Re: AZFS file system proposal

[Dmitri Vorobiev <dmitri.vorobiev@movial.fi>]

Maxim Shchetynin wrote:
> Hello,
> 
> there are some users which have interest on such kind of file system like azfs. Please, have a look at this version of a diff file which introduces a first version of azfs to 2.6.26. This file system may be useful for example on IBM CellBlades where user can mount DDR2 memory of Axon controller as a disk and to be able to access it directly without any caching mechanism in between.
> 
> Subject: azfs: initial submit of azfs, a non-buffered filesystem
> 
> From: Maxim Shchetynin <maxim@de.ibm.com>
> 
> Non-buffered filesystem for block devices with a gendisk and
> with direct_access() method in gendisk->fops.
> AZFS does not buffer outgoing traffic and is doing no read ahead.
> It supports mount options (given with -o) bs=x,uid=x,gid=x.
> If block-size (bs) is not specified AZFS uses block-size used
> by block device. Though mmap() method is available only if
> block-size equals to or is greater than the system page size.
> 
> Signed-off-by: Maxim Shchetynin <maxim@de.ibm.com>
> 
> diff -Nuar linux-2.6.26-rc5/arch/powerpc/configs/cell_defconfig linux-2.6.26-rc5-azfs/arch/powerpc/configs/cell_defconfig
> --- linux-2.6.26-rc5/arch/powerpc/configs/cell_defconfig	2008-06-05 05:10:44.000000000 +0200
> +++ linux-2.6.26-rc5-azfs/arch/powerpc/configs/cell_defconfig	2008-06-06 11:53:34.000000000 +0200
> @@ -240,6 +240,7 @@
>  # CPU Frequency drivers
>  #
>  CONFIG_AXON_RAM=m
> +CONFIG_AZ_FS=m
>  # CONFIG_FSL_ULI1575 is not set
>  
>  #
> diff -Nuar linux-2.6.26-rc5/fs/Kconfig linux-2.6.26-rc5-azfs/fs/Kconfig
> --- linux-2.6.26-rc5/fs/Kconfig	2008-06-05 05:10:44.000000000 +0200
> +++ linux-2.6.26-rc5-azfs/fs/Kconfig	2008-06-06 16:55:11.616419992 +0200
> @@ -360,6 +360,17 @@
>  	  If you are not using a security module that requires using
>  	  extended attributes for file security labels, say N.
>  
> +config AZ_FS
> +	tristate "AZFS filesystem support"
> +	help
> +	  Non-buffered filesystem for block devices with a gendisk and
> +	  with direct_access() method in gendisk->fops.
> +	  AZFS does not buffer outgoing traffic and is doing no read ahead.
> +	  It supports mount options (given with -o) bs=x,uid=x,gid=x.
> +	  If block-size (bs) is not specified AZFS uses block-size used
> +	  by block device. Though mmap() method is available only if
> +	  block-size equals to or is greater than system page size.
> +
>  config JFS_FS
>  	tristate "JFS filesystem support"
>  	select NLS
> diff -Nuar linux-2.6.26-rc5/fs/Makefile linux-2.6.26-rc5-azfs/fs/Makefile
> --- linux-2.6.26-rc5/fs/Makefile	2008-06-05 05:10:44.000000000 +0200
> +++ linux-2.6.26-rc5-azfs/fs/Makefile	2008-06-06 11:53:34.000000000 +0200
> @@ -119,3 +119,4 @@
>  obj-$(CONFIG_DEBUG_FS)		+= debugfs/
>  obj-$(CONFIG_OCFS2_FS)		+= ocfs2/
>  obj-$(CONFIG_GFS2_FS)           += gfs2/
> +obj-$(CONFIG_AZ_FS)		+= azfs.o
> diff -Nuar linux-2.6.26-rc5/fs/azfs.c linux-2.6.26-rc5-azfs/fs/azfs.c
> --- linux-2.6.26-rc5/fs/azfs.c	1970-01-01 01:00:00.000000000 +0100
> +++ linux-2.6.26-rc5-azfs/fs/azfs.c	2008-06-06 17:46:23.587653053 +0200
> @@ -0,0 +1,1179 @@
> +/*
> + * (C) Copyright IBM Deutschland Entwicklung GmbH 2007
> + *
> + * Author: Maxim Shchetynin <maxim@de.ibm.com>
> + *
> + * Non-buffered filesystem driver.
> + * It registers a filesystem which may be used for all kind of block devices
> + * which have a direct_access() method in block_device_operations.
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License as published by
> + * the Free Software Foundation; either version 2, or (at your option)
> + * any later version.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program; if not, write to the Free Software
> + * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
> + */
> +
> +#include <linux/backing-dev.h>
> +#include <linux/blkdev.h>
> +#include <linux/cache.h>
> +#include <linux/dcache.h>
> +#include <linux/device.h>
> +#include <linux/err.h>
> +#include <linux/fs.h>
> +#include <linux/genhd.h>
> +#include <linux/kernel.h>
> +#include <linux/limits.h>
> +#include <linux/list.h>
> +#include <linux/module.h>
> +#include <linux/mount.h>
