xref: /kernel/linux/linux-6.6/Documentation/filesystems/caching/cachefiles.rst

.. SPDX-License-Identifier: GPL-2.0

===================================
Cache on Already Mounted Filesystem
===================================

.. Contents:

 (*) Overview.

 (*) Requirements.

 (*) Configuration.

 (*) Starting the cache.

 (*) Things to avoid.

 (*) Cache culling.

 (*) Cache structure.

 (*) Security model and SELinux.

 (*) A note on security.

 (*) Statistical information.

 (*) Debugging.

 (*) On-demand Read.


Overview
========

CacheFiles is a caching backend that's meant to use as a cache a directory on
an already mounted filesystem of a local type (such as Ext3).

CacheFiles uses a userspace daemon to do some of the cache management - such as
reaping stale nodes and culling.  This is called cachefilesd and lives in
/sbin.

The filesystem and data integrity of the cache are only as good as those of the
filesystem providing the backing services.  Note that CacheFiles does not
attempt to journal anything since the journalling interfaces of the various
filesystems are very specific in nature.

CacheFiles creates a misc character device - "/dev/cachefiles" - that is used
to communication with the daemon.  Only one thing may have this open at once,
and while it is open, a cache is at least partially in existence.  The daemon
opens this and sends commands down it to control the cache.

CacheFiles is currently limited to a single cache.

CacheFiles attempts to maintain at least a certain percentage of free space on
the filesystem, shrinking the cache by culling the objects it contains to make
space if necessary - see the "Cache Culling" section.  This means it can be
placed on the same medium as a live set of data, and will expand to make use of
spare space and automatically contract when the set of data requires more
space.



Requirements
============

The use of CacheFiles and its daemon requires the following features to be
available in the system and in the cache filesystem:

	- dnotify.

	- extended attributes (xattrs).

	- openat() and friends.

	- bmap() support on files in the filesystem (FIBMAP ioctl).

	- The use of bmap() to detect a partial page at the end of the file.

It is strongly recommended that the "dir_index" option is enabled on Ext3
filesystems being used as a cache.


Configuration
=============

The cache is configured by a script in /etc/cachefilesd.conf.  These commands
set up cache ready for use.  The following script commands are available:

 brun <N>%, bcull <N>%, bstop <N>%, frun <N>%, fcull <N>%, fstop <N>%
	Configure the culling limits.  Optional.  See the section on culling
	The defaults are 7% (run), 5% (cull) and 1% (stop) respectively.

	The commands beginning with a 'b' are file space (block) limits, those
	beginning with an 'f' are file count limits.

 dir <path>
	Specify the directory containing the root of the cache.  Mandatory.

 tag <name>
	Specify a tag to FS-Cache to use in distinguishing multiple caches.
	Optional.  The default is "CacheFiles".

 debug <mask>
	Specify a numeric bitmask to control debugging in the kernel module.
	Optional.  The default is zero (all off).  The following values can be
	OR'd into the mask to collect various information:

		==	=================================================
		1	Turn on trace of function entry (_enter() macros)
		2	Turn on trace of function exit (_leave() macros)
		4	Turn on trace of internal debug points (_debug())
		==	=================================================

	This mask can also be set through sysfs, eg::

		echo 5 >/sys/modules/cachefiles/parameters/debug


Starting the Cache
==================

The cache is started by running the daemon.  The daemon opens the cache device,
configures the cache and tells it to begin caching.  At that point the cache
binds to fscache and the cache becomes live.

The daemon is run as follows::

	/sbin/cachefilesd [-d]* [-s] [-n] [-f <configfile>]

The flags are:

 ``-d``
	Increase the debugging level.  This can be specified multiple times and
	is cumulative with itself.

 ``-s``
	Send messages to stderr instead of syslog.

 ``-n``
	Don't daemonise and go into background.

 ``-f <configfile>``
	Use an alternative configuration file rather than the default one.


