Programming Project 7:

File-Related Syscalls

 

 

 

 

Due Date: ______________________________

Project Duration:  One week

 

 

 

Overview and Goal

 

In this project, you will implement the syscalls relating to file I/O: Open, Read, Write, Seek, and Close.  These syscalls will allow user-level processes to read and write to files stored on the BLITZ DISK file.  The goal is for you to understand the syscall mechanism in more detail, to understand what happens when several processes operate on a shared file system, and to understand how the kernel buffers and moves data between the file system on the disk and the user-level processes.

 

 

 

Download New Files

 

The files for this project are available in:

 

            http://www.cs.pdx.edu/~harry/Blitz/OSProject/p7/

 

The following files are new to this project:

 

     TestProgram4.h

     TestProgram4.c

     Program1.h

     Program1.c

     Program2.h

     Program2.c

 

The following files have been modified from the last project:

 

     makefile

     DISK

 

The makefile has been modified to compile TestProgram4, Program1, and Program2.  The DISK file has been enlarged, since the previous version was too small to accommodate TestProgram4, Program1, and Program2.

 

All remaining files are unchanged from the last project.

 

 

 

The Task:

 

Implement the following syscalls:

 

            Open

            Read

            Write

            Seek

            Close

 

Update your code as necessary to account for the possibility of open files.  You’ll probably need to modify these routines, too:

 

            Handle_Sys_Fork

            ProcessFinish

 

Note that files may be open in a process that invokes the Exec syscall.  These files should remain open in the process after the Exec completes successfully.  Does this require a change to your code?

 

Take a look at the project 5 document for details on the specifications of each of these syscalls.  Please re-read that document thoroughly.  In particular, the following sections of that document contain important information that you’ll need to complete this project.

 

            The “Stub” File System

            The “diskUtil” Tool

            The FileManager

            FileControlBlock (FCB) and OpenFile

 

 

 

Limitations to the “Stub” Filesystem:

 

New files cannot be added to the disk.  We will not implement the Create syscall in this project.  All files must already be on the DISK before they can be opened and accessed.  (Files can be added to the BLITZ DISK with the diskUtil command.)

 

Also, we will not be able to change the size of a file.  If the user-level program invokes Write with arguments that would cause the bytes transferred to be beyond the current end of the file, the bytes that are not before the file end should be transferred, but the bytes beyond the file end will not be transferred.  The Write will return the number of bytes successfully written.  For example, if the file size is 10 and Write is invoked with a length of 5 on a file whose current position is 7, then 3 bytes should be transferred and the 2 bytes beyond the end of the file will not be transferred.  The Write should return 3.

 

 

 

Implementing Handle-Sys-Open:

 

Here are some actions you’ll need to take when to implementing the Open syscall:

 

            Copy the filename string from virtual space to a small buffer in this routine.

            Make sure the length of the filename does not exceed MAX_STRING_SIZE

            Locate an empty slot in this process’s fileDescriptor array.

            If there are no empty slots, return –1.

            Allocate an OpenFile object (see FileManager.Open).

            If this fails, return –1.

            Set the entry in the fileDescriptor array to point to the OpenFile object.

            Return the index of the fileDescriptor array element.

 

 

 

Suggestions Concerning the Algorithm For Handle-Sys-Read:

 

Here is a possible approach to implementing the Read syscall.

 

You’ll need to begin by checking that the fileDesc argument really is valid.  The user must provide a legal index into the fileDescriptor array and the file referenced must have been previously opened.  If not, the Read syscall should return –1.

 

You’ll also need to check that the requested number of bytes (the sizeInBytes argument) is not negative.  If it is, then return –1.

 

We’ll discuss problems related to the buffer argument later.

 

When a user-level program invokes the Read syscall, it asks for a sequence of bytes to be fetched from a file on disk.  Unfortunately, this sequence may span several sectors in the disk file, so several calls to DiskDriver.SynchReadSector will be needed to read all the data.

 

Each call to DiskDriver.SynchReadSector will read an entire 8K sector from the disk into memory.  The caller will provide the address of where in memory to read the sector, which we can refer to as the “sector buffer” area in memory.

