On Wed 01-10-14 15:05:46, Andreas Dilger wrote: > On Oct 1, 2014, at 1:31 PM, Jan Kara <jack@xxxxxxx> 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@xxxxxxx> > > --- > > 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@xxxxxxxxxxxxxxx > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > > Cheers, Andreas > > > > > -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html