Things to Avoid
===============

Do not mount other things within the cache as this will cause problems.  The
kernel module contains its own very cut-down path walking facility that ignores
mountpoints, but the daemon can't avoid them.

Do not create, rename or unlink files and directories in the cache while the
cache is active, as this may cause the state to become uncertain.

Renaming files in the cache might make objects appear to be other objects (the
filename is part of the lookup key).

Do not change or remove the extended attributes attached to cache files by the
cache as this will cause the cache state management to get confused.

Do not create files or directories in the cache, lest the cache get confused or
serve incorrect data.

Do not chmod files in the cache.  The module creates things with minimal
permissions to prevent random users being able to access them directly.


Cache Culling
=============

The cache may need culling occasionally to make space.  This involves
discarding objects from the cache that have been used less recently than
anything else.  Culling is based on the access time of data objects.  Empty
directories are culled if not in use.

Cache culling is done on the basis of the percentage of blocks and the
percentage of files available in the underlying filesystem.  There are six
"limits":

 brun, frun
     If the amount of free space and the number of available files in the cache
     rises above both these limits, then culling is turned off.

 bcull, fcull
     If the amount of available space or the number of available files in the
     cache falls below either of these limits, then culling is started.

 bstop, fstop
     If the amount of available space or the number of available files in the
     cache falls below either of these limits, then no further allocation of
     disk space or files is permitted until culling has raised things above
     these limits again.

These must be configured thusly::

	0 <= bstop < bcull < brun < 100
	0 <= fstop < fcull < frun < 100

Note that these are percentages of available space and available files, and do
_not_ appear as 100 minus the percentage displayed by the "df" program.

The userspace daemon scans the cache to build up a table of cullable objects.
These are then culled in least recently used order.  A new scan of the cache is
started as soon as space is made in the table.  Objects will be skipped if
their atimes have changed or if the kernel module says it is still using them.


Cache Structure
===============

The CacheFiles module will create two directories in the directory it was
given:

 * cache/
 * graveyard/

The active cache objects all reside in the first directory.  The CacheFiles
kernel module moves any retired or culled objects that it can't simply unlink
to the graveyard from which the daemon will actually delete them.

The daemon uses dnotify to monitor the graveyard directory, and will delete
anything that appears therein.


The module represents index objects as directories with the filename "I..." or
"J...".  Note that the "cache/" directory is itself a special index.

Data objects are represented as files if they have no children, or directories
if they do.  Their filenames all begin "D..." or "E...".  If represented as a
directory, data objects will have a file in the directory called "data" that
actually holds the data.

Special objects are similar to data objects, except their filenames begin
"S..." or "T...".


If an object has children, then it will be represented as a directory.
Immediately in the representative directory are a collection of directories
named for hash values of the child object keys with an '@' prepended.  Into
this directory, if possible, will be placed the representations of the child
objects::

	 /INDEX    /INDEX     /INDEX                            /DATA FILES
	/=========/==========/=================================/================
	cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400
	cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...DB1ry
	cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...N22ry
	cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...FP1ry


If the key is so long that it exceeds NAME_MAX with the decorations added on to
it, then it will be cut into pieces, the first few of which will be used to
make a nest of directories, and the last one of which will be the objects
inside the last directory.  The names of the intermediate directories will have
'+' prepended::

	J1223/@23/+xy...z/+kl...m/Epqr


Note that keys are raw data, and not only may they exceed NAME_MAX in size,
they may also contain things like '/' and NUL characters, and so they may not
be suitable for turning directly into a filename.

To handle this, CacheFiles will use a suitably printable filename directly and
"base-64" encode ones that aren't directly suitable.  The two versions of
object filenames indicate the encoding:

	===============	===============	===============
	OBJECT TYPE	PRINTABLE	ENCODED
	===============	===============	===============
	Index		"I..."		"J..."
	Data		"D..."		"E..."
	Special		"S..."		"T..."
	===============	===============	===============

Intermediate directories are always "@" or "+" as appropriate.


