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- ==========================
- FS-CACHE CACHE BACKEND API
- ==========================
-
-The FS-Cache system provides an API by which actual caches can be supplied to
-FS-Cache for it to then serve out to network filesystems and other interested
-parties.
-
-This API is declared in <linux/fscache-cache.h>.
-
-
-====================================
-INITIALISING AND REGISTERING A CACHE
-====================================
-
-To start off, a cache definition must be initialised and registered for each
-cache the backend wants to make available. For instance, CacheFS does this in
-the fill_super() operation on mounting.
-
-The cache definition (struct fscache_cache) should be initialised by calling:
-
- void fscache_init_cache(struct fscache_cache *cache,
- struct fscache_cache_ops *ops,
- const char *idfmt,
- ...);
-
-Where:
-
- (*) "cache" is a pointer to the cache definition;
-
- (*) "ops" is a pointer to the table of operations that the backend supports on
- this cache; and
-
- (*) "idfmt" is a format and printf-style arguments for constructing a label
- for the cache.
-
-
-The cache should then be registered with FS-Cache by passing a pointer to the
-previously initialised cache definition to:
-
- int fscache_add_cache(struct fscache_cache *cache,
- struct fscache_object *fsdef,
- const char *tagname);
-
-Two extra arguments should also be supplied:
-
- (*) "fsdef" which should point to the object representation for the FS-Cache
- master index in this cache. Netfs primary index entries will be created
- here. FS-Cache keeps the caller's reference to the index object if
- successful and will release it upon withdrawal of the cache.
-
- (*) "tagname" which, if given, should be a text string naming this cache. If
- this is NULL, the identifier will be used instead. For CacheFS, the
- identifier is set to name the underlying block device and the tag can be
- supplied by mount.
-
-This function may return -ENOMEM if it ran out of memory or -EEXIST if the tag
-is already in use. 0 will be returned on success.
-
-
-=====================
-UNREGISTERING A CACHE
-=====================
-
-A cache can be withdrawn from the system by calling this function with a
-pointer to the cache definition:
-
- void fscache_withdraw_cache(struct fscache_cache *cache);
-
-In CacheFS's case, this is called by put_super().
-
-
-========
-SECURITY
-========
-
-The cache methods are executed one of two contexts:
-
- (1) that of the userspace process that issued the netfs operation that caused
- the cache method to be invoked, or
-
- (2) that of one of the processes in the FS-Cache thread pool.
-
-In either case, this may not be an appropriate context in which to access the
-cache.
-
-The calling process's fsuid, fsgid and SELinux security identities may need to
-be masqueraded for the duration of the cache driver's access to the cache.
-This is left to the cache to handle; FS-Cache makes no effort in this regard.
-
-
-===================================
-CONTROL AND STATISTICS PRESENTATION
-===================================
-
-The cache may present data to the outside world through FS-Cache's interfaces
-in sysfs and procfs - the former for control and the latter for statistics.
-
-A sysfs directory called /sys/fs/fscache/<cachetag>/ is created if CONFIG_SYSFS
-is enabled. This is accessible through the kobject struct fscache_cache::kobj
-and is for use by the cache as it sees fit.
-
-
-========================
-RELEVANT DATA STRUCTURES
-========================
-
- (*) Index/Data file FS-Cache representation cookie:
-
- struct fscache_cookie {
- struct fscache_object_def *def;
- struct fscache_netfs *netfs;
- void *netfs_data;
- ...
- };
-
- The fields that might be of use to the backend describe the object
- definition, the netfs definition and the netfs's data for this cookie.
- The object definition contain functions supplied by the netfs for loading
- and matching index entries; these are required to provide some of the
- cache operations.
-
-
- (*) In-cache object representation:
-
- struct fscache_object {
- int debug_id;
- enum {
- FSCACHE_OBJECT_RECYCLING,
- ...
- } state;
- spinlock_t lock
- struct fscache_cache *cache;
- struct fscache_cookie *cookie;
- ...
- };
-
- Structures of this type should be allocated by the cache backend and
- passed to FS-Cache when requested by the appropriate cache operation. In
- the case of CacheFS, they're embedded in CacheFS's internal object
- structures.
-
- The debug_id is a simple integer that can be used in debugging messages
- that refer to a particular object. In such a case it should be printed
- using "OBJ%x" to be consistent with FS-Cache.
