[PATCH] fs: Add romfs version 2

From: Lindsay Roberts
Date: Fri Jul 13 2007 - 02:02:15 EST

* Increases romfs partition size limit from 2GB to 4GB.
* Adds new derivative of romfs filesystem (rom2fs) with
block aligned regular file data to bring performance
parity with ext2/3. This is about 225% of the read
speed of the existing romfs.

Signed-off-by: Lindsay Roberts <lindsay.roberts.os@xxxxxxxxx>
---
genromfs patch available at http://rom2fs.googlepages.com .

Documentation/filesystems/romfs.txt | 34 +++++--
fs/romfs/inode.c | 195 +++++++++++++++++++++++++++++------
include/linux/romfs_fs.h | 1 +
3 files changed, 187 insertions(+), 43 deletions(-)

diff --git a/Documentation/filesystems/romfs.txt
b/Documentation/filesystems/romfs.txt
index 2d2a7b2..170b1cc 100644
--- a/Documentation/filesystems/romfs.txt
+++ b/Documentation/filesystems/romfs.txt
@@ -7,6 +7,10 @@ similar feature, and even the possibility of a small
kernel, with a
file system which doesn't take up useful memory from the router
functions in the basement of your office.

+The romfs version 2 filesystem is a slight derivation created to fix
+performance issues with file data unaligned to logical disk blocks.
+It differs only in its placement of regular file data.
+
For comparison, both the older minix and xiafs (the latter is now
defunct) filesystems, compiled as module need more than 20000 bytes,
while romfs is less than a page, about 4000 bytes (assuming i586
@@ -18,7 +22,10 @@ with romfs, it needed 3079 blocks.

To create such a file system, you'll need a user program named
genromfs. It is available via anonymous ftp on sunsite.unc.edu and
-its mirrors, in the /pub/Linux/system/recovery/ directory.
+its mirrors, in the /pub/Linux/system/recovery/ directory, as well as
+at the sourceforge project http://romfs.sourceforge.net/ . A genromfs
+patch to support version 2 is available at
+http://rom2fs.googlepages.com/ .

As the name suggests, romfs could be also used (space-efficiently) on
various read-only media, like (E)EPROM disks if someone will have the
@@ -43,6 +50,11 @@ from the network, and you will have all the
tools/modules available
from a nearby server, so you don't want to carry two disks for this
purpose, just because it won't fit into ext2.

+romfs also has a secondary use in reproducibility. The absence of
+both timestamps and permission information coupled with the read-only
+nature of the file system gives it amazing capability as a byte
+reproducible medium for a given directory structure.
+
romfs operates on block devices as you can expect, and the underlying
structure is very simple. Every accessible structure begins on 16
byte boundaries for fast access. The minimum space a file will take
@@ -50,7 +62,8 @@ is 32 bytes (this is an empty file, with a less than
16 character
name). The maximum overhead for any non-empty file is the header, and
the 16 byte padding for the name and the contents, also 16+14+15 = 45
bytes. This is quite rare however, since most file names are longer
-than 3 bytes, and shorter than 15 bytes.
+than 3 bytes, and shorter than 15 bytes. romfs version 2 adds
+additional overhead in order to align file data to (1k) disk blocks.

The layout of the filesystem is the following:

@@ -59,7 +72,7 @@ offset content
+---+---+---+---+
0 | - | r | o | m | \
+---+---+---+---+ The ASCII representation of those bytes
- 4 | 1 | f | s | - | / (i.e. "-rom1fs-")
+ 4 | 1 | f | s | - | / (i.e. "-rom1fs-" or "-rom2fs-")
+---+---+---+---+
8 | full size | The number of accessible bytes in this fs.
+---+---+---+---+
@@ -101,8 +114,10 @@ offset content
16 | file name | The zero terminated name of the file,
: : padded to 16 byte boundary
+---+---+---+---+
- xx | file data |
- : :
+ xx | file data | In the case of romfs version 2 - regular
+ : : files, this offset is padded to the next
+ 1024 byte block relative to the start of
+ the filesystem.

Since the file headers begin always at a 16 byte boundary, the lowest
4 bits would be always zero in the next filehdr pointer. These four
@@ -169,11 +184,12 @@ solutions: implement write access as a
compile-time option, or a new,
similarly small writable filesystem for RAM disks.

- Since the files are only required to have alignment on a 16 byte
-boundary, it is currently possibly suboptimal to read or execute files
-from the filesystem. It might be resolved by reordering file data to
-have most of it (i.e. except the start and the end) laying at "natural"
+boundary, it is currently suboptimal to read or execute files from the
+filesystem. It might be resolved by reordering file data to have most
+of it (i.e. except the start and the end) laying at "natural"
boundaries, thus it would be possible to directly map a big portion of
-the file contents to the mm subsystem.
+the file contents to the mm subsystem. This is addressed by romfs
+version 2.

