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Date:	Tue, 16 Aug 2011 15:22:27 -0600
From:	Andreas Dilger <adilger@...mcloud.com>
To:	Bernd Schubert <bernd.schubert@...m.fraunhofer.de>
Cc:	linux-nfs@...r.kernel.org, linux-ext4@...r.kernel.org,
	hch@...radead.org, yong.fan@...mcloud.com,
	linux-fsdevel@...r.kernel.org, tytso@....edu
Subject: Re: [PATCH 2/6] Return 32/64-bit dir name hash according to usage type

On 2011-08-16, at 5:25 AM, Bernd Schubert wrote:

> From: Fan Yong <yong.fan@...mcloud.com>
> 
> Traditionally ext2/3/4 has returned a 32-bit hash value from llseek()
> to appease NFSv2, which can only handle a 32-bit cookie for seekdir()
> and telldir().  However, this causes problems if there are 32-bit hash
> collisions, since the NFSv2 server can get stuck resending the same
> entries from the directory repeatedly.
> 
> Allow ext4 to return a full 64-bit hash (both major and minor) for
> telldir to decrease the chance of hash collisions.  This still needs
> integration on the NFS side.
> 
> Patch-updated-by: Bernd Schubert <bernd.schubert@...m.fraunhofer.de>
> (blame me if something is not correct)
> 
> Signed-off-by: Fan Yong <yong.fan@...mcloud.com>
> Signed-off-by: Andreas Dilger <adilger@...mcloud.com>
> Signed-off-by: Bernd Schubert <bernd.schubert@...m.fraunhofer.de>
> ---
> fs/ext4/dir.c  |  185 ++++++++++++++++++++++++++++++++++++++++++++------------
> fs/ext4/ext4.h |    6 ++
> fs/ext4/hash.c |    4 +
> 3 files changed, 154 insertions(+), 41 deletions(-)
> 
> diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
> index 164c560..cc47087 100644
> --- a/fs/ext4/dir.c
> +++ b/fs/ext4/dir.c
> @@ -32,24 +32,8 @@ static unsigned char ext4_filetype_table[] = {
> 	DT_UNKNOWN, DT_REG, DT_DIR, DT_CHR, DT_BLK, DT_FIFO, DT_SOCK, DT_LNK
> };
> 
> -static int ext4_readdir(struct file *, void *, filldir_t);
> static int ext4_dx_readdir(struct file *filp,
> 			   void *dirent, filldir_t filldir);
> -static int ext4_release_dir(struct inode *inode,
> -				struct file *filp);
> -
> -const struct file_operations ext4_dir_operations = {
> -	.llseek		= ext4_llseek,
> -	.read		= generic_read_dir,
> -	.readdir	= ext4_readdir,		/* we take BKL. needed?*/
> -	.unlocked_ioctl = ext4_ioctl,
> -#ifdef CONFIG_COMPAT
> -	.compat_ioctl	= ext4_compat_ioctl,
> -#endif
> -	.fsync		= ext4_sync_file,
> -	.release	= ext4_release_dir,
> -};
> -
> 
> static unsigned char get_dtype(struct super_block *sb, int filetype)
> {
> @@ -254,22 +238,134 @@ out:
> 	return ret;
> }
> 
> +static inline int is_32bit_api(void)
> +{
> +#ifdef HAVE_IS_COMPAT_TASK
> +	return is_compat_task();

Looking more closely, this should actually be "#ifdef CONFIG_COMPAT" in the mainline kernel.

HAVE_IS_COMPAT_TASK is from the Lustre configure script for detecting which kernel is_compat_task() was added in, since it appeared in some kernels at 2.6.17 but wasn't in most arches until 2.6.29.

Sorry I didn't notice this earlier.