 

The data requested by the user-level process will not necessarily begin or end on a sector boundary.  So some buffering and movement of the data will be necessary.  Take a look at FileManager.SynchRead, which will perform a lot of the work you’ll need to do when implementing the Read syscall.  In particular, it will call DiskDriver.SynchReadSector to read a sector into the sector buffer and then it will copy the desired bytes from the sector buffer to wherever they should go.  (This is called the “target address.”)

 

Each FileControlBlock already has a sector buffer associated with it.  This is an 8K memory region that was allocated from the pool of memory frames when the FileManager was initialized.  [You do not need to allocate any memory buffers in this project.]

 

If the requested sequence of bytes spans several sectors, FileManager.SynchRead will read as many sectors as necessary (re-using the one sector buffer) and, after each disk I/O completes, it will copy bytes in several chunks to the target area.  (Also, if the sector buffer is dirty before it starts, it will first write the old sector out to disk.)

 

When a user requests a sequence of bytes to be read from a file, the user provides a pointer to a “user-space target area,” which tells the kernel where to copy this data to.  User-level code often refers to its target area as a “buffer” but be careful to avoid confusing this area with the “sector buffer,” which is part of the FileControlBlock in kernel space.  The user will supply a virtual address for the user target area by supplying values called buffer and sizeInBytes to the Read syscall.

 

When implementing the Read syscall, you’ll need to translate the target address from a virtual address into a physical address, so you can know where in memory to move the data.

 

Unfortunately, the user target area may cross page boundaries in the virtual address space.  In general, the pages of the user’s address space will not be in contiguous memory frames.  FileManager.SynchRead cannot deal with this; it expects its target area to be one contiguous region in memory.

 

This means that you cannot simply call FileManager.SynchRead once to get the job done.  You’ll need to break the user target area into chunks such that each chunk is entirely within one page.  Then, you can call FileManager.SynchRead to read each of these chunks in a loop.

 

Below is some pseudo-code showing how this loop might work.  It works by breaking the entire sequence of bytes to be read into several chunks.  Each chunk is entirely on one page and does not cross a page boundary.  It computes the length and starting address of each chunk.  It translates the starting address into a physical address.  Then it calls FileManager.SynchRead to read the chunk and moves on the next chunk.

 

The key variables are:

 

virtPage             Virtual address into which to read the next chunk (virtual address page number)

offset                  Virtual address into which to read the next chunk (offset into the page)

chunkSize         The number of bytes to be read for this chunk

nextPosInFile   The position in the file from which to read the next chunk

copiedSoFar     The number of bytes read from disk so far

 

At the beginning of each loop iteration, virtPage and offset tell where the first byte in the next chunk should go.  Each iteration computes the size of the next chunk, does the read, and then adjusts all the variables.  nextPosInFile tells where in the file the next chunk to be read begins.  Initially, it will be given by the current position in the file, but will be adjusted after each chunk is read.

 

virtAddr = buffer asInteger

virtPage = virtAddr / PAGE_SIZE

offset = virtAddr % PAGE_SIZE

copiedSoFar = 0

nextPosInFile = ...

 

-- Each iteration will compute the size of the next chunk

-- and process it...

while true

 

  -- Compute the size of this chunk...

  thisChunkSize = PAGE_SIZE-offset

  if nextPosInFile + thisChunkSize > sizeOfFile

    thisChunkSize = sizeOfFile - nextPosInFile

  if copiedSoFar + thisChunkSize > sizeInBytes

    thisChunkSize = sizeInBytes - copiedSoFar

 

  -- See if we are done...

  if thisChunkSize <= 0

    exit loop

 

  -- Check for errors...

  if (virtPage < 0) or

     (page number is too large) or

     (page is not valid) or

     (page is not writable)

    deal with errors

 

  -- Do the read...

  Set “DirtyBit” for this page

  Set “ReferencedBit” for this page

  Compute destAddr = addrSpace.ExtractFrameAddr (virtPage) + offset

  Perform read into destAddr, nextPosInFile, chunkSize bytes

 

  -- Increment...

  nextPosInFile = nextPosInFile + thisChunkSize

  copiedSoFar = copiedSoFar + thisChunkSize

  virtPage = virtPage + 1

  offset = 0

 

  -- See if we are done...

  if copiedSoFar == sizeInBytes

    exit loop

 

endWhile

 

Note that we are marking the page in the user’s virtual address space as “dirty.”  Think carefully about why a “read” would mark the page dirty.  When implementing the Write syscall, the operation will not cause the page to become dirty!