- Compression might be an useful feature, but memory is quite a
limiting factor in my eyes.
diff --git a/fs/romfs/inode.c b/fs/romfs/inode.c
index 2284e03..9fa99d2 100644
--- a/fs/romfs/inode.c
+++ b/fs/romfs/inode.c
@@ -49,6 +49,7 @@
* Aug 1999 2.3.16 __initfunc() => __init change
* Oct 1999 2.3.24 page->owner hack obsoleted
* Nov 1999 2.3.27 2.3.25+ page->offset => index change
+ * Jul 2007 added romfs version 2
*/

/* todo:
@@ -75,6 +76,7 @@
#include <linux/smp_lock.h>
#include <linux/buffer_head.h>
#include <linux/vfs.h>
+#include <linux/mpage.h>

#include <asm/uaccess.h>

@@ -84,10 +86,25 @@ struct romfs_inode_info {
struct inode vfs_inode;
};

-/* instead of private superblock data */
-static inline unsigned long romfs_maxsize(struct super_block *sb)
+struct romfs_fs_info {
+ u32 version;
+ u32 size;
+};
+
+enum {
+ ROMFS_VERSION_1 = 1,
+ ROMFS_VERSION_2 = 2,
+};
+
+/* access private superblock data */
+static inline struct romfs_fs_info *get_romfs_priv(struct super_block *sb)
{
- return (unsigned long)sb->s_fs_info;
+ return (struct romfs_fs_info *) sb->s_fs_info;
+}
+
+static inline u32 romfs_maxsize(struct super_block *sb)
+{
+ return get_romfs_priv(sb)->size;
}

static inline struct romfs_inode_info *ROMFS_I(struct inode *inode)
@@ -95,6 +112,13 @@ static inline struct romfs_inode_info
*ROMFS_I(struct inode *inode)
return list_entry(inode, struct romfs_inode_info, vfs_inode);
}

+/* Returns the block number containing the given byte offset */
+static __u32
+romfs_blocknum(struct super_block *sb, __u32 offset)
+{
+ return (offset - 1) >> sb->s_blocksize_bits;
+}
+
static __u32
romfs_checksum(void *data, int size)
{
@@ -117,7 +141,8 @@ static int romfs_fill_super(struct super_block *s,
void *data, int silent)
struct buffer_head *bh;
struct romfs_super_block *rsb;
struct inode *root;
- int sz;
+ u32 sz;
+ struct romfs_fs_info * rom_info = NULL;

/* I would parse the options here, but there are none.. :) */

@@ -127,39 +152,51 @@ static int romfs_fill_super(struct super_block
*s, void *data, int silent)
bh = sb_bread(s, 0);
if (!bh) {
/* XXX merge with other printk? */
- printk ("romfs: unable to read superblock\n");
+ printk ("romfs: unable to read superblock\n");
goto outnobh;
}

rsb = (struct romfs_super_block *)bh->b_data;
sz = be32_to_cpu(rsb->size);
- if (rsb->word0 != ROMSB_WORD0 || rsb->word1 != ROMSB_WORD1
+ if (rsb->word0 != ROMSB_WORD0
+ || (rsb->word1 != ROMSB_WORD1
+ && rsb->word1 != ROM2SB_WORD1)
|| sz < ROMFH_SIZE) {
if (!silent)
printk ("VFS: Can't find a romfs filesystem on dev "
"%s.\n", s->s_id);
goto out;
}
- if (romfs_checksum(rsb, min_t(int, sz, 512))) {
+ if (romfs_checksum(rsb, min_t(u32, sz, 512))) {
printk ("romfs: bad initial checksum on dev "
"%s.\n", s->s_id);
goto out;
}

s->s_magic = ROMFS_MAGIC;
- s->s_fs_info = (void *)(long)sz;
+
+ rom_info = kmalloc (sizeof(struct romfs_fs_info), GFP_KERNEL);
+ if (!rom_info) {
+ printk ("romfs: not enough memory\n");
+ goto out;
+ }
+ s->s_fs_info = rom_info;
+ rom_info->size = sz;
+ rom_info->version = ROMFS_VERSION_1; /* Default to original romfs */
+
+ if (rsb->word1 == ROM2SB_WORD1) {
+ rom_info->version = ROMFS_VERSION_2;
+ }
+
+ sz=ROMFH_SIZE+((ROMFH_PAD+strnlen(rsb->name,ROMFS_MAXFN)+1)&ROMFH_MASK);

s->s_flags |= MS_RDONLY;