Each object in the cache has an extended attribute label that holds the object
type ID (required to distinguish special objects) and the auxiliary data from
the netfs.  The latter is used to detect stale objects in the cache and update
or retire them.


Note that CacheFiles will erase from the cache any file it doesn't recognise or
any file of an incorrect type (such as a FIFO file or a device file).


Security Model and SELinux
==========================

CacheFiles is implemented to deal properly with the LSM security features of
the Linux kernel and the SELinux facility.

One of the problems that CacheFiles faces is that it is generally acting on
behalf of a process, and running in that process's context, and that includes a
security context that is not appropriate for accessing the cache - either
because the files in the cache are inaccessible to that process, or because if
the process creates a file in the cache, that file may be inaccessible to other
processes.

The way CacheFiles works is to temporarily change the security context (fsuid,
fsgid and actor security label) that the process acts as - without changing the
security context of the process when it the target of an operation performed by
some other process (so signalling and suchlike still work correctly).


When the CacheFiles module is asked to bind to its cache, it:

 (1) Finds the security label attached to the root cache directory and uses
     that as the security label with which it will create files.  By default,
     this is::

	cachefiles_var_t

 (2) Finds the security label of the process which issued the bind request
     (presumed to be the cachefilesd daemon), which by default will be::

	cachefilesd_t

     and asks LSM to supply a security ID as which it should act given the
     daemon's label.  By default, this will be::

	cachefiles_kernel_t

     SELinux transitions the daemon's security ID to the module's security ID
     based on a rule of this form in the policy::

	type_transition <daemon's-ID> kernel_t : process <module's-ID>;

     For instance::

	type_transition cachefilesd_t kernel_t : process cachefiles_kernel_t;


The module's security ID gives it permission to create, move and remove files
and directories in the cache, to find and access directories and files in the
cache, to set and access extended attributes on cache objects, and to read and
write files in the cache.

The daemon's security ID gives it only a very restricted set of permissions: it
may scan directories, stat files and erase files and directories.  It may
not read or write files in the cache, and so it is precluded from accessing the
data cached therein; nor is it permitted to create new files in the cache.


There are policy source files available in:

	https://people.redhat.com/~dhowells/fscache/cachefilesd-0.8.tar.bz2

and later versions.  In that tarball, see the files::

	cachefilesd.te
	cachefilesd.fc
	cachefilesd.if

They are built and installed directly by the RPM.

If a non-RPM based system is being used, then copy the above files to their own
directory and run::

	make -f /usr/share/selinux/devel/Makefile
	semodule -i cachefilesd.pp

You will need checkpolicy and selinux-policy-devel installed prior to the
build.


By default, the cache is located in /var/fscache, but if it is desirable that
it should be elsewhere, than either the above policy files must be altered, or
an auxiliary policy must be installed to label the alternate location of the
cache.

For instructions on how to add an auxiliary policy to enable the cache to be
located elsewhere when SELinux is in enforcing mode, please see::

	/usr/share/doc/cachefilesd-*/move-cache.txt

When the cachefilesd rpm is installed; alternatively, the document can be found
in the sources.


A Note on Security
==================

CacheFiles makes use of the split security in the task_struct.  It allocates
its own task_security structure, and redirects current->cred to point to it
when it acts on behalf of another process, in that process's context.

The reason it does this is that it calls vfs_mkdir() and suchlike rather than
bypassing security and calling inode ops directly.  Therefore the VFS and LSM
may deny the CacheFiles access to the cache data because under some
circumstances the caching code is running in the security context of whatever
process issued the original syscall on the netfs.

Furthermore, should CacheFiles create a file or directory, the security
parameters with that object is created (UID, GID, security label) would be
derived from that process that issued the system call, thus potentially
preventing other processes from accessing the cache - including CacheFiles's
cache management daemon (cachefilesd).