> +#include <linux/mm.h>
> +#include <linux/mm_types.h>
> +#include <linux/mutex.h>
> +#include <linux/namei.h>
> +#include <linux/pagemap.h>
> +#include <linux/parser.h>
> +#include <linux/slab.h>
> +#include <linux/spinlock.h>
> +#include <linux/stat.h>
> +#include <linux/statfs.h>
> +#include <linux/string.h>
> +#include <linux/time.h>
> +#include <linux/types.h>
> +#include <linux/aio.h>
> +#include <linux/uio.h>
> +#include <asm/bug.h>
> +#include <asm/page.h>
> +#include <asm/pgtable.h>
> +#include <asm/string.h>
> +
> +#define AZFS_FILESYSTEM_NAME		"azfs"
> +#define AZFS_FILESYSTEM_FLAGS		FS_REQUIRES_DEV
> +
> +#define AZFS_SUPERBLOCK_MAGIC		0xABBA1972
> +#define AZFS_SUPERBLOCK_FLAGS		MS_NOEXEC | \
> +					MS_SYNCHRONOUS | \
> +					MS_DIRSYNC | \
> +					MS_ACTIVE
> +
> +#define AZFS_BDI_CAPABILITIES		BDI_CAP_NO_ACCT_DIRTY | \
> +					BDI_CAP_NO_WRITEBACK | \
> +					BDI_CAP_MAP_COPY | \
> +					BDI_CAP_MAP_DIRECT | \
> +					BDI_CAP_VMFLAGS
> +
> +#define AZFS_CACHE_FLAGS		SLAB_HWCACHE_ALIGN | \
> +					SLAB_RECLAIM_ACCOUNT | \
> +					SLAB_MEM_SPREAD
> +
> +enum azfs_direction {
> +	AZFS_MMAP,
> +	AZFS_READ,
> +	AZFS_WRITE
> +};
> +
> +struct azfs_super {
> +	struct list_head		list;
> +	unsigned long			media_size;
> +	unsigned long			block_size;
> +	unsigned short			block_shift;
> +	unsigned long			sector_size;
> +	unsigned short			sector_shift;
> +	uid_t				uid;
> +	gid_t				gid;
> +	unsigned long			ph_addr;
> +	unsigned long			io_addr;
> +	struct block_device		*blkdev;
> +	struct dentry			*root;
> +	struct list_head		block_list;
> +	rwlock_t			lock;
> +};
> +
> +struct azfs_super_list {
> +	struct list_head		head;
> +	spinlock_t			lock;
> +};
> +
> +struct azfs_block {
> +	struct list_head		list;
> +	unsigned long			id;
> +	unsigned long			count;
> +};
> +
> +struct azfs_znode {
> +	struct list_head		block_list;
> +	rwlock_t			lock;
> +	loff_t				size;
> +	struct inode			vfs_inode;
> +};
> +
> +static struct azfs_super_list		super_list;
> +static struct kmem_cache		*azfs_znode_cache __read_mostly = NULL;
> +static struct kmem_cache		*azfs_block_cache __read_mostly = NULL;
> +
> +#define I2Z(inode) \
> +	container_of(inode, struct azfs_znode, vfs_inode)
> +
> +#define for_each_block(block, block_list) \
> +	list_for_each_entry(block, block_list, list)
> +#define for_each_block_reverse(block, block_list) \
> +	list_for_each_entry_reverse(block, block_list, list)
> +#define for_each_block_safe(block, ding, block_list) \
> +	list_for_each_entry_safe(block, ding, block_list, list)
> +#define for_each_block_safe_reverse(block, ding, block_list) \
> +	list_for_each_entry_safe_reverse(block, ding, block_list, list)
> +
> +/**
> + * azfs_block_init - create and initialise a new block in a list
> + * @block_list: destination list
> + * @id: block id
> + * @count: size of a block
> + */
> +static inline struct azfs_block*
> +azfs_block_init(struct list_head *block_list,
> +		unsigned long id, unsigned long count)
> +{
> +	struct azfs_block *block;
> +
> +	block = kmem_cache_alloc(azfs_block_cache, GFP_KERNEL);
> +	if (!block)
> +		return NULL;
> +
> +	block->id = id;
> +	block->count = count;
> +
> +	INIT_LIST_HEAD(&block->list);
> +	list_add_tail(&block->list, block_list);
> +
> +	return block;
> +}
> +
> +/**
> + * azfs_block_free - remove block from a list and free it back in cache
> + * @block: block to be removed
> + */
> +static inline void
> +azfs_block_free(struct azfs_block *block)
> +{
> +	list_del(&block->list);
> +	kmem_cache_free(azfs_block_cache, block);
> +}
> +
> +/**
> + * azfs_block_move - move block to another list
> + * @block: block to be moved
> + * @block_list: destination list
> + */
> +static inline void
> +azfs_block_move(struct azfs_block *block, struct list_head *block_list)
> +{
> +	list_move_tail(&block->list, block_list);
> +}
> +
> +/**
> + * azfs_recherche - get real address of a part of a file
> + * @inode: inode
> + * @direction: data direction
> + * @from: offset for read/write operation
> + * @size: pointer to a value of the amount of data to be read/written
> + */
> +static unsigned long
> +azfs_recherche(struct inode *inode, enum azfs_direction direction,