- /* Find the start of the fs */
- sz = (ROMFH_SIZE +
- strnlen(rsb->name, ROMFS_MAXFN) + 1 + ROMFH_PAD)
- & ROMFH_MASK;

s->s_op = &romfs_ops;
root = iget(s, sz);
if (!root)
- goto out;
+ goto outkfree;

s->s_root = d_alloc_root(root);
if (!s->s_root)
@@ -170,12 +207,21 @@ static int romfs_fill_super(struct super_block
*s, void *data, int silent)

outiput:
iput(root);
+outkfree:
+ kfree(rom_info);
out:
brelse(bh);
outnobh:
return -EINVAL;
}

+static void
+romfs_put_super(struct super_block * sb)
+{
+ kfree(sb->s_fs_info);
+ sb->s_fs_info = NULL;
+}
+
/* That's simple too. */

static int
@@ -233,6 +279,36 @@ romfs_strnlen(struct inode *i, unsigned long
offset, unsigned long count)
return res;
}

+/* Fills the private romfs inode struct with version specific data */
+static void
+romfs_fill_private_inode(struct inode *inode, int is_regular)
+{
+ struct romfs_inode_info *romfs_inode = ROMFS_I(inode);
+ int info_len;
+
+ info_len = romfs_strnlen(inode,
+ (inode->i_ino & ROMFH_MASK) + ROMFH_SIZE,
+ ROMFS_MAXFN);
+
+ if (likely(info_len >= 0))
+ info_len = ((ROMFH_SIZE+info_len+1+ROMFH_PAD)&ROMFH_MASK);
+ else
+ info_len = 0;
+
+ romfs_inode->i_metasize = info_len;
+
+ if (get_romfs_priv(inode->i_sb)->version > ROMFS_VERSION_1
+ && is_regular) {
+ /* Data is on logical block directly after end of header */
+ romfs_inode->i_dataoffset =
+ romfs_blocknum(inode->i_sb,
+ inode->i_ino+romfs_inode->i_metasize)+1;
+ } else {
+ /* Data is on 16 byte boundry after header */
+ romfs_inode->i_dataoffset = info_len+(inode->i_ino&ROMFH_MASK);
+ }
+}
+
static int
romfs_copyfrom(struct inode *i, void *dest, unsigned long offset,
unsigned long count)
{
@@ -240,11 +316,13 @@ romfs_copyfrom(struct inode *i, void *dest,
unsigned long offset, unsigned long
unsigned long avail, maxsize, res;

maxsize = romfs_maxsize(i->i_sb);
- if (offset >= maxsize || count > maxsize || offset+count>maxsize)
+ if (unlikely( offset >= maxsize
+ || count > maxsize
+ || offset + count > maxsize))
return -1;

bh = sb_bread(i->i_sb, offset>>ROMBSBITS);
- if (!bh)
+ if (unlikely(!bh))
return -1; /* error */

avail = ROMBSIZE - (offset & ROMBMASK);
@@ -259,7 +337,7 @@ romfs_copyfrom(struct inode *i, void *dest,
unsigned long offset, unsigned long
dest += maxsize;

bh = sb_bread(i->i_sb, offset>>ROMBSBITS);
- if (!bh)
+ if (unlikely(!bh))
return -1;
maxsize = min_t(unsigned long, count - res, ROMBSIZE);
memcpy(dest, bh->b_data, maxsize);
@@ -410,8 +488,8 @@ out: unlock_kernel();
}

/*
- * Ok, we do readpage, to be able to execute programs. Unfortunately,
- * we can't use bmap, since we may have looser alignments.
+ * romfs version one readpage function. File data is unaligned
+ * to logical block, must manually copy to kmap'd page.
*/

static int
@@ -457,12 +535,64 @@ err_out:
return result;
}

+/*
+ * Retrieves the disk logical block given a block relative to a file.
+ * Conforms to include/linux/fs.h:get_block_t
+ */
+int rom2fs_get_block(struct inode *inode, sector_t iblock, struct
buffer_head *bh_result, int create)
+{
+ sector_t disk_block = iblock + ROMFS_I(inode)->i_dataoffset;
+ map_bh(bh_result, inode->i_sb, disk_block);
+ return 0;
+}
+
+
+static int
+rom2fs_readpage(struct file *file, struct page * page)
+{
+ return mpage_readpage(page, rom2fs_get_block);
+}
+
+static int
+rom2fs_readpages(struct file *file, struct address_space * addr_space,
+ struct list_head *pages, unsigned nr_pages)
+{
+ return mpage_readpages(addr_space, pages, nr_pages, rom2fs_get_block);
+}
+
+static sector_t
+rom2fs_bmap(struct address_space *mapping, sector_t block)
+{
+ return generic_block_bmap(mapping, block, rom2fs_get_block);
+}
+
+static ssize_t
+rom2fs_direct_IO(int rw, struct kiocb *iocb, const struct iovec *iov,
+ loff_t offset, unsigned long nr_segs)
+{
+ struct file *file = iocb->ki_filp;
+ struct inode *inode = file->f_mapping->host;
+
+ return blockdev_direct_IO(rw, iocb, inode, inode->i_sb->s_bdev, iov,
+ offset, nr_segs, rom2fs_get_block, NULL);
+}
+
/* Mapping from our types to the kernel */

