[linux-cifs-client] Making CIFS FS-Cache capable

[David Howells]

Hi Steve,

To make CIFS FS-Cache capable, we need to consider a number of things:

 (1) We need an index hierarchy through which we can find cached data when a
     file is reopened after being dropped from the inode cache.

 (2) We need coherency data so that we can tell whether the cache for a file
     we're reopening is still valid.

 (3) We need a way to tell CIFS that it should use FS-Cache for a particular
     mount.


INDEX HIERARCHY
===============

As far as (1) goes, that doesn't need to be a very deep hierarchy.  NFS's has
three levels:

      - Server (nfs_fscache_server_index_def)
      - Exported FS (nfs_fscache_super_index_def)
      - Inode (nfs_fscache_inode_object_def)

The objects of the first type are keyed by NFS version, address and port;
objects of the second type are keyed by the FSID and various mount parameters,
including an optional uniquifier string; and objects of the third type are
keyed on NFS file handles.

AFS, on the other hand, has a four level hierarchy:

	- Cell (afs_cell_cache_index_def)
	- Volume location (afs_vlocation_cache_index_def)
	- Volume instance (afs_volume_cache_index_def)
	- Inode (afs_vnode_cache_index_def)

Keyed by cell name, volume name, volume type and vnode ID respectively.


For CIFS, (1) might be a little more complicated.  We need to identify a set of
files, and then the files within that set.  Perhaps a three level hierarchy:

	- Server
	- Share
	- File

Where server might be the IP address and name of a server, and share may be the
name of a share on that server.  Files are more interesting, since, as far as I
know, the primary way of identifying files on a server is by filename.

However, we could limit cacheable CIFS files to just those that have a
UniqueId.

Failing that, we'd have to create an index for every directory, and install the
files into the appropriate directory index.  Note that it is permissible to (a)
mix different types of objects within an index, and (b) have coherency data
attached to an index.  Files and directories would then be keyed by their
names.


COHERENCY DATA
==============

With respect to coherency data, NFS uses i_size, i_mtime and i_ctime plus
change_attr if it's available.  AFS uses the FID uniquifier and the data
version number.

CIFS could use CreationTime, LastModificationTime/LastWriteTime,
LastStatusChange/ChangeTime, NumOfBytes and EndOfFile.


CONFIGURING CIFS
================

The obvious way to enable FS-Caching for a CIFS mount would be to add a mount
option in a similar way to NFS (NFS has -o fsc or -o fsc=<uniquifier>).

Finer grained caching configuration is something on my to-do list, and will
probably be available on two levels:

 (a) By directory (where the config is inherited by the children of that dir).

 (b) By simple pattern match on names of regular files.


Thoughts?

David