At the risk of being damned by the entire francophone world, I'd still
suggest using an English keyword for the function name here.

> +	       unsigned long from, unsigned long *size)
> +{
> +	struct azfs_super *super;
> +	struct azfs_znode *znode;
> +	struct azfs_block *block;
> +	unsigned long block_id, west, east;
> +
> +	super = inode->i_sb->s_fs_info;
> +	znode = I2Z(inode);
> +
> +	if (from + *size > znode->size) {
> +		i_size_write(inode, from + *size);
> +		inode->i_op->truncate(inode);
> +	}
> +
> +	read_lock(&znode->lock);
> +
> +	if (list_empty(&znode->block_list)) {
> +		read_unlock(&znode->lock);
> +		return 0;
> +	}
> +
> +	block_id = from >> super->block_shift;
> +
> +	for_each_block(block, &znode->block_list) {
> +		if (block->count > block_id)
> +			break;
> +		block_id -= block->count;
> +	}
> +
> +	west = from % super->block_size;
> +	east = ((block->count - block_id) << super->block_shift) - west;
> +
> +	if (*size > east)
> +		*size = east;
> +
> +	block_id = ((block->id + block_id) << super->block_shift) + west;
> +
> +	read_unlock(&znode->lock);
> +
> +	block_id += direction == AZFS_MMAP ? super->ph_addr : super->io_addr;
> +
> +	return block_id;
> +}
> +
> +static struct inode*
> +azfs_new_inode(struct super_block *, struct inode *, int, dev_t);

Would it not be better to place this function prototype along with the
bunch of macro definitions you have above into a private header?

> +
> +/**
> + * azfs_mknod - mknod() method for inode_operations
> + * @dir, @dentry, @mode, @dev: see inode_operations methods
> + */
> +static int
> +azfs_mknod(struct inode *dir, struct dentry *dentry, int mode, dev_t dev)
> +{
> +	struct inode *inode;
> +
> +	inode = azfs_new_inode(dir->i_sb, dir, mode, dev);
> +	if (!inode)
> +		return -ENOSPC;
> +
> +	if (S_ISREG(mode))
> +		I2Z(inode)->size = 0;
> +
> +	dget(dentry);
> +	d_instantiate(dentry, inode);
> +
> +	return 0;
> +}
> +
> +/**
> + * azfs_create - create() method for inode_operations
> + * @dir, @dentry, @mode, @nd: see inode_operations methods
> + */
> +static int
> +azfs_create(struct inode *dir, struct dentry *dentry, int mode,
> +	    struct nameidata *nd)
> +{
> +	return azfs_mknod(dir, dentry, mode | S_IFREG, 0);
> +}
> +
> +/**
> + * azfs_mkdir - mkdir() method for inode_operations
> + * @dir, @dentry, @mode: see inode_operations methods
> + */
> +static int
> +azfs_mkdir(struct inode *dir, struct dentry *dentry, int mode)
> +{
> +	int rc;
> +
> +	rc = azfs_mknod(dir, dentry, mode | S_IFDIR, 0);
> +	if (rc == 0)

Maybe "if (!rc)" ?

> +		inc_nlink(dir);
> +
> +	return rc;
> +}
> +
> +/**
> + * azfs_symlink - symlink() method for inode_operations
> + * @dir, @dentry, @name: see inode_operations methods
> + */
> +static int
> +azfs_symlink(struct inode *dir, struct dentry *dentry, const char *name)
> +{
> +	struct inode *inode;
> +	int rc;
> +
> +	inode = azfs_new_inode(dir->i_sb, dir, S_IFLNK | S_IRWXUGO, 0);
> +	if (!inode)
> +		return -ENOSPC;
> +
> +	rc = page_symlink(inode, name, strlen(name) + 1);
> +	if (rc) {
> +		iput(inode);
> +		return rc;
> +	}
> +
> +	dget(dentry);
> +	d_instantiate(dentry, inode);
> +
> +	return 0;
> +}
> +
> +/**
> + * azfs_aio_read - aio_read() method for file_operations
> + * @iocb, @iov, @nr_segs, @pos: see file_operations methods
> + */
> +static ssize_t
> +azfs_aio_read(struct kiocb *iocb, const struct iovec *iov,
> +	      unsigned long nr_segs, loff_t pos)
> +{
> +	struct inode *inode;
> +	void *ziel;

void *target?

> +	unsigned long pin;
> +	unsigned long size, todo, step;
> +	ssize_t rc;
> +
> +	inode = iocb->ki_filp->f_mapping->host;
> +
> +	mutex_lock(&inode->i_mutex);
> +
> +	if (pos >= i_size_read(inode)) {
> +		rc = 0;
> +		goto out;
> +	}
> +
> +	ziel = iov->iov_base;
> +	todo = min((loff_t) iov->iov_len, i_size_read(inode) - pos);
> +
> +	for (step = todo; step; step -= size) {
> +		size = step;
> +		pin = azfs_recherche(inode, AZFS_READ, pos, &size);
> +		if (!pin) {
> +			rc = -ENOSPC;
> +			goto out;
> +		}
> +		if (copy_to_user(ziel, (void*) pin, size)) {
> +			rc = -EFAULT;
> +			goto out;
> +		}
> +
> +		iocb->ki_pos += size;
> +		pos += size;
> +		ziel += size;
> +	}
> +
> +	rc = todo;
> +
> +out:
> +	mutex_unlock(&inode->i_mutex);
> +
> +	return rc;
> +}
> +
> +/**
> + * azfs_aio_write - aio_write() method for file_operations
> + * @iocb, @iov, @nr_segs, @pos: see file_operations methods
> + */
> +static ssize_t
> +azfs_aio_write(struct kiocb *iocb, const struct iovec *iov,
> +	       unsigned long nr_segs, loff_t pos)
> +{
> +	struct inode *inode;
> +	void *quell;

void *source?