static const struct address_space_operations romfs_aops = {
.readpage = romfs_readpage
};

+
+/* Operations for rom2fs - make use of block aligned file data */
+
+static const struct address_space_operations rom2fs_aops = {
+ .readpage = rom2fs_readpage,
+ .readpages = rom2fs_readpages,
+ .bmap = rom2fs_bmap,
+ .direct_IO = rom2fs_direct_IO,
+};
+
static const struct file_operations romfs_dir_operations = {
.read = generic_read_dir,
.readdir = romfs_readdir,
@@ -508,21 +638,14 @@ romfs_read_inode(struct inode *i)
i->i_mtime.tv_nsec = i->i_atime.tv_nsec = i->i_ctime.tv_nsec = 0;
i->i_uid = i->i_gid = 0;

- /* Precalculate the data offset */
- ino = romfs_strnlen(i, ino+ROMFH_SIZE, ROMFS_MAXFN);
- if (ino >= 0)
- ino = ((ROMFH_SIZE+ino+1+ROMFH_PAD)&ROMFH_MASK);
- else
- ino = 0;
-
- ROMFS_I(i)->i_metasize = ino;
- ROMFS_I(i)->i_dataoffset = ino+(i->i_ino&ROMFH_MASK);
+ /* Precalculate the data offset */
+ romfs_fill_private_inode(i, ((nextfh & ROMFH_TYPE) == ROMFH_REG));

- /* Compute permissions */
- ino = romfs_modemap[nextfh & ROMFH_TYPE];
+ /* Compute permissions */
+ ino = romfs_modemap[nextfh & ROMFH_TYPE];
/* only "normal" files have ops */
switch (nextfh & ROMFH_TYPE) {
- case 1:
+ case ROMFH_DIR:
i->i_size = ROMFS_I(i)->i_metasize;
i->i_op = &romfs_dir_inode_operations;
i->i_fop = &romfs_dir_operations;
@@ -530,14 +653,17 @@ romfs_read_inode(struct inode *i)
ino |= S_IXUGO;
i->i_mode = ino;
break;
- case 2:
+ case ROMFH_REG:
i->i_fop = &generic_ro_fops;
- i->i_data.a_ops = &romfs_aops;
+ if (get_romfs_priv(i->i_sb)->version == ROMFS_VERSION_1)
+ i->i_data.a_ops = &romfs_aops;
+ else
+ i->i_data.a_ops = &rom2fs_aops;
if (nextfh & ROMFH_EXEC)
ino |= S_IXUGO;
i->i_mode = ino;
break;
- case 3:
+ case ROMFH_SYM:
i->i_op = &page_symlink_inode_operations;
i->i_data.a_ops = &romfs_aops;
i->i_mode = ino | S_IRWXUGO;
@@ -602,6 +728,7 @@ static const struct super_operations romfs_ops = {
.read_inode = romfs_read_inode,
.statfs = romfs_statfs,
.remount_fs = romfs_remount,
+ .put_super = romfs_put_super,
};

static int romfs_get_sb(struct file_system_type *fs_type,
@@ -624,7 +751,7 @@ static int __init init_romfs_fs(void)
int err = init_inodecache();
if (err)
goto out1;
- err = register_filesystem(&romfs_fs_type);
+ err = register_filesystem(&romfs_fs_type);
if (err)
goto out;
return 0;
diff --git a/include/linux/romfs_fs.h b/include/linux/romfs_fs.h
index e20bbf9..ab7164b 100644
--- a/include/linux/romfs_fs.h
+++ b/include/linux/romfs_fs.h
@@ -15,6 +15,7 @@
#define __mk4(a,b,c,d) cpu_to_be32(__mkl(__mkw(a,b),__mkw(c,d)))
#define ROMSB_WORD0 __mk4('-','r','o','m')
#define ROMSB_WORD1 __mk4('1','f','s','-')
+#define ROM2SB_WORD1 __mk4('2','f','s','-')

/* On-disk "super block" */
-
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