What is required is to temporarily override the security of the process that
issued the system call.  We can't, however, just do an in-place change of the
security data as that affects the process as an object, not just as a subject.
This means it may lose signals or ptrace events for example, and affects what
the process looks like in /proc.

So CacheFiles makes use of a logical split in the security between the
objective security (task->real_cred) and the subjective security (task->cred).
The objective security holds the intrinsic security properties of a process and
is never overridden.  This is what appears in /proc, and is what is used when a
process is the target of an operation by some other process (SIGKILL for
example).

The subjective security holds the active security properties of a process, and
may be overridden.  This is not seen externally, and is used when a process
acts upon another object, for example SIGKILLing another process or opening a
file.

LSM hooks exist that allow SELinux (or Smack or whatever) to reject a request
for CacheFiles to run in a context of a specific security label, or to create
files and directories with another security label.


Statistical Information
=======================

If FS-Cache is compiled with the following option enabled::

	CONFIG_CACHEFILES_HISTOGRAM=y

then it will gather certain statistics and display them through a proc file.

 /proc/fs/cachefiles/histogram

     ::

	cat /proc/fs/cachefiles/histogram
	JIFS  SECS  LOOKUPS   MKDIRS    CREATES
	===== ===== ========= ========= =========

     This shows the breakdown of the number of times each amount of time
     between 0 jiffies and HZ-1 jiffies a variety of tasks took to run.  The
     columns are as follows:

	=======		=======================================================
	COLUMN		TIME MEASUREMENT
	=======		=======================================================
	LOOKUPS		Length of time to perform a lookup on the backing fs
	MKDIRS		Length of time to perform a mkdir on the backing fs
	CREATES		Length of time to perform a create on the backing fs
	=======		=======================================================

     Each row shows the number of events that took a particular range of times.
     Each step is 1 jiffy in size.  The JIFS column indicates the particular
     jiffy range covered, and the SECS field the equivalent number of seconds.


Debugging
=========

If CONFIG_CACHEFILES_DEBUG is enabled, the CacheFiles facility can have runtime
debugging enabled by adjusting the value in::

	/sys/module/cachefiles/parameters/debug

This is a bitmask of debugging streams to enable:

	=======	=======	===============================	=======================
	BIT	VALUE	STREAM				POINT
	=======	=======	===============================	=======================
	0	1	General				Function entry trace
	1	2					Function exit trace
	2	4					General
	=======	=======	===============================	=======================

The appropriate set of values should be OR'd together and the result written to
the control file.  For example::

	echo $((1|4|8)) >/sys/module/cachefiles/parameters/debug

will turn on all function entry debugging.


On-demand Read
==============

When working in its original mode, CacheFiles serves as a local cache for a
remote networking fs - while in on-demand read mode, CacheFiles can boost the
scenario where on-demand read semantics are needed, e.g. container image
distribution.

The essential difference between these two modes is seen when a cache miss
occurs: In the original mode, the netfs will fetch the data from the remote
server and then write it to the cache file; in on-demand read mode, fetching
the data and writing it into the cache is delegated to a user daemon.

``CONFIG_CACHEFILES_ONDEMAND`` should be enabled to support on-demand read mode.


Protocol Communication
----------------------

The on-demand read mode uses a simple protocol for communication between kernel
and user daemon. The protocol can be modeled as::

	kernel --[request]--> user daemon --[reply]--> kernel

CacheFiles will send requests to the user daemon when needed.  The user daemon
should poll the devnode ('/dev/cachefiles') to check if there's a pending
request to be processed.  A POLLIN event will be returned when there's a pending
request.

The user daemon then reads the devnode to fetch a request to process.  It should
be noted that each read only gets one request. When it has finished processing
the request, the user daemon should write the reply to the devnode.

Each request starts with a message header of the form::

	struct cachefiles_msg {
		__u32 msg_id;
		__u32 opcode;
		__u32 len;
		__u32 object_id;
		__u8  data[];
	};

where:

	* ``msg_id`` is a unique ID identifying this request among all pending
	  requests.