> +	unsigned long pin;
> +	unsigned long size, todo, step;
> +	ssize_t rc;
> +
> +	inode = iocb->ki_filp->f_mapping->host;
> +
> +	quell = iov->iov_base;
> +	todo = iov->iov_len;
> +
> +	mutex_lock(&inode->i_mutex);
> +
> +	for (step = todo; step; step -= size) {
> +		size = step;
> +		pin = azfs_recherche(inode, AZFS_WRITE, pos, &size);
> +		if (!pin) {
> +			rc = -ENOSPC;
> +			goto out;
> +		}
> +		if (copy_from_user((void*) pin, quell, size)) {
> +			rc = -EFAULT;
> +			goto out;
> +		}
> +
> +		iocb->ki_pos += size;
> +		pos += size;
> +		quell += size;
> +	}
> +
> +	rc = todo;
> +
> +out:
> +	mutex_unlock(&inode->i_mutex);
> +
> +	return rc;
> +}
> +
> +/**
> + * azfs_open - open() method for file_operations
> + * @inode, @file: see file_operations methods
> + */
> +static int
> +azfs_open(struct inode *inode, struct file *file)
> +{
> +	file->private_data = inode;
> +
> +	if (file->f_flags & O_TRUNC) {
> +		i_size_write(inode, 0);
> +		inode->i_op->truncate(inode);
> +	}
> +	if (file->f_flags & O_APPEND)
> +		inode->i_fop->llseek(file, 0, SEEK_END);
> +
> +	return 0;
> +}
> +
> +/**
> + * azfs_mmap - mmap() method for file_operations
> + * @file, @vm: see file_operations methods
> + */
> +static int
> +azfs_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> +	struct azfs_super *super;
> +	struct azfs_znode *znode;
> +	struct inode *inode;
> +	unsigned long cursor, pin;
> +	unsigned long todo, size, vm_start;
> +	pgprot_t page_prot;
> +
> +	inode = file->private_data;
> +	znode = I2Z(inode);
> +	super = inode->i_sb->s_fs_info;
> +
> +	if (super->block_size < PAGE_SIZE)
> +		return -EINVAL;
> +
> +	cursor = vma->vm_pgoff << super->block_shift;
> +	todo = vma->vm_end - vma->vm_start;
> +
> +	if (cursor + todo > i_size_read(inode))
> +		return -EINVAL;
> +
> +	page_prot = pgprot_val(vma->vm_page_prot);
> +	page_prot |= (_PAGE_NO_CACHE | _PAGE_RW);
> +	page_prot &= ~_PAGE_GUARDED;
> +	vma->vm_page_prot = __pgprot(page_prot);
> +
> +	vm_start = vma->vm_start;
> +	for (size = todo; todo; todo -= size, size = todo) {
> +		pin = azfs_recherche(inode, AZFS_MMAP, cursor, &size);
> +		if (!pin)
> +			return -EAGAIN;
> +		pin >>= PAGE_SHIFT;
> +		if (remap_pfn_range(vma, vm_start, pin, size, vma->vm_page_prot))
> +			return -EAGAIN;
> +
> +		vm_start += size;
> +		cursor += size;
> +	}
> +
> +	return 0;
> +}
> +
> +/**
> + * azfs_truncate - truncate() method for inode_operations
> + * @inode: see inode_operations methods
> + */
> +static void
> +azfs_truncate(struct inode *inode)
> +{
> +	struct azfs_super *super;
> +	struct azfs_znode *znode;
> +	struct azfs_block *block, *ding, *knoten, *west, *east;

The risk of me getting damned increases with that, but maybe it would be
better to use an English keyword for "knoten"?