-
- Each object contains a pointer to the cookie that represents the object it
- is backing. An object should retired when put_object() is called if it is
- in state FSCACHE_OBJECT_RECYCLING. The fscache_object struct should be
- initialised by calling fscache_object_init(object).
-
-
- (*) FS-Cache operation record:
-
- struct fscache_operation {
- atomic_t usage;
- struct fscache_object *object;
- unsigned long flags;
- #define FSCACHE_OP_EXCLUSIVE
- void (*processor)(struct fscache_operation *op);
- void (*release)(struct fscache_operation *op);
- ...
- };
-
- FS-Cache has a pool of threads that it uses to give CPU time to the
- various asynchronous operations that need to be done as part of driving
- the cache. These are represented by the above structure. The processor
- method is called to give the op CPU time, and the release method to get
- rid of it when its usage count reaches 0.
-
- An operation can be made exclusive upon an object by setting the
- appropriate flag before enqueuing it with fscache_enqueue_operation(). If
- an operation needs more processing time, it should be enqueued again.
-
-
- (*) FS-Cache retrieval operation record:
-
- struct fscache_retrieval {
- struct fscache_operation op;
- struct address_space *mapping;
- struct list_head *to_do;
- ...
- };
-
- A structure of this type is allocated by FS-Cache to record retrieval and
- allocation requests made by the netfs. This struct is then passed to the
- backend to do the operation. The backend may get extra refs to it by
- calling fscache_get_retrieval() and refs may be discarded by calling
- fscache_put_retrieval().
-
- A retrieval operation can be used by the backend to do retrieval work. To
- do this, the retrieval->op.processor method pointer should be set
- appropriately by the backend and fscache_enqueue_retrieval() called to
- submit it to the thread pool. CacheFiles, for example, uses this to queue
- page examination when it detects PG_lock being cleared.
-
- The to_do field is an empty list available for the cache backend to use as
- it sees fit.
-
-
- (*) FS-Cache storage operation record:
-
- struct fscache_storage {
- struct fscache_operation op;
- pgoff_t store_limit;
- ...
- };
-
- A structure of this type is allocated by FS-Cache to record outstanding
- writes to be made. FS-Cache itself enqueues this operation and invokes
- the write_page() method on the object at appropriate times to effect
- storage.
-
-
-================
-CACHE OPERATIONS
-================
-
-The cache backend provides FS-Cache with a table of operations that can be
-performed on the denizens of the cache. These are held in a structure of type:
-
- struct fscache_cache_ops
-
- (*) Name of cache provider [mandatory]:
-
- const char *name
-
- This isn't strictly an operation, but should be pointed at a string naming
- the backend.
-
-
- (*) Allocate a new object [mandatory]:
-
- struct fscache_object *(*alloc_object)(struct fscache_cache *cache,
- struct fscache_cookie *cookie)
-
- This method is used to allocate a cache object representation to back a
- cookie in a particular cache. fscache_object_init() should be called on
- the object to initialise it prior to returning.
-
- This function may also be used to parse the index key to be used for
- multiple lookup calls to turn it into a more convenient form. FS-Cache
- will call the lookup_complete() method to allow the cache to release the
- form once lookup is complete or aborted.
-
-
- (*) Look up and create object [mandatory]:
-
- void (*lookup_object)(struct fscache_object *object)
-
- This method is used to look up an object, given that the object is already
- allocated and attached to the cookie. This should instantiate that object
- in the cache if it can.
-
- The method should call fscache_object_lookup_negative() as soon as
- possible if it determines the object doesn't exist in the cache. If the
- object is found to exist and the netfs indicates that it is valid then
- fscache_obtained_object() should be called once the object is in a
- position to have data stored in it. Similarly, fscache_obtained_object()
- should also be called once a non-present object has been created.
-
- If a lookup error occurs, fscache_object_lookup_error() should be called
- to abort the lookup of that object.
-
-
- (*) Release lookup data [mandatory]:
-
- void (*lookup_complete)(struct fscache_object *object)
-
- This method is called to ask the cache to release any resources it was
- using to perform a lookup.
-
-
- (*) Increment object refcount [mandatory]:
-
- struct fscache_object *(*grab_object)(struct fscache_object *object)
-
- This method is called to increment the reference count on an object. It
- may fail (for instance if the cache is being withdrawn) by returning NULL.
