xfs: abstract the differences in dir2/dir3 via an ops vector

Lots of the dir code now goes through switches to determine what is
the correct on-disk format to parse. It generally involves a
"xfs_sbversion_hasfoo" check, deferencing the superblock version and
feature fields and hence touching several cache lines per operation
in the process. Some operations do multiple checks because they nest
conditional operations and they don't pass the information in a
direct fashion between each other.

Hence, add an ops vector to the xfs_inode structure that is
configured when the inode is initialised to point to all the correct
decode and encoding operations.  This will significantly reduce the
branchiness and cacheline footprint of the directory object decoding
and encoding.

This is the first patch in a series of conversion patches. It will
introduce the ops structure, the setup of it and add the first
operation to the vector. Subsequent patches will convert directory
ops one at a time to keep the changes simple and obvious.

Just this patch shows the benefit of such an approach on code size.
Just converting the two shortform dir operations as this patch does
decreases the built binary size by ~1500 bytes:

$ size fs/xfs/xfs.o.orig fs/xfs/xfs.o.p1
   text    data     bss     dec     hex filename
 794490   96802    1096  892388   d9de4 fs/xfs/xfs.o.orig
 792986   96802    1096  890884   d9804 fs/xfs/xfs.o.p1
$

That's a significant decrease in the instruction cache footprint of
the directory code for such a simple change, and indicates that this
approach is definitely worth pursuing further.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Myers <bpm@sgi.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/fs/xfs/xfs_dir2.h b/fs/xfs/xfs_dir2.h
index 9910401327d..1909d9faff7 100644
--- a/fs/xfs/xfs_dir2.h
+++ b/fs/xfs/xfs_dir2.h
@@ -32,6 +32,20 @@ struct xfs_dir2_data_unused;
 extern struct xfs_name	xfs_name_dotdot;
 
 /*
+ * directory operations vector for encode/decode routines
+ */
+struct xfs_dir_ops {
+	int	(*sf_entsize)(struct xfs_dir2_sf_hdr *hdr, int len);
+	struct xfs_dir2_sf_entry *
+		(*sf_nextentry)(struct xfs_dir2_sf_hdr *hdr,
+				struct xfs_dir2_sf_entry *sfep);
+};
+
+extern const struct xfs_dir_ops xfs_dir2_ops;
+extern const struct xfs_dir_ops xfs_dir2_ftype_ops;
+extern const struct xfs_dir_ops xfs_dir3_ops;
+
+/*
  * Generic directory interface routines
  */
 extern void xfs_dir_startup(void);