> +#else
> +	return (BITS_PER_LONG == 32);
> +#endif
> +}
> +
> /*
>  * These functions convert from the major/minor hash to an f_pos
> - * value.
> + * value for dx directories
> + *
> + * Upper layer (for example NFS) should specify FMODE_32BITHASH or
> + * FMODE_64BITHASH explicitly. On the other hand, we allow ext4 to be mounted
> + * directly on both 32-bit and 64-bit nodes, under such case, neither
> + * FMODE_32BITHASH nor FMODE_64BITHASH is specified.
> + */
> +static inline loff_t hash2pos(struct file *filp, __u32 major, __u32 minor)
> +{
> +	if ((filp->f_flags & FMODE_32BITHASH) ||
> +	    (!(filp->f_flags & FMODE_64BITHASH) && is_32bit_api()))
> +		return major >> 1;
> +	else
> +		return ((__u64)(major >> 1) << 32) | (__u64)minor;
> +}
> +
> +static inline __u32 pos2maj_hash(struct file *filp, loff_t pos)
> +{
> +	if ((filp->f_flags & FMODE_32BITHASH) ||
> +	    (!(filp->f_mode & FMODE_64BITHASH) && is_32bit_api()))
> +		return (pos << 1) & 0xffffffff;
> +	else
> +		return ((pos >> 32) << 1) & 0xffffffff;
> +}
> +
> +static inline __u32 pos2min_hash(struct file *filp, loff_t pos)
> +{
> +	if ((filp->f_flags & FMODE_32BITHASH) ||
> +	    (!(filp->f_flags & FMODE_64BITHASH) && is_32bit_api()))
> +		return 0;
> +	else
> +		return pos & 0xffffffff;
> +}
> +
> +/*
> + * Return 32- or 64-bit end-of-file for dx directories
> + */
> +static inline loff_t ext4_get_htree_eof(struct file *filp)
> +{
> +	if ((filp->f_mode & FMODE_32BITHASH) ||
> +	    (!(filp->f_mode & FMODE_64BITHASH) && is_32bit_api()))
> +		return EXT4_HTREE_EOF_32BIT;
> +	else
> +		return EXT4_HTREE_EOF_64BIT;
> +}
> +
> +
> +/*
> + * ext4_dir_llseek() based on generic_file_llseek() to handle both
> + * non-htree and htree directories, where the "offset" is in terms
> + * of the filename hash value instead of the byte offset.
>  *
> - * Currently we only use major hash numer.  This is unfortunate, but
> - * on 32-bit machines, the same VFS interface is used for lseek and
> - * llseek, so if we use the 64 bit offset, then the 32-bit versions of
> - * lseek/telldir/seekdir will blow out spectacularly, and from within
> - * the ext2 low-level routine, we don't know if we're being called by
> - * a 64-bit version of the system call or the 32-bit version of the
> - * system call.  Worse yet, NFSv2 only allows for a 32-bit readdir
> - * cookie.  Sigh.
> + * NOTE: offsets obtained *before* ext4_set_inode_flag(dir, EXT4_INODE_INDEX)
> + *       will be invalid once the directory was converted into a dx directory
>  */
> -#define hash2pos(major, minor)	(major >> 1)
> -#define pos2maj_hash(pos)	((pos << 1) & 0xffffffff)
> -#define pos2min_hash(pos)	(0)
> +loff_t ext4_dir_llseek(struct file *file, loff_t offset, int origin)
> +{
> +	struct inode *inode = file->f_mapping->host;
> +	loff_t ret = -EINVAL;
> +	int is_dx_dir = ext4_test_inode_flag(inode, EXT4_INODE_INDEX);
> +
> +	mutex_lock(&inode->i_mutex);
> +
> +	/* NOTE: relative offsets with dx directories might not work
> +	 *       as expected, as it is difficult to figure out the
> +	 *       correct offset between dx hashes */
> +
> +	switch (origin) {
> +	case SEEK_END:
> +		if (unlikely(offset > 0))
> +			goto out_err; /* not supported for directories */
> +
> +		/* so only negative offsets are left, does that have a
> +		 * meaning for directories at all? */
> +		if (is_dx_dir)
> +			offset += ext4_get_htree_eof(file);
> +		else
> +			offset += inode->i_size;
> +		break;
> +	case SEEK_CUR:
> +		/*
> +		 * Here we special-case the lseek(fd, 0, SEEK_CUR)
> +		 * position-querying operation.  Avoid rewriting the "same"
> +		 * f_pos value back to the file because a concurrent read(),
> +		 * write() or lseek() might have altered it
> +		 */
> +		if (offset == 0) {
> +			offset = file->f_pos;
> +			goto out_ok;
> +		}
> +
> +		offset += file->f_pos;
> +		break;
> +	}
> +
> +	if (unlikely(offset < 0))
> +		goto out_err;
> +
> +	if (!is_dx_dir) {
> +		if (offset > inode->i_sb->s_maxbytes)
> +			goto out_err;
> +	} else if (offset > ext4_get_htree_eof(file))
> +		goto out_err;
> +
> +	/* Special lock needed here? */
> +	if (offset != file->f_pos) {