- It should return the object pointer if successful.
-
-
- (*) Lock/Unlock object [mandatory]:
-
- void (*lock_object)(struct fscache_object *object)
- void (*unlock_object)(struct fscache_object *object)
-
- These methods are used to exclusively lock an object. It must be possible
- to schedule with the lock held, so a spinlock isn't sufficient.
-
-
- (*) Pin/Unpin object [optional]:
-
- int (*pin_object)(struct fscache_object *object)
- void (*unpin_object)(struct fscache_object *object)
-
- These methods are used to pin an object into the cache. Once pinned an
- object cannot be reclaimed to make space. Return -ENOSPC if there's not
- enough space in the cache to permit this.
-
-
- (*) Update object [mandatory]:
-
- int (*update_object)(struct fscache_object *object)
-
- This is called to update the index entry for the specified object. The
- new information should be in object->cookie->netfs_data. This can be
- obtained by calling object->cookie->def->get_aux()/get_attr().
-
-
- (*) Discard object [mandatory]:
-
- void (*drop_object)(struct fscache_object *object)
-
- This method is called to indicate that an object has been unbound from its
- cookie, and that the cache should release the object's resources and
- retire it if it's in state FSCACHE_OBJECT_RECYCLING.
-
- This method should not attempt to release any references held by the
- caller. The caller will invoke the put_object() method as appropriate.
-
-
- (*) Release object reference [mandatory]:
-
- void (*put_object)(struct fscache_object *object)
-
- This method is used to discard a reference to an object. The object may
- be freed when all the references to it are released.
-
-
- (*) Synchronise a cache [mandatory]:
-
- void (*sync)(struct fscache_cache *cache)
-
- This is called to ask the backend to synchronise a cache with its backing
- device.
-
-
- (*) Dissociate a cache [mandatory]:
-
- void (*dissociate_pages)(struct fscache_cache *cache)
-
- This is called to ask a cache to perform any page dissociations as part of
- cache withdrawal.
-
-
- (*) Notification that the attributes on a netfs file changed [mandatory]:
-
- int (*attr_changed)(struct fscache_object *object);
-
- This is called to indicate to the cache that certain attributes on a netfs
- file have changed (for example the maximum size a file may reach). The
- cache can read these from the netfs by calling the cookie's get_attr()
- method.
-
- The cache may use the file size information to reserve space on the cache.
- It should also call fscache_set_store_limit() to indicate to FS-Cache the
- highest byte it's willing to store for an object.
-
- This method may return -ve if an error occurred or the cache object cannot
- be expanded. In such a case, the object will be withdrawn from service.
-
- This operation is run asynchronously from FS-Cache's thread pool, and
- storage and retrieval operations from the netfs are excluded during the
- execution of this operation.
-
-
- (*) Reserve cache space for an object's data [optional]:
-
- int (*reserve_space)(struct fscache_object *object, loff_t size);
-
- This is called to request that cache space be reserved to hold the data
- for an object and the metadata used to track it. Zero size should be
- taken as request to cancel a reservation.
-
- This should return 0 if successful, -ENOSPC if there isn't enough space
- available, or -ENOMEM or -EIO on other errors.
-
- The reservation may exceed the current size of the object, thus permitting
- future expansion. If the amount of space consumed by an object would
- exceed the reservation, it's permitted to refuse requests to allocate
- pages, but not required. An object may be pruned down to its reservation
- size if larger than that already.
-
-
- (*) Request page be read from cache [mandatory]:
-
- int (*read_or_alloc_page)(struct fscache_retrieval *op,
- struct page *page,
- gfp_t gfp)
-
- This is called to attempt to read a netfs page from the cache, or to
- reserve a backing block if not. FS-Cache will have done as much checking
- as it can before calling, but most of the work belongs to the backend.
-
- If there's no page in the cache, then -ENODATA should be returned if the
- backend managed to reserve a backing block; -ENOBUFS or -ENOMEM if it
- didn't.
-
- If there is suitable data in the cache, then a read operation should be
- queued and 0 returned. When the read finishes, fscache_end_io() should be
- called.
-
- The fscache_mark_pages_cached() should be called for the page if any cache
- metadata is retained. This will indicate to the netfs that the page needs
- explicit uncaching. This operation takes a pagevec, thus allowing several
- pages to be marked at once.