> +	unsigned long id, count;
> +	signed long delta;
> +
> +	super = inode->i_sb->s_fs_info;
> +	znode = I2Z(inode);
> +
> +	delta = i_size_read(inode) + (super->block_size - 1);
> +	delta >>= super->block_shift;
> +	delta -= inode->i_blocks;
> +
> +	if (delta == 0) {
> +		znode->size = i_size_read(inode);
> +		return;
> +	}
> +
> +	write_lock(&znode->lock);
> +
> +	while (delta > 0) {
> +		west = east = NULL;
> +
> +		write_lock(&super->lock);
> +
> +		if (list_empty(&super->block_list)) {
> +			write_unlock(&super->lock);
> +			break;
> +		}
> +
> +		for (count = delta; count; count--) {
> +			for_each_block(block, &super->block_list)
> +				if (block->count >= count) {
> +					east = block;
> +					break;
> +				}
> +			if (east)
> +				break;
> +		}
> +
> +		for_each_block_reverse(block, &znode->block_list) {
> +			if (block->id + block->count == east->id)
> +				west = block;
> +			break;
> +		}
> +
> +		if (east->count == count) {
> +			if (west) {
> +				west->count += east->count;
> +				azfs_block_free(east);
> +			} else {
> +				azfs_block_move(east, &znode->block_list);
> +			}
> +		} else {
> +			if (west) {
> +				west->count += count;
> +			} else {
> +				if (!azfs_block_init(&znode->block_list,
> +						east->id, count)) {
> +					write_unlock(&super->lock);
> +					break;
> +				}
> +			}
> +
> +			east->id += count;
> +			east->count -= count;
> +		}
> +
> +		write_unlock(&super->lock);
> +
> +		inode->i_blocks += count;
> +
> +		delta -= count;
> +	}
> +
> +	while (delta < 0) {
> +		for_each_block_safe_reverse(block, knoten, &znode->block_list) {
> +			id = block->id;
> +			count = block->count;
> +			if ((signed long) count + delta > 0) {
> +				block->count += delta;
> +				id += block->count;
> +				count -= block->count;
> +				block = NULL;
> +			}
> +
> +			west = east = NULL;
> +
> +			write_lock(&super->lock);
> +
> +			for_each_block(ding, &super->block_list) {
> +				if (!west && (ding->id + ding->count == id))
> +					west = ding;
> +				else if (!east && (id + count == ding->id))
> +					east = ding;
> +				if (west && east)
> +					break;
> +			}
> +
> +			if (west && east) {
> +				west->count += count + east->count;
> +				azfs_block_free(east);
> +				if (block)
> +					azfs_block_free(block);
> +			} else if (west) {
> +				west->count += count;
> +				if (block)
> +					azfs_block_free(block);
> +			} else if (east) {
> +				east->id -= count;
> +				east->count += count;
> +				if (block)
> +					azfs_block_free(block);
> +			} else {
> +				if (!block) {
> +					if (!azfs_block_init(&super->block_list,
> +							id, count)) {
> +						write_unlock(&super->lock);
> +						break;
> +					}
> +				} else {
> +					azfs_block_move(block, &super->block_list);
> +				}
> +			}
> +
> +			write_unlock(&super->lock);
> +
> +			inode->i_blocks -= count;
> +
> +			delta += count;
> +
> +			break;
> +		}
> +	}
> +
> +	write_unlock(&znode->lock);
> +
> +	znode->size = min(i_size_read(inode),
> +			(loff_t) inode->i_blocks << super->block_shift);
> +}
> +
> +/**
> + * azfs_getattr - getattr() method for inode_operations
> + * @mnt, @dentry, @stat: see inode_operations methods
> + */
> +static int
> +azfs_getattr(struct vfsmount *mnt, struct dentry *dentry, struct kstat *stat)
> +{
> +	struct azfs_super *super;
> +	struct inode *inode;
> +	unsigned short shift;
> +
> +	inode = dentry->d_inode;
> +	super = inode->i_sb->s_fs_info;
> +
> +	generic_fillattr(inode, stat);
> +	stat->blocks = inode->i_blocks;
> +	shift = super->block_shift - super->sector_shift;
> +	if (shift)
> +		stat->blocks <<= shift;
> +
> +	return 0;
> +}
> +
> +static const struct address_space_operations azfs_aops = {
> +	.write_begin	= simple_write_begin,
> +	.write_end	= simple_write_end
> +};
> +
> +static struct backing_dev_info azfs_bdi = {
> +	.ra_pages	= 0,
> +	.capabilities	= AZFS_BDI_CAPABILITIES
> +};
> +
> +static struct inode_operations azfs_dir_iops = {
> +	.create		= azfs_create,
> +	.lookup		= simple_lookup,
> +	.link		= simple_link,
> +	.unlink		= simple_unlink,
> +	.symlink	= azfs_symlink,
> +	.mkdir		= azfs_mkdir,
> +	.rmdir		= simple_rmdir,
> +	.mknod		= azfs_mknod,
> +	.rename		= simple_rename
> +};
> +
> +static const struct file_operations azfs_reg_fops = {
> +	.llseek		= generic_file_llseek,
> +	.aio_read	= azfs_aio_read,
> +	.aio_write	= azfs_aio_write,
> +	.open		= azfs_open,
> +	.mmap		= azfs_mmap,
> +	.fsync		= simple_sync_file,
> +};
> +
> +static struct inode_operations azfs_reg_iops = {
> +	.truncate	= azfs_truncate,
> +	.getattr	= azfs_getattr
> +};
> +
> +/**
> + * azfs_new_inode - cook a new inode
> + * @sb: super-block
> + * @dir: parent directory
> + * @mode: file mode
> + * @dev: to be forwarded to init_special_inode()
> + */
> +static struct inode*
> +azfs_new_inode(struct super_block *sb, struct inode *dir, int mode, dev_t dev)
> +{
> +	struct azfs_super *super;
> +	struct inode *inode;
> +
> +	inode = new_inode(sb);
> +	if (!inode)
> +		return NULL;
> +
> +	inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
> +
> +	inode->i_mode = mode;
> +	if (dir) {
> +		dir->i_mtime = dir->i_ctime = inode->i_mtime;
> +		inode->i_uid = current->fsuid;
> +		if (dir->i_mode & S_ISGID) {
> +			if (S_ISDIR(mode))
> +				inode->i_mode |= S_ISGID;
> +			inode->i_gid = dir->i_gid;
> +		} else {
> +			inode->i_gid = current->fsgid;
