GFS2 - GeekOS FileSystem #2

The purpose of this project is to add a writable filesystem, GFS2, to GeekOS. Unlike the existing PFAT, GFS2 includes directories, inodes, direntries, etc. The new filesystem will reside on the second IDE disk drive in the emulator. The PFAT drive will continue to hold user programs while you format and test your implementation of GFS2. The second IDE disk's image is called diskd.img. It is 10MB by default.

VFS and file operations

Since GeekOS will have two types of filesystems (PFAT and GFS2), it will have a virtual filesystem layer (VFS) to direct requests to an appropriate filesystem (see figure below). We have provided an implementation of the VFS layer in the file vfs.c. The VFS layer will call the appropriate GFS2 routines when a file operation refers to a file in GFS2.

The implementation of PFAT is in pfat.c. You will implement GFS2 in gfs2.c and relevant system calls in syscall.c.

VFS picks the functions to call based on supplied structures containing function pointers, one for each operation. For example, see Init_PFAT in pfat.c: this initializes the PFAT filesystem with VFS by passing it a pointer to s_pfatFilesystemOps, a structure that contains function pointers to PFAT routines for mounting (and formatting) a filesystem. Other PFAT functions are stored in different structures (e.g., look at the PFAT_Open routine, which passes the s_pfatFileOps structure to VFS). You will analogously use s_gfs2FilesystemOps, s_gfs2MountPointOps, s_gosfsDirOps, and s_gfs2FileOps in gfs2.c. You should also add a call to Init_GFS2 provided gfs2.c in main.c to register the GFS2 filesystem. In general, use the PFAT implementation as your guide to interfacing GFS2 with VFS.

Open files are tracked by the kernel using a file descriptor, defined as struct File. Each user space process will have an associated file descriptor table that records which files the process can currently read and write. A user process can have up to USER_MAX_FILES files open at once. The file descriptor table is implemented as a struct File *fileList[USER_MAX_FILES] array that you must add to struct User_Context. Note that not all the entries in the fileList are necessarily open files, since usually a process has less than USER_MAX_FILES files open at once. If fileList[i] is NULL, it represents a free slot (file descriptor is not used). This descriptor will be filled out by the code in VFS; e.g., see Open in vfs.h, whose pFile argument is a pointer to a free slot in the table.

GFS2 Data Types

Primitive types

gfs2_blocknum

The number of the block (not sector) on disk.

gfs2_inodenum

The number of the inode. (not the block containing the inode.)

Inode

You know the inode. The size of the inode is constrained so that an even number of inodes fit into each block (inodes won't span blocks).

Type can be either:

GFS2_DIRECTORY

It's a directory. It won't be read or written directly by applications.

GFS2_FILE

It's a file.

Dirent

Figure caveat: the lengths aren't pointers; the alignment padding isn't explicit.

Directories are files consisting of appended records of type gfs2_dirent. Note that you will be overflowing the array to write names of any length; sizeof(struct gfs2_dirent) will very likely confuse. To calculate the size for a new dirent, round 4 + 2 + name_length to the nearest multiple of four. A helper function would be a good idea here.

Some code for seeking through the list in a directory is in lecture notes: to find an entry, first look for entries that match the length.

You do have to support deletion; I believe there is an obvious way to remove from the middle of this list without rewriting the directory, so I leave it to you to determine.

Superblock

The superblock contains the file system parameters. It must be stored at offset 1024 (512-byte block #3). You can expect it to be there on mount. If it's not there, the file system is broken and would have to be repaired before being mounted.

A replica of the superblock will, when you format, be added at number-of-blocks / 2. That's it. On format, just one. The block index of the replica superblock is in the filesystem-numbered block size, not the 512-byte block space (like block #3). 512-block #3 is assumed. All the other replicas will be at block "0", which means not replicated.

The Empty Directory

When creating a directory, it is not entirely empty. Each directory has both '.' and '..' entries. The root directory will have contents:

If you're wondering why inode 1 is represented using 01 00 00 00, smack yourself. LSB.

I encourage you to not write these 16 bytes, but instead to write a dirent constructor function to use to build the directory.