 

When a user-program reads data from disk to memory, the page in memory is changed.  So the page that receives the data from disk must be marked as dirty.  When a user-program writes data from memory to disk, the page in memory is unchanged.  The page would not be marked dirty.

 

You also need to set the Referenced Bit for all pages accessed, regardless of whether the user is invoking the Read or Write syscall, since in either case, the page has been used.

 

Normally, the MMU will set the Dirty Bit and Referenced Bit when appropriate, but this will only occur when paging is turned on.  When the kernel handles the Read and Write syscalls, it will be accessing the pages directly, using their physical memory addresses.  This will bypass the MMU, so the kernel code will need to change the bits explicitly.

 

[Setting the Dirty Bit correctly will be necessary if we implement virtual memory in a later project.  A failure to set the dirty bit correctly might occasionally result in data that gets lost when a page that should be copied to disk is not copied.  A failure to set the referenced bit may have repercussions for the page replacement algorithm, causing it to malfunction.  The resulting thrashing or poor performance might be very, very difficult to debug and fix!]

 

It is possible that the user will provide bad arguments to the Read syscall.  For example, the user target area may lie outside of the virtual address space or may be in a page that is not writable.  In such a case, the kernel should detect the error and return –1.  Furthermore, if such a problem with the parameters occurs, the kernel should not perform any disk operation.  In other words, the user target area should be entirely unchanged.

 

This is not how the above code works.  If, for example, the user target area runs past the end of the virtual address space, the above code may read several chunks successfully before encountering the error and aborting.

 

To perform correctly, you’ll actually need to do the work using two loops.  You’ll execute one loop to completion, then execute the second loop.  Both loops will be just like the one shown above, except that the first loop will not actually perform the reading.  The sole purpose of the first loop is to check the user target area and return from the syscall if problems.  The second loop will repeat the computations exactly and will also perform the reading operations.

 

FileManager.SynchRead will never fail; as coded it always returns true.

 

A similar approach to Handle_Sys_Read can be taken for Handle_Sys_Write.

 

 

 

Implementing Handle-Sys-Seek:

 

Here are some actions you’ll need to take when to implementing the Seek syscall:

 

      Lock the FileManager since we will be updating shared data (the OpenFile.currentPos)

      Check the fileDesc argument and get a pointer to the OpenFile object.

      Make sure the file is open.  (Recall that a null entry in fileDescriptors means “not open.”)

      Deal with the possibility that the new current position is equal to –1, which has a special meaning.

      Deal with the possibility that the new current position is less that –1.  (Zero is okay.)

      Deal with the possibility that the new current position is greater than the file size.

      Update the current position.

      Return the new current position.

 

Remember to unlock the FileManager, regardless of whether you are making a normal return or an error return.

 

 

 

The User-Level Programs

 

The p7 directory contains a new user-level program called:

 

      TestProgram4

 

Please change InitUserProcess to load TestProgram4 as the initial process.  Then run it  several times, once for each of the tests it contains.

 

After you have finished coding and debugging, please run each test once and hand in the output from each test.  A separate document shows more-or-less what the correct output should look like.

 

Use the same code to execute all tests.  Please hand in only one copy of your Kernel.c code and do not hand in any output that was produced by a different version of your code!

 

Do not change TestProgram4, except to uncomment one of the lines in the main function.

 

During your testing, it may be convenient to modify the tests as you try to see what is going on and get things to work.  Before you make your final test runs, please recopy TestProgram4.c from our directory, so that you get a fresh, unaltered version.

 

Please hand in hardcopy of Kernel.c.  You only need to hand in...

 


            Handle_Sys_Open

            Handle_Sys_Close

            Handle_Sys_Read

            Handle_Sys_Write

            Handle_Sys_Seek

 

and any other code you wrote / modified, like Handle_Sys_Fork and ProcessFinish.

 

 

 

 

Desired Output

 

There is some sample output, collected together into a separate file called DesiredOutput.pdf in the p7 directory.