Re: [PATCH 1/2] Implement trivial struct feature macros

[Christoph Hellwig <hch@infradead.org>]

On Thu, Apr 30, 2009 at 07:31:17PM +0200, Jan Kara wrote:
> Sometimes it is useful to have a generic main structure (like struct inode)
> and then with some of these structures you want to associate additional
> information (like quota information, block device information etc.).
> Traditionally, we solved this by having a pointer to associated data in
> the generic structure. This gets inefficient when there are lots of
> various kinds of data that can be associated with the generic structure
> (we need one pointer per type of associated information). This is an
> attempt to address the issue.
> 
> The idea is simple. There is some chunk of allocated memory that contains
> the generic structure and all the associated information. The generic
> structure has a pointer to the (usually static) table of offsets (from
> the beginning of the generic structure) of associated structures for
> each possible associated structure type.
> 
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  include/linux/fs.h             |    5 +++++
>  include/linux/struct_feature.h |   12 ++++++++++++
>  2 files changed, 17 insertions(+), 0 deletions(-)
>  create mode 100644 include/linux/struct_feature.h
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 5bed436..2404b16 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -710,6 +710,10 @@ static inline int mapping_writably_mapped(struct address_space *mapping)
>  #define i_size_ordered_init(inode) do { } while (0)
>  #endif
>  
> +enum {
> +	INODE_FEATURES
> +};
> +
>  struct inode {
>  	struct hlist_node	i_hash;
>  	struct list_head	i_list;
> @@ -775,6 +779,7 @@ struct inode {
>  	void			*i_security;
>  #endif
>  	void			*i_private; /* fs or device private pointer */
> +	int			*feature_table;
>  };
>  
>  /*
> diff --git a/include/linux/struct_feature.h b/include/linux/struct_feature.h
> new file mode 100644
> index 0000000..0849d3d
> --- /dev/null
> +++ b/include/linux/struct_feature.h
> @@ -0,0 +1,12 @@
> +#ifndef _LINUX_STRUCT_FEATURE_H
> +#define _LINUX_STRUCT_FEATURE_H
> +
> +
> +#define FEATURE_OFFSET(str, generic, feature) (offsetof(str, feature) - offsetof(str, generic))
> +
> +#define GET_FEATURE(generic, feature) \
> +	((generic)->feature_table && (generic)->feature_table[feature] ? \
> +	((void *)(((char *)(generic)) + (generic)->feature_table[feature])) : \
> +	NULL)

These should be inlines.

Also I don't think we should make it super-generic.  If we'll eventually
need it it should be specific to struct inode, something like:

static void *get_inode_extension(struct inode *inode, emum inode_extension ext)
{
	if (!inode->i_extensions[ext])
		return NULL;
	return (char *)inode + inode->i_extensions[ext];
}