Free Blocks Bitmap

Track free disk blocks using a bit vector. A library called bitset is provided (see bitset.h and bitset.c) that manages a set of bits and provides functions to find bits that are 0, which correspond to free disk blocks (note that this is the opposite of the textbook, which uses a 1 to indicate a free block).

The free disk blocks map is stored within the file system using inode 2.

Mounting

The mount system call allows you to associate a filesystem with a place in the file name hierarchy. The Mount call is implemented as part of the VFS code we supply (see Mount function in vfs.c); you will implement your Init_GFS2 function so that VFS's mount code will call your function GFS2_Mount() in gfs2.c. Among other things, it must ensure that the filesystem being mounted is GFS2.

Buffer Cache

The buffer cache allows you to keep recently-accessed blocks in memory for a while (for example, the inode for a directory), and hold in-progress modifications to blocks until all the writes to that block are done and can be committed back.

Each buffer in the cache has a block number (the backing store block number), the buffer in memory, and two flags:

in use

Don't touch it, it's in use. Some thread is reading from it or writing to it. Or it's going to disk. The flag is set when you Get_FS_Buffer(), and cleared when you Release_FS_Buffer(). (Releasing doesn't remove the buffer from cache, it just marks it not in use.) If the flag has been set when Get_FS_Buffer() is called, you'll block, so pretend it is a lock acquisition.

dirty

It has been modified. Mark a buffer dirty using Modify_FS_Buffer(). The buffer cache will take care of eventually writing the block back to disk, in whatever order it chooses.

Call Create_FS_Buffer_Cache() to make a new buffer cache for the device; it is given a block size at creation time.

Call Sync_FS_Buffer() to push back specific blocks, and Sync_FS_Buffer_Cache() to push them all back. Ideally, one would use Sync_FS_Buffer for flushing the blocks of a file on close or of filesystem metadata eagerly. One would use Sync_FS_Buffer_Cache to sync the whole filesystem. (i.e., you've run the "sync" program before, right?)

There's a Destroy_FS_Buffer_Cache() function too. I can't see a purpose to it in this assignment.

Format

1. Get the drive's device name, size, and block size from the command line.
2. Figure out the Free Blocks Bitmap size, mark the inode-bearing blocks (first 1/64th) used, mark the replica superblock used. You may either mark the blocks that will store the free blocks bitmap and root directory as used now, or mark them when you allocate those blocks.
3. Create a valid, but empty directory in inode 1. Mark its ref count as 1.
4. Create a the free blocks bitmap file in inode 2. Mark its ref count as 1.
5. Create a the free list of inodes. For inode 3..n, make the self inode pointer the i+1, until for inode n it is zero. Ensure each of the unused inodes has reference count zero.
6. Write the superblocks as the last step.

New System Calls

You will implement new system calls as described below. As you can see, the semantics is very similar to the UNIX file system. Some notes:

• All user-supplied pointers (e.g. strings, buffers) must be checked for validity.
• Some of the checks in 'Reason for failure' column are already done by vfs.c so you don't have to perform the check if it's implemented for you already.

