1	     ====================================================
2	     IN-KERNEL CACHE OBJECT REPRESENTATION AND MANAGEMENT
3	     ====================================================
4
5By: David Howells <dhowells@redhat.com>
6
7Contents:
8
9 (*) Representation
10
11 (*) Object management state machine.
12
13     - Provision of cpu time.
14     - Locking simplification.
15
16 (*) The set of states.
17
18 (*) The set of events.
19
20
21==============
22REPRESENTATION
23==============
24
25FS-Cache maintains an in-kernel representation of each object that a netfs is
26currently interested in.  Such objects are represented by the fscache_cookie
27struct and are referred to as cookies.
28
29FS-Cache also maintains a separate in-kernel representation of the objects that
30a cache backend is currently actively caching.  Such objects are represented by
31the fscache_object struct.  The cache backends allocate these upon request, and
32are expected to embed them in their own representations.  These are referred to
33as objects.
34
35There is a 1:N relationship between cookies and objects.  A cookie may be
36represented by multiple objects - an index may exist in more than one cache -
37or even by no objects (it may not be cached).
38
39Furthermore, both cookies and objects are hierarchical.  The two hierarchies
40correspond, but the cookies tree is a superset of the union of the object trees
41of multiple caches:
42
43	    NETFS INDEX TREE               :      CACHE 1     :      CACHE 2
44	                                   :                  :
45	                                   :   +-----------+  :
46	                          +----------->|  IObject  |  :
47	      +-----------+       |        :   +-----------+  :
48	      |  ICookie  |-------+        :         |        :
49	      +-----------+       |        :         |        :   +-----------+
50	            |             +------------------------------>|  IObject  |
51	            |                      :         |        :   +-----------+
52	            |                      :         V        :         |
53	            |                      :   +-----------+  :         |
54	            V             +----------->|  IObject  |  :         |
55	      +-----------+       |        :   +-----------+  :         |
56	      |  ICookie  |-------+        :         |        :         V
57	      +-----------+       |        :         |        :   +-----------+
58	            |             +------------------------------>|  IObject  |
59	      +-----+-----+                :         |        :   +-----------+
60	      |           |                :         |        :         |
61	      V           |                :         V        :         |
62	+-----------+     |                :   +-----------+  :         |
63	|  ICookie  |------------------------->|  IObject  |  :         |
64	+-----------+     |                :   +-----------+  :         |
65	      |           V                :         |        :         V
66	      |     +-----------+          :         |        :   +-----------+
67	      |     |  ICookie  |-------------------------------->|  IObject  |
68	      |     +-----------+          :         |        :   +-----------+
69	      V           |                :         V        :         |
70	+-----------+     |                :   +-----------+  :         |
71	|  DCookie  |------------------------->|  DObject  |  :         |
72	+-----------+     |                :   +-----------+  :         |
73	                  |                :                  :         |
74	          +-------+-------+        :                  :         |
75	          |               |        :                  :         |
76	          V               V        :                  :         V
77	    +-----------+   +-----------+  :                  :   +-----------+
78	    |  DCookie  |   |  DCookie  |------------------------>|  DObject  |
79	    +-----------+   +-----------+  :                  :   +-----------+
80	                                   :                  :
81
82In the above illustration, ICookie and IObject represent indices and DCookie
83and DObject represent data storage objects.  Indices may have representation in
84multiple caches, but currently, non-index objects may not.  Objects of any type
85may also be entirely unrepresented.
86
87As far as the netfs API goes, the netfs is only actually permitted to see
88pointers to the cookies.  The cookies themselves and any objects attached to
89those cookies are hidden from it.
90
91
92===============================
93OBJECT MANAGEMENT STATE MACHINE
94===============================
95
96Within FS-Cache, each active object is managed by its own individual state
97machine.  The state for an object is kept in the fscache_object struct, in
98object->state.  A cookie may point to a set of objects that are in different
99states.
100
101Each state has an action associated with it that is invoked when the machine
102wakes up in that state.  There are four logical sets of states:
103
104 (1) Preparation: states that wait for the parent objects to become ready.  The
105     representations are hierarchical, and it is expected that an object must
106     be created or accessed with respect to its parent object.
