the bytes read.  The
 two always sum to @code{PGSIZE} (4,096).  The handling of a page depends
 on these variables' values:
 
 @itemize @bullet
 @item

-If @code{read_bytes} equals @code{PGSIZE}, the page should be demand
+If @code{page_read_bytes} equals @code{PGSIZE}, the page should be demand

 paged from disk on its first access.
 
 @item
 paged from disk on its first access.
 
 @item

-If @code{zero_bytes} equals @code{PGSIZE}, the page does not need to
+If @code{page_zero_bytes} equals @code{PGSIZE}, the page does not need to

 be read from disk at all because it is all zeroes.  You should handle
 such pages by creating a new page consisting of all zeroes at the
 first page fault.
 
 @item
 be read from disk at all because it is all zeroes.  You should handle
 such pages by creating a new page consisting of all zeroes at the
 first page fault.
 
 @item

-If neither @code{read_bytes} nor @code{zero_bytes} equals
+If neither @code{page_read_bytes} nor @code{page_zero_bytes} equals

 @code{PGSIZE}, then part of the page is to be read from disk and the
 remainder zeroed.  This is a special case.  You are allowed to handle
 it by reading the partial page from disk at executable load time and
 @code{PGSIZE}, then part of the page is to be read from disk and the
 remainder zeroed.  This is a special case.  You are allowed to handle
 it by reading the partial page from disk at executable load time and


@@ -483,8 +483,7 @@ 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
 
 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
 
 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


@@ -500,9 +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.
 
 @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.
 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
 
 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


@@ -580,9 +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.
 
 @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 
 
 @verbatim
  Makefile.build       |    4 


@@ -605,7 +625,7 @@ summary of project 2.
  17 files changed, 1532 insertions(+), 104 deletions(-)
 @end verbatim
 
  17 files changed, 1532 insertions(+), 104 deletions(-)
 @end verbatim
 

-@item Do we need a working HW 2 to implement HW 3?
+@item Do we need a working Project 2 to implement Project 3?

 
 Yes.
 
 
 Yes.
 


@@ -649,7 +669,7 @@ kernel functions need to obtain memory.
 You can layer some other allocator on top of @func{palloc_get_page} if
 you like, but it should be the underlying mechanism.
 
 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{-u} option to @command{pintos} to limit
+Also, you can use the @option{-ul} 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.
 
 the size of the user pool, which makes it easy to test your VM
 implementation with various user memory sizes.
 


@@ -692,7 +712,7 @@ with the file mapped into your address space, you can directly address
 it like so:
 
 @example
 it like so:
 
 @example

-write (addr, 64, STDOUT_FILENO);
+write (STDOUT_FILENO, addr, 64);

 @end example
 
 Similarly, if you wanted to replace the first byte of the file,
 @end example
 
 Similarly, if you wanted to replace the first byte of the file,