> +		}
> +	} else {
> +		super = sb->s_fs_info;
> +		inode->i_uid = super->uid;
> +		inode->i_gid = super->gid;
> +	}
> +
> +	inode->i_blocks = 0;
> +	inode->i_mapping->a_ops = &azfs_aops;
> +	inode->i_mapping->backing_dev_info = &azfs_bdi;
> +
> +	switch (mode & S_IFMT) {
> +	case S_IFDIR:
> +		inode->i_op = &azfs_dir_iops;
> +		inode->i_fop = &simple_dir_operations;
> +		inc_nlink(inode);
> +		break;
> +
> +	case S_IFREG:
> +		inode->i_op = &azfs_reg_iops;
> +		inode->i_fop = &azfs_reg_fops;
> +		break;
> +
> +	case S_IFLNK:
> +		inode->i_op = &page_symlink_inode_operations;
> +		break;
> +
> +	default:
> +		init_special_inode(inode, mode, dev);
> +		break;
> +	}
> +
> +	return inode;
> +}
> +
> +/**
> + * azfs_alloc_inode - alloc_inode() method for super_operations
> + * @sb: see super_operations methods
> + */
> +static struct inode*
> +azfs_alloc_inode(struct super_block *sb)
> +{
> +	struct azfs_znode *znode;
> +
> +	znode = kmem_cache_alloc(azfs_znode_cache, GFP_KERNEL);
> +
> +	INIT_LIST_HEAD(&znode->block_list);
> +	rwlock_init(&znode->lock);
> +
> +	inode_init_once(&znode->vfs_inode);
> +
> +	return znode ? &znode->vfs_inode : NULL;
> +}
> +
> +/**
> + * azfs_destroy_inode - destroy_inode() method for super_operations
> + * @inode: see super_operations methods
> + */
> +static void
> +azfs_destroy_inode(struct inode *inode)
> +{
> +	kmem_cache_free(azfs_znode_cache, I2Z(inode));
> +}
> +
> +/**
> + * azfs_delete_inode - delete_inode() method for super_operations
> + * @inode: see super_operations methods
> + */
> +static void
> +azfs_delete_inode(struct inode *inode)
> +{
> +	if (S_ISREG(inode->i_mode)) {
> +		i_size_write(inode, 0);
> +		azfs_truncate(inode);
> +	}
> +	truncate_inode_pages(&inode->i_data, 0);
> +	clear_inode(inode);
> +}
> +
> +/**
> + * azfs_statfs - statfs() method for super_operations
> + * @dentry, @stat: see super_operations methods
> + */
> +static int
> +azfs_statfs(struct dentry *dentry, struct kstatfs *stat)
> +{
> +	struct super_block *sb;
> +	struct azfs_super *super;
> +	struct inode *inode;
> +	unsigned long inodes, blocks;
> +
> +	sb = dentry->d_sb;
> +	super = sb->s_fs_info;
> +
> +	inodes = blocks = 0;
> +	mutex_lock(&sb->s_lock);
> +	list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
> +		inodes++;
> +		blocks += inode->i_blocks;
> +	}
> +	mutex_unlock(&sb->s_lock);
> +
> +	stat->f_type = AZFS_SUPERBLOCK_MAGIC;
> +	stat->f_bsize = super->block_size;
> +	stat->f_blocks = super->media_size >> super->block_shift;
> +	stat->f_bfree = stat->f_blocks - blocks;
> +	stat->f_bavail = stat->f_blocks - blocks;
> +	stat->f_files = inodes + blocks;
> +	stat->f_ffree = blocks + 1;
> +	stat->f_namelen = NAME_MAX;
> +
> +	return 0;
> +}
> +
> +static struct super_operations azfs_ops = {
> +	.alloc_inode	= azfs_alloc_inode,
> +	.destroy_inode	= azfs_destroy_inode,
> +	.drop_inode	= generic_delete_inode,
> +	.delete_inode	= azfs_delete_inode,
> +	.statfs		= azfs_statfs
> +};
> +
> +enum {
> +	Opt_blocksize_short,
> +	Opt_blocksize_long,
> +	Opt_uid,
> +	Opt_gid,
> +	Opt_err
> +};
> +
> +static match_table_t tokens = {
> +	{Opt_blocksize_short, "bs=%u"},
> +	{Opt_blocksize_long, "blocksize=%u"},
> +	{Opt_uid, "uid=%u"},
> +	{Opt_gid, "gid=%u"},
> +	{Opt_err, NULL}
> +};
> +
> +/**
> + * azfs_parse_mount_parameters - parse options given to mount with -o
> + * @sb: super block
> + * @options: comma separated options
> + */
> +static int
> +azfs_parse_mount_parameters(struct super_block *sb, char *options)
> +{
> +	struct azfs_super *super;
> +	char *option;
> +	int token, value;
> +	substring_t args[MAX_OPT_ARGS];
> +
> +	super = sb->s_fs_info;
> +
> +	while ((option = strsep(&options, ",")) != NULL) {
> +		if (!*option)
> +			continue;
> +
> +		token = match_token(option, tokens, args);
> +		switch (token) {
> +		case Opt_blocksize_short:
> +		case Opt_blocksize_long:
> +			if (match_int(&args[0], &value))
> +				goto syntax_error;
> +			super->block_size = value;
> +			break;
> +
> +		case Opt_uid:
> +			if (match_int(&args[0], &value))
> +				goto syntax_error;
> +			super->uid = value;
> +			break;
> +
> +		case Opt_gid:
> +			if (match_int(&args[0], &value))
> +				goto syntax_error;
> +			super->gid = value;
> +			break;
> +
> +		default:
> +			goto syntax_error;
> +		}
> +	}
> +
> +	return 1;
> +
> +syntax_error:
> +	printk(KERN_ERR "%s: invalid mount option\n",
> +			AZFS_FILESYSTEM_NAME);
> +
> +	return 0;
> +}
> +
> +/**
> + * azfs_fill_super - fill_super routine for get_sb
> + * @sb, @data, @silent: see file_system_type methods
> + */
> +static int
> +azfs_fill_super(struct super_block *sb, void *data, int silent)
> +{
> +	struct gendisk *disk;
> +	struct azfs_super *super = NULL, *knoten;
> +	struct azfs_block *block = NULL;
> +	struct inode *inode = NULL;
> +	void *kaddr;
> +	unsigned long pfn;
> +	int rc;
> +
> +	BUG_ON(!sb->s_bdev);
> +
> +	disk = sb->s_bdev->bd_disk;
> +
> +	if (!disk || !disk->queue) {
> +		printk(KERN_ERR "%s needs a block device which has a gendisk "
> +				"with a queue\n",
> +				AZFS_FILESYSTEM_NAME);
> +		return -ENOSYS;
> +	}
> +
> +	if (!disk->fops->direct_access) {
> +		printk(KERN_ERR "%s needs a block device with a "
> +				"direct_access() method\n",
> +				AZFS_FILESYSTEM_NAME);
> +		return -ENOSYS;
> +	}
> +
> +	if (!get_device(disk->driverfs_dev)) {
