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 {

+	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)