Call	User Function	Return on success	Return on failure	Reasons for failure	Comment
SYS_MOUNT	Mount(char *dev, char *prefix, char *fstype)	0	-1	a filesystem already mounted under name illegal value for one of the parameters
SYS_OPEN	Open(char *path, int mode)	new file descriptor number	-1	name does not exist (if permissions don't include O_CREATE ) path to name does not exist (if permissions include O_CREATE ) attempt to open a directory, (should use OpenDirectory)	there's no create syscall, so setting O_CREATE will create the file. If the file exists, the call succeeds (return >= 0) but its data contents is not affected. Should NOT create directories; e.g. if you do Open("/d/d1/d2/d3/xFile", O_CREATE) and the leading path /d/d1/d2/d3 does not exist, the syscall fails, returning -1 You should use the Allocate_File in vfs.c to get a File when opening. The permissions values are flags and may be or'ed together in a call. For example: O_CREATE | O_READ O_READ | O_WRITE O_CREATE | O_READ | O_WRITE
SYS_OPENDIRECTORY	Open_Directory(char *path)	new file descriptor number	-1	path does not exist attempt to open a file, (should use Open)	Should NOT create directories, use CreateDirectory instead. You should use the Allocate_File in vfs.c to get a File when opening.
SYS_CLOSE	Close(int fd)	0	-1	fd not within range 0-(USER_MAX_FILES-1) fd is not an open file
SYS_DELETE	Delete(char *path)	0	-1	name does not exist name is a non-empty directory	if Delete(file) is called and file is still open in other threads or even in the thread that called Delete(), all the subsequent operations on that file (except Close()) should fail
SYS_READ	Read(int fd, char *buffer, int length)	number of bytes read	-1	fd not in range fd is not an open file fd was not open with O_READ flag fd is a directory	it's OK if return value < length, for instance reading close to end of file increase the filePos (the current position in the file) by the number of bytes read (if successful)
SYS_READENTRY	Read_Entry(int fd, VFS_Dir_Entry *entry)	0 if entry is valid >0 if end of directory reached	-1	fd not in range fd is not an open directory	copy over directory entry info from a GOSF_Dir_Entry into a VFS_Dir_Entry skip over unused entries increase the filePos, if successful VFS_Dir_Entry is defined in fileio.h
SYS_WRITE	Write(int fd, char *buffer, int length)	number of bytes written	-1	fd not in range fd is not an open file fd was not open with O_WRITE flag fd is a directory	increases filePos by the number of bytes written, if successful "Grow on write"- allocate blocks "on the fly" if past end of file "Grow minimum" - allocate only the needed blocks. A seek to a position past the end of the file should be allowed and a write will allocate only a single block rather than all blocks in between.
SYS_STAT	Stat(char *path, VFS_File_Stat *stat)	0	-1	path is not valid	VFS_File_Stat is defined in fileio.h
SYS_FSTAT	FStat(int fd, VFS_File_Stat *stat)	0	-1	fd not in range fd is not an open file	VFS_File_Stat is defined in fileio.h
SYS_SEEK	Seek(int fd, int offset)	0	-1	fd not in range fd is not an open file	offset is an absolute position; for files: could be equal to fileSize or greater, then Write appends, see above. The behavior of seek() on gfs2 directories shall be undefined; the behavior of readdir after seek similarly undefined.
SYS_CREATEDIR	Create_Directory(char *path)	0	-1	name already exists, as file or directory regular file encountered on the path to name	Should NOT create directories recursively; e.g. CreateDirectory("/d/d1/d2/d3/d4"), will fail if /d/d1/d2/d3 does not exist already.
SYS_SYNC	Sync(void)	0	-1	None	Forces any outstanding operations to be written to disk.

Testing and Requirements

Requirements

Here are some must do's for the project:

• Make sure your Mount() works well, so that we can test your project. If we cannot Mount() a GFS2, we cannot grade your project.
• You might also want to mount /d to ide1 automatically in main() to speed up your testing, but the code you submit should not mount /d automatically.
• You should support disk sizes of at least 32 MB. More than 32 MB is optional. You can create an arbitrarily-sized diskd.img using the following procedure:
  • Change the size/disk geometry by changing the diskd line in .bochsrc
  • Change the argument to $(ZEROFILE) in Makefile. For $(ZEROFILE) one block is 512, so 4096 blocks = 2 MB, 65536 blocks = 32 MB and so on.
  In your submitted project, when someone types gmake, a 32 MB diskd.img file should be created.
• You should support file sizes of between 0 and at least XXXX (was 5MB) MB. More than XXXX MB is optional.

Testing

Study questions

Consider the difference between this mini inode and a more realistic inode; what do others maintain, and why does gfs2 not include it?

Consider the difference between the original unix file system and FFS; in your implementation, how often do you jump between reading inodes and reading data blocks?

Consider readent (SYS_READENTRY); does it make sense to have this be a system call? "strace ls" on a linux machine. What system call does it use instead? Check the man page and explain why that's a better design.

Using your experience from this project, what does it mean for a filesystem to have been cleanly unmounted? How does the kernel know if a filesystem is clean when it is being mounted? What would have to be added to this project to allow that decision? How would one check and repair GFS2?