> +		printk(KERN_ERR "%s cannot get reference to device driver\n",
> +				AZFS_FILESYSTEM_NAME);
> +		return -EFAULT;
> +	}
> +
> +	sb->s_magic = AZFS_SUPERBLOCK_MAGIC;
> +	sb->s_flags = AZFS_SUPERBLOCK_FLAGS;
> +	sb->s_op = &azfs_ops;
> +	sb->s_maxbytes = get_capacity(disk) * disk->queue->hardsect_size;
> +	sb->s_time_gran = 1;
> +
> +	spin_lock(&super_list.lock);
> +	list_for_each_entry(knoten, &super_list.head, list)
> +		if (knoten->blkdev == sb->s_bdev) {
> +			super = knoten;
> +			break;
> +		}
> +	spin_unlock(&super_list.lock);
> +
> +	if (super) {
> +		if (strlen((char*) data))
> +			printk(KERN_WARNING "/dev/%s was already mounted with "
> +					"%s before, it will be mounted with "
> +					"mount options used last time, "
> +					"options just given would be ignored\n",
> +					disk->disk_name, AZFS_FILESYSTEM_NAME);
> +		sb->s_fs_info = super;
> +	} else {
> +		super = kzalloc(sizeof(struct azfs_super), GFP_KERNEL);
> +		if (!super) {
> +			rc = -ENOMEM;
> +			goto failed;
> +		}
> +		sb->s_fs_info = super;
> +
> +		if (!azfs_parse_mount_parameters(sb, (char*) data)) {
> +			rc = -EINVAL;
> +			goto failed;
> +		}
> +
> +		inode = azfs_new_inode(sb, NULL, S_IFDIR | S_IRWXUGO, 0);
> +		if (!inode) {
> +			rc = -ENOMEM;
> +			goto failed;
> +		}
> +
> +		super->root = d_alloc_root(inode);
> +		if (!super->root) {
> +			rc = -ENOMEM;
> +			goto failed;
> +		}
> +		dget(super->root);
> +
> +		INIT_LIST_HEAD(&super->list);
> +		INIT_LIST_HEAD(&super->block_list);
> +		rwlock_init(&super->lock);
> +
> +		super->media_size = sb->s_maxbytes;
> +
> +		if (!super->block_size)
> +			super->block_size = sb->s_blocksize;
> +		super->block_shift = blksize_bits(super->block_size);
> +
> +		super->sector_size = disk->queue->hardsect_size;
> +		super->sector_shift = blksize_bits(super->sector_size);
> +
> +		super->blkdev = sb->s_bdev;
> +
> +		block = azfs_block_init(&super->block_list,
> +				0, super->media_size >> super->block_shift);
> +		if (!block) {
> +			rc = -ENOMEM;
> +			goto failed;
> +		}
> +
> +		rc = disk->fops->direct_access(super->blkdev, 0, &kaddr, &pfn);
> +		if (rc < 0) {
> +			rc = -EFAULT;
> +			goto failed;
> +		}
> +		super->ph_addr = (unsigned long) kaddr;
> +
> +		super->io_addr = (unsigned long) ioremap_flags(
> +				super->ph_addr, super->media_size, _PAGE_NO_CACHE);
> +		if (!super->io_addr) {
> +			rc = -EFAULT;
> +			goto failed;
> +		}
> +
> +		spin_lock(&super_list.lock);
> +		list_add(&super->list, &super_list.head);
> +		spin_unlock(&super_list.lock);
> +	}
> +
> +	sb->s_root = super->root;
> +	disk->driverfs_dev->driver_data = super;
> +	disk->driverfs_dev->platform_data = sb;
> +
> +	if (super->block_size < PAGE_SIZE)
> +		printk(KERN_INFO "Block size on %s is smaller then system "
> +				"page size: mmap() would not be supported\n",
> +				disk->disk_name);
> +
> +	return 0;
> +
> +failed:
> +	if (super) {
> +		sb->s_root = NULL;
> +		sb->s_fs_info = NULL;
> +		if (block)
> +			azfs_block_free(block);
> +		if (super->root)
> +			dput(super->root);
> +		if (inode)
> +			iput(inode);
> +		disk->driverfs_dev->driver_data = NULL;
> +		kfree(super);
> +		disk->driverfs_dev->platform_data = NULL;
> +		put_device(disk->driverfs_dev);
> +	}
> +
> +	return rc;
> +}
> +
> +/**
> + * azfs_get_sb - get_sb() method for file_system_type
> + * @fs_type, @flags, @dev_name, @data, @mount: see file_system_type methods
> + */
> +static int
> +azfs_get_sb(struct file_system_type *fs_type, int flags,
> +	    const char *dev_name, void *data, struct vfsmount *mount)
> +{
> +	return get_sb_bdev(fs_type, flags,
> +			dev_name, data, azfs_fill_super, mount);
> +}
> +
> +/**
> + * azfs_kill_sb - kill_sb() method for file_system_type
> + * @sb: see file_system_type methods
> + */
> +static void
> +azfs_kill_sb(struct super_block *sb)
> +{
> +	sb->s_root = NULL;
> +	kill_block_super(sb);
> +}
> +
> +static struct file_system_type azfs_fs = {
> +	.owner		= THIS_MODULE,
> +	.name		= AZFS_FILESYSTEM_NAME,
> +	.get_sb		= azfs_get_sb,
> +	.kill_sb	= azfs_kill_sb,
> +	.fs_flags	= AZFS_FILESYSTEM_FLAGS
> +};
> +
> +/**
> + * azfs_init
> + */
> +static int __init
> +azfs_init(void)
> +{
> +	int rc;
> +
> +	INIT_LIST_HEAD(&super_list.head);
> +	spin_lock_init(&super_list.lock);
> +
> +	azfs_znode_cache = kmem_cache_create("azfs_znode_cache",
> +			sizeof(struct azfs_znode), 0, AZFS_CACHE_FLAGS, NULL);
> +	if (!azfs_znode_cache) {
> +		printk(KERN_ERR "Could not allocate inode cache for %s\n",
> +				AZFS_FILESYSTEM_NAME);
> +		rc = -ENOMEM;
> +		goto failed;
> +	}
> +
> +	azfs_block_cache = kmem_cache_create("azfs_block_cache",
> +			sizeof(struct azfs_block), 0, AZFS_CACHE_FLAGS, NULL);
> +	if (!azfs_block_cache) {
> +		printk(KERN_ERR "Could not allocate block cache for %s\n",
> +				AZFS_FILESYSTEM_NAME);
> +		rc = -ENOMEM;
> +		goto failed;
> +	}
> +
> +	rc = register_filesystem(&azfs_fs);
> +	if (rc != 0) {
> +		printk(KERN_ERR "Could not register %s\n",
> +				AZFS_FILESYSTEM_NAME);
> +		goto failed;
> +	}
> +
> +	return 0;
> +
> +failed:
> +	if (azfs_block_cache)
> +		kmem_cache_destroy(azfs_block_cache);
> +
> +	if (azfs_znode_cache)
> +		kmem_cache_destroy(azfs_znode_cache);
> +
> +	return rc;
> +}
> +
> +/**
> + * azfs_exit
> + */
> +static void __exit
> +azfs_exit(void)
> +{
> +	struct azfs_super *super, *SUPER;