107
108 (2) Initialisation: states that perform lookups in the cache and validate
109     what's found and that create on disk any missing metadata.
110
111 (3) Normal running: states that allow netfs operations on objects to proceed
112     and that update the state of objects.
113
114 (4) Termination: states that detach objects from their netfs cookies, that
115     delete objects from disk, that handle disk and system errors and that free
116     up in-memory resources.
117
118
119In most cases, transitioning between states is in response to signalled events.
120When a state has finished processing, it will usually set the mask of events in
121which it is interested (object->event_mask) and relinquish the worker thread.
122Then when an event is raised (by calling fscache_raise_event()), if the event
123is not masked, the object will be queued for processing (by calling
124fscache_enqueue_object()).
125
126
127PROVISION OF CPU TIME
128---------------------
129
130The work to be done by the various states was given CPU time by the threads of
131the slow work facility.  This was used in preference to the workqueue facility
132because:
133
134 (1) Threads may be completely occupied for very long periods of time by a
135     particular work item.  These state actions may be doing sequences of
136     synchronous, journalled disk accesses (lookup, mkdir, create, setxattr,
137     getxattr, truncate, unlink, rmdir, rename).
138
139 (2) Threads may do little actual work, but may rather spend a lot of time
140     sleeping on I/O.  This means that single-threaded and 1-per-CPU-threaded
141     workqueues don't necessarily have the right numbers of threads.
142
143
144LOCKING SIMPLIFICATION
145----------------------
146
147Because only one worker thread may be operating on any particular object's
148state machine at once, this simplifies the locking, particularly with respect
149to disconnecting the netfs's representation of a cache object (fscache_cookie)
150from the cache backend's representation (fscache_object) - which may be
151requested from either end.
152
153
154=================
155THE SET OF STATES
156=================
157
158The object state machine has a set of states that it can be in.  There are
159preparation states in which the object sets itself up and waits for its parent
160object to transit to a state that allows access to its children:
161
162 (1) State FSCACHE_OBJECT_INIT.
163
164     Initialise the object and wait for the parent object to become active.  In
165     the cache, it is expected that it will not be possible to look an object
166     up from the parent object, until that parent object itself has been looked
167     up.
168
169There are initialisation states in which the object sets itself up and accesses
170disk for the object metadata:
171
172 (2) State FSCACHE_OBJECT_LOOKING_UP.
173
174     Look up the object on disk, using the parent as a starting point.
175     FS-Cache expects the cache backend to probe the cache to see whether this
176     object is represented there, and if it is, to see if it's valid (coherency
177     management).
178
179     The cache should call fscache_object_lookup_negative() to indicate lookup
180     failure for whatever reason, and should call fscache_obtained_object() to
181     indicate success.
182
183     At the completion of lookup, FS-Cache will let the netfs go ahead with
184     read operations, no matter whether the file is yet cached.  If not yet
185     cached, read operations will be immediately rejected with ENODATA until
186     the first known page is uncached - as to that point there can be no data
187     to be read out of the cache for that file that isn't currently also held
188     in the pagecache.
189
190 (3) State FSCACHE_OBJECT_CREATING.
191
192     Create an object on disk, using the parent as a starting point.  This
193     happens if the lookup failed to find the object, or if the object's
194     coherency data indicated what's on disk is out of date.  In this state,
195     FS-Cache expects the cache to create
196
197     The cache should call fscache_obtained_object() if creation completes
198     successfully, fscache_object_lookup_negative() otherwise.
199
200     At the completion of creation, FS-Cache will start processing write
201     operations the netfs has queued for an object.  If creation failed, the
202     write ops will be transparently discarded, and nothing recorded in the
203     cache.
204
205There are some normal running states in which the object spends its time
206servicing netfs requests:
207
208 (4) State FSCACHE_OBJECT_AVAILABLE.
209
210     A transient state in which pending operations are started, child objects
211     are permitted to advance from FSCACHE_OBJECT_INIT state, and temporary
212     lookup data is freed.
