Re: [PATCH 04/12] fs: Generic infrastructure for optional inode fields

Jan Kara
2014-10-08T08:45:14Z

On Wed 01-10-14 15:05:46, Andreas Dilger wrote:
> On Oct 1, 2014, at 1:31 PM, Jan Kara <jack@suse.cz> wrote:
> > There are parts of struct inode which are used only by a few filesystems
> > (e.g. i_dquot pointers, i_mapping->private_list, ...). Thus all the
> > other filesystems are just wasting memory with these fields. On the
> > other hand it isn't simple to just move these fields to filesystem
> > specific part of inode because there is generic code which needs to peek
> > into the fields and it is cumbersome to provide helpers into which fs
> > has to stuff the field it is storing elsewhere.
> >
> > We create a simple infrastructure which allows for optional inode fields
> > stored in the fs-specific part of the inode. Accessing these fields has
> > a slightly worse performance as we have to lookup their offset in the
> > offset table stored in the superblock but in most cases this is
> > acceptable. Notably, this offset-table mechanism is faster than having
> > fs-specific hook functions which would need to be called to provide
> > pointers to desired fields.
> >
> > Signed-off-by: Jan Kara <jack@suse.cz>
> > ---
> > include/linux/fs.h | 24 ++++++++++++++++++++++++
> > 1 file changed, 24 insertions(+)
> >
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index 94187721ad41..977f8fb6ca88 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -615,6 +615,11 @@ struct inode {
> > void *i_private; /* fs or device private pointer */
> > };
> >
> > +/* Optional inode fields (stored in filesystems inode if the fs needs them) */
> > +enum {
>
> This should be a named enum, like "enum inode_field" or similar, so it
> can be referenced below.
>
> > + IF_FIELD_NR /* Number of optional inode fields */
> > +};
> > +
> > static inline int inode_unhashed(struct inode *inode)
> > {
> > return hlist_unhashed(&inode->i_hash);
> > @@ -1236,6 +1241,11 @@ struct super_block {
> > void *s_fs_info; /* Filesystem private info */
> > unsigned int s_max_links;
> > fmode_t s_mode;
> > + /*
> > + * We could have here just a pointer to the offsets array but this
> > + * way we save one dereference when looking up field offsets
> > + */
> > + int s_inode_fields[IF_FIELD_NR];
> >
> > /* Granularity of c/m/atime in ns.
> > Cannot be worse than a second */
> > @@ -1286,6 +1296,20 @@ struct super_block {
> > struct rcu_head rcu;
> > };
> >
> > +static inline void *inode_field(const struct inode *inode, int field)
>
> This should use "enum inode_field" instead of int, so the compiler could
> warn about invalid parameter values. It might make sense to add a check:
OK, makes sense.

> if (field < IF_FIELD_NR)
>
> but I'm not sure if the overhead is worthwhile, unless it can always be
> resolved at compile time. That might be possible since this is a static
> inline function.
Well, I can stick there BUG_ON(field >= IF_FIELD_NR). That looks like a
sensible debugging measure with negligible overhead.

Honza
> > +{
> > + int offset = inode->i_sb->s_inode_fields[field];
> > +
> > + if (!offset) /* Field not present? */
> > + return NULL;
>
> > + return ((char *)inode) + offset;
> > +}
> > +
> > +static inline void sb_init_inode_fields(struct super_block *sb, int *fields)
> > +{
> > + memcpy(sb->s_inode_fields, fields, sizeof(int) * IF_FIELD_NR);
> > +}
> > +
> > extern struct timespec current_fs_time(struct super_block *sb);
> >
> > /*
> > --
> > 1.8.1.4
> >
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at http://vger.kernel.org/majordomo-info.html
>
>
> Cheers, Andreas
>
>
>
>
>

--
Jan Kara <jack@suse.cz>
SUSE Labs, CR