I think that yelling in deep desperation like that is not quite in
agreement with the kernel coding style.

> +	struct azfs_block *block, *knoten;
> +	struct gendisk *disk;
> +
> +	spin_lock(&super_list.lock);
> +	list_for_each_entry_safe(super, SUPER, &super_list.head, list) {
> +		disk = super->blkdev->bd_disk;
> +		list_del(&super->list);
> +		iounmap((void*) super->io_addr);
> +		write_lock(&super->lock);
> +		for_each_block_safe(block, knoten, &super->block_list)
> +			azfs_block_free(block);
> +		write_unlock(&super->lock);
> +		disk->driverfs_dev->driver_data = NULL;
> +		disk->driverfs_dev->platform_data = NULL;
> +		kfree(super);
> +		put_device(disk->driverfs_dev);
> +	}
> +	spin_unlock(&super_list.lock);
> +
> +	unregister_filesystem(&azfs_fs);
> +
> +	kmem_cache_destroy(azfs_block_cache);
> +	kmem_cache_destroy(azfs_znode_cache);
> +}
> +
> +module_init(azfs_init);
> +module_exit(azfs_exit);
> +
> +MODULE_LICENSE("GPL");
> +MODULE_AUTHOR("Maxim Shchetynin <maxim@de.ibm.com>");
> +MODULE_DESCRIPTION("Non-buffered file system for IO devices");
> 

An unprecedented lack of comments in this driver can hardly boost the
reader's attention. Besides, I personally think that a kind of a design
document could be extremely useful - basically, explain the purpose of
the filesystem, the basic idead behind it, etc.

Thanks,
Dmitri