213
214 (5) State FSCACHE_OBJECT_ACTIVE.
215
216     The normal running state.  In this state, requests the netfs makes will be
217     passed on to the cache.
218
219 (6) State FSCACHE_OBJECT_INVALIDATING.
220
221     The object is undergoing invalidation.  When the state comes here, it
222     discards all pending read, write and attribute change operations as it is
223     going to clear out the cache entirely and reinitialise it.  It will then
224     continue to the FSCACHE_OBJECT_UPDATING state.
225
226 (7) State FSCACHE_OBJECT_UPDATING.
227
228     The state machine comes here to update the object in the cache from the
229     netfs's records.  This involves updating the auxiliary data that is used
230     to maintain coherency.
231
232And there are terminal states in which an object cleans itself up, deallocates
233memory and potentially deletes stuff from disk:
234
235 (8) State FSCACHE_OBJECT_LC_DYING.
236
237     The object comes here if it is dying because of a lookup or creation
238     error.  This would be due to a disk error or system error of some sort.
239     Temporary data is cleaned up, and the parent is released.
240
241 (9) State FSCACHE_OBJECT_DYING.
242
243     The object comes here if it is dying due to an error, because its parent
244     cookie has been relinquished by the netfs or because the cache is being
245     withdrawn.
246
247     Any child objects waiting on this one are given CPU time so that they too
248     can destroy themselves.  This object waits for all its children to go away
249     before advancing to the next state.
250
251(10) State FSCACHE_OBJECT_ABORT_INIT.
252
253     The object comes to this state if it was waiting on its parent in
254     FSCACHE_OBJECT_INIT, but its parent died.  The object will destroy itself
255     so that the parent may proceed from the FSCACHE_OBJECT_DYING state.
256
257(11) State FSCACHE_OBJECT_RELEASING.
258(12) State FSCACHE_OBJECT_RECYCLING.
259
260     The object comes to one of these two states when dying once it is rid of
261     all its children, if it is dying because the netfs relinquished its
262     cookie.  In the first state, the cached data is expected to persist, and
263     in the second it will be deleted.
264
265(13) State FSCACHE_OBJECT_WITHDRAWING.
266
267     The object transits to this state if the cache decides it wants to
268     withdraw the object from service, perhaps to make space, but also due to
269     error or just because the whole cache is being withdrawn.
270
271(14) State FSCACHE_OBJECT_DEAD.
272
273     The object transits to this state when the in-memory object record is
274     ready to be deleted.  The object processor shouldn't ever see an object in
275     this state.
276
277
278THE SET OF EVENTS
279-----------------
280
281There are a number of events that can be raised to an object state machine:
282
283 (*) FSCACHE_OBJECT_EV_UPDATE
284
285     The netfs requested that an object be updated.  The state machine will ask
286     the cache backend to update the object, and the cache backend will ask the
287     netfs for details of the change through its cookie definition ops.
288
289 (*) FSCACHE_OBJECT_EV_CLEARED
290
291     This is signalled in two circumstances:
292
293     (a) when an object's last child object is dropped and
294
295     (b) when the last operation outstanding on an object is completed.
296
297     This is used to proceed from the dying state.
298
299 (*) FSCACHE_OBJECT_EV_ERROR
300
301     This is signalled when an I/O error occurs during the processing of some
302     object.
303
304 (*) FSCACHE_OBJECT_EV_RELEASE
305 (*) FSCACHE_OBJECT_EV_RETIRE
306
307     These are signalled when the netfs relinquishes a cookie it was using.
308     The event selected depends on whether the netfs asks for the backing
309     object to be retired (deleted) or retained.
310
311 (*) FSCACHE_OBJECT_EV_WITHDRAW
312
313     This is signalled when the cache backend wants to withdraw an object.
314     This means that the object will have to be detached from the netfs's
315     cookie.
316
317Because the withdrawing releasing/retiring events are all handled by the object
318state machine, it doesn't matter if there's a collision with both ends trying
319to sever the connection at the same time.  The state machine can just pick
320which one it wants to honour, and that effects the other.
321