> +		file->f_pos = offset;
> +		file->f_version = 0;
> +	}
> +
> +out_ok:
> +	ret = offset;
> +out_err:
> +	mutex_unlock(&inode->i_mutex);
> +
> +	return ret;
> +}
> 
> /*
>  * This structure holds the nodes of the red-black tree used to store
> @@ -330,15 +426,16 @@ static void free_rb_tree_fname(struct rb_root *root)
> }
> 
> 
> -static struct dir_private_info *ext4_htree_create_dir_info(loff_t pos)
> +static struct dir_private_info *ext4_htree_create_dir_info(struct file *filp,
> +							   loff_t pos)
> {
> 	struct dir_private_info *p;
> 
> 	p = kzalloc(sizeof(struct dir_private_info), GFP_KERNEL);
> 	if (!p)
> 		return NULL;
> -	p->curr_hash = pos2maj_hash(pos);
> -	p->curr_minor_hash = pos2min_hash(pos);
> +	p->curr_hash = pos2maj_hash(filp, pos);
> +	p->curr_minor_hash = pos2min_hash(filp, pos);
> 	return p;
> }
> 
> @@ -429,7 +526,7 @@ static int call_filldir(struct file *filp, void *dirent,
> 		       "null fname?!?\n");
> 		return 0;
> 	}
> -	curr_pos = hash2pos(fname->hash, fname->minor_hash);
> +	curr_pos = hash2pos(filp, fname->hash, fname->minor_hash);
> 	while (fname) {
> 		error = filldir(dirent, fname->name,
> 				fname->name_len, curr_pos,
> @@ -454,13 +551,13 @@ static int ext4_dx_readdir(struct file *filp,
> 	int	ret;
> 
> 	if (!info) {
> -		info = ext4_htree_create_dir_info(filp->f_pos);
> +		info = ext4_htree_create_dir_info(filp, filp->f_pos);
> 		if (!info)
> 			return -ENOMEM;
> 		filp->private_data = info;
> 	}
> 
> -	if (filp->f_pos == EXT4_HTREE_EOF)
> +	if (filp->f_pos == ext4_get_htree_eof(filp))
> 		return 0;	/* EOF */
> 
> 	/* Some one has messed with f_pos; reset the world */
> @@ -468,8 +565,8 @@ static int ext4_dx_readdir(struct file *filp,
> 		free_rb_tree_fname(&info->root);
> 		info->curr_node = NULL;
> 		info->extra_fname = NULL;
> -		info->curr_hash = pos2maj_hash(filp->f_pos);
> -		info->curr_minor_hash = pos2min_hash(filp->f_pos);
> +		info->curr_hash = pos2maj_hash(filp, filp->f_pos);
> +		info->curr_minor_hash = pos2min_hash(filp, filp->f_pos);
> 	}
> 
> 	/*
> @@ -501,7 +598,7 @@ static int ext4_dx_readdir(struct file *filp,
> 			if (ret < 0)
> 				return ret;
> 			if (ret == 0) {
> -				filp->f_pos = EXT4_HTREE_EOF;
> +				filp->f_pos = ext4_get_htree_eof(filp);
> 				break;
> 			}
> 			info->curr_node = rb_first(&info->root);
> @@ -521,7 +618,7 @@ static int ext4_dx_readdir(struct file *filp,
> 			info->curr_minor_hash = fname->minor_hash;
> 		} else {
> 			if (info->next_hash == ~0) {
> -				filp->f_pos = EXT4_HTREE_EOF;
> +				filp->f_pos = ext4_get_htree_eof(filp);
> 				break;
> 			}
> 			info->curr_hash = info->next_hash;
> @@ -540,3 +637,15 @@ static int ext4_release_dir(struct inode *inode, struct file *filp)
> 
> 	return 0;
> }
> +
> +const struct file_operations ext4_dir_operations = {
> +	.llseek		= ext4_dir_llseek,
> +	.read		= generic_read_dir,
> +	.readdir	= ext4_readdir,
> +	.unlocked_ioctl = ext4_ioctl,
> +#ifdef CONFIG_COMPAT
> +	.compat_ioctl	= ext4_compat_ioctl,
> +#endif
> +	.fsync		= ext4_sync_file,
> +	.release	= ext4_release_dir,
> +};
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index e717dfd..31d9ba0 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -1560,7 +1560,11 @@ struct dx_hash_info
> 	u32		*seed;
> };
> 
> -#define EXT4_HTREE_EOF	0x7fffffff
> +
> +/* 32 and 64 bit signed EOF for dx directories */
> +#define EXT4_HTREE_EOF_32BIT   ((1UL  << (32 - 1)) - 1)
> +#define EXT4_HTREE_EOF_64BIT   ((1ULL << (64 - 1)) - 1)
> +
> 
> /*
>  * Control parameters used by ext4_htree_next_block
> diff --git a/fs/ext4/hash.c b/fs/ext4/hash.c
> index ac8f168..fa8e491 100644
> --- a/fs/ext4/hash.c
> +++ b/fs/ext4/hash.c
> @@ -200,8 +200,8 @@ int ext4fs_dirhash(const char *name, int len, struct dx_hash_info *hinfo)
> 		return -1;
> 	}
> 	hash = hash & ~1;
> -	if (hash == (EXT4_HTREE_EOF << 1))
> -		hash = (EXT4_HTREE_EOF-1) << 1;
> +	if (hash == (EXT4_HTREE_EOF_32BIT << 1))
> +		hash = (EXT4_HTREE_EOF_32BIT - 1) << 1;
> 	hinfo->hash = hash;
> 	hinfo->minor_hash = minor_hash;
> 	return 0;
> 


Cheers, Andreas
--
Andreas Dilger 
Principal Engineer
Whamcloud, Inc.



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