	* ``opcode`` indicates the type of this request.

	* ``object_id`` is a unique ID identifying the cache file operated on.

	* ``data`` indicates the payload of this request.

	* ``len`` indicates the whole length of this request, including the
	  header and following type-specific payload.


Turning on On-demand Mode
-------------------------

An optional parameter becomes available to the "bind" command::

	bind [ondemand]

When the "bind" command is given no argument, it defaults to the original mode.
When it is given the "ondemand" argument, i.e. "bind ondemand", on-demand read
mode will be enabled.


The OPEN Request
----------------

When the netfs opens a cache file for the first time, a request with the
CACHEFILES_OP_OPEN opcode, a.k.a an OPEN request will be sent to the user
daemon.  The payload format is of the form::

	struct cachefiles_open {
		__u32 volume_key_size;
		__u32 cookie_key_size;
		__u32 fd;
		__u32 flags;
		__u8  data[];
	};

where:

	* ``data`` contains the volume_key followed directly by the cookie_key.
	  The volume key is a NUL-terminated string; the cookie key is binary
	  data.

	* ``volume_key_size`` indicates the size of the volume key in bytes.

	* ``cookie_key_size`` indicates the size of the cookie key in bytes.

	* ``fd`` indicates an anonymous fd referring to the cache file, through
	  which the user daemon can perform write/llseek file operations on the
	  cache file.


The user daemon can use the given (volume_key, cookie_key) pair to distinguish
the requested cache file.  With the given anonymous fd, the user daemon can
fetch the data and write it to the cache file in the background, even when
kernel has not triggered a cache miss yet.

Be noted that each cache file has a unique object_id, while it may have multiple
anonymous fds.  The user daemon may duplicate anonymous fds from the initial
anonymous fd indicated by the @fd field through dup().  Thus each object_id can
be mapped to multiple anonymous fds, while the usr daemon itself needs to
maintain the mapping.

When implementing a user daemon, please be careful of RLIMIT_NOFILE,
``/proc/sys/fs/nr_open`` and ``/proc/sys/fs/file-max``.  Typically these needn't
be huge since they're related to the number of open device blobs rather than
open files of each individual filesystem.

The user daemon should reply the OPEN request by issuing a "copen" (complete
open) command on the devnode::

	copen <msg_id>,<cache_size>

where:

	* ``msg_id`` must match the msg_id field of the OPEN request.

	* When >= 0, ``cache_size`` indicates the size of the cache file;
	  when < 0, ``cache_size`` indicates any error code encountered by the
	  user daemon.


The CLOSE Request
-----------------

When a cookie withdrawn, a CLOSE request (opcode CACHEFILES_OP_CLOSE) will be
sent to the user daemon.  This tells the user daemon to close all anonymous fds
associated with the given object_id.  The CLOSE request has no extra payload,
and shouldn't be replied.


The READ Request
----------------

When a cache miss is encountered in on-demand read mode, CacheFiles will send a
READ request (opcode CACHEFILES_OP_READ) to the user daemon. This tells the user
daemon to fetch the contents of the requested file range.  The payload is of the
form::

	struct cachefiles_read {
		__u64 off;
		__u64 len;
	};

where:

	* ``off`` indicates the starting offset of the requested file range.

	* ``len`` indicates the length of the requested file range.


When it receives a READ request, the user daemon should fetch the requested data
and write it to the cache file identified by object_id.

When it has finished processing the READ request, the user daemon should reply
by using the CACHEFILES_IOC_READ_COMPLETE ioctl on one of the anonymous fds
associated with the object_id given in the READ request.  The ioctl is of the
form::

	ioctl(fd, CACHEFILES_IOC_READ_COMPLETE, msg_id);

where:

	* ``fd`` is one of the anonymous fds associated with the object_id
	  given.

	* ``msg_id`` must match the msg_id field of the READ request.