-
- The retrieval record pointed to by op should be retained for each page
- queued and released when I/O on the page has been formally ended.
- fscache_get/put_retrieval() are available for this purpose.
-
- The retrieval record may be used to get CPU time via the FS-Cache thread
- pool. If this is desired, the op->op.processor should be set to point to
- the appropriate processing routine, and fscache_enqueue_retrieval() should
- be called at an appropriate point to request CPU time. For instance, the
- retrieval routine could be enqueued upon the completion of a disk read.
- The to_do field in the retrieval record is provided to aid in this.
-
- If an I/O error occurs, fscache_io_error() should be called and -ENOBUFS
- returned if possible or fscache_end_io() called with a suitable error
- code..
-
-
- (*) Request pages be read from cache [mandatory]:
-
- int (*read_or_alloc_pages)(struct fscache_retrieval *op,
- struct list_head *pages,
- unsigned *nr_pages,
- gfp_t gfp)
-
- This is like the read_or_alloc_page() method, except it is handed a list
- of pages instead of one page. Any pages on which a read operation is
- started must be added to the page cache for the specified mapping and also
- to the LRU. Such pages must also be removed from the pages list and
- *nr_pages decremented per page.
-
- If there was an error such as -ENOMEM, then that should be returned; else
- if one or more pages couldn't be read or allocated, then -ENOBUFS should
- be returned; else if one or more pages couldn't be read, then -ENODATA
- should be returned. If all the pages are dispatched then 0 should be
- returned.
-
-
- (*) Request page be allocated in the cache [mandatory]:
-
- int (*allocate_page)(struct fscache_retrieval *op,
- struct page *page,
- gfp_t gfp)
-
- This is like the read_or_alloc_page() method, except that it shouldn't
- read from the cache, even if there's data there that could be retrieved.
- It should, however, set up any internal metadata required such that
- the write_page() method can write to the cache.
-
- If there's no backing block available, then -ENOBUFS should be returned
- (or -ENOMEM if there were other problems). If a block is successfully
- allocated, then the netfs page should be marked and 0 returned.
-
-
- (*) Request pages be allocated in the cache [mandatory]:
-
- int (*allocate_pages)(struct fscache_retrieval *op,
- struct list_head *pages,
- unsigned *nr_pages,
- gfp_t gfp)
-
- This is an multiple page version of the allocate_page() method. pages and
- nr_pages should be treated as for the read_or_alloc_pages() method.
-
-
- (*) Request page be written to cache [mandatory]:
-
- int (*write_page)(struct fscache_storage *op,
- struct page *page);
-
- This is called to write from a page on which there was a previously
- successful read_or_alloc_page() call or similar. FS-Cache filters out
- pages that don't have mappings.
-
- This method is called asynchronously from the FS-Cache thread pool. It is
- not required to actually store anything, provided -ENODATA is then
- returned to the next read of this page.
-
- If an error occurred, then a negative error code should be returned,
- otherwise zero should be returned. FS-Cache will take appropriate action
- in response to an error, such as withdrawing this object.
-
- If this method returns success then FS-Cache will inform the netfs
- appropriately.
-
-
- (*) Discard retained per-page metadata [mandatory]:
-
- void (*uncache_page)(struct fscache_object *object, struct page *page)
-
- This is called when a netfs page is being evicted from the pagecache. The
- cache backend should tear down any internal representation or tracking it
- maintains for this page.
-
-
-==================
-FS-CACHE UTILITIES
-==================
-
-FS-Cache provides some utilities that a cache backend may make use of:
-
- (*) Note occurrence of an I/O error in a cache:
-
- void fscache_io_error(struct fscache_cache *cache)
-
- This tells FS-Cache that an I/O error occurred in the cache. After this
- has been called, only resource dissociation operations (object and page
- release) will be passed from the netfs to the cache backend for the
- specified cache.
-
- This does not actually withdraw the cache. That must be done separately.
-
-
- (*) Invoke the retrieval I/O completion function:
-
- void fscache_end_io(struct fscache_retrieval *op, struct page *page,
- int error);
-
- This is called to note the end of an attempt to retrieve a page. The
- error value should be 0 if successful and an error otherwise.
-
-
- (*) Set highest store limit:
-
- void fscache_set_store_limit(struct fscache_object *object,
- loff_t i_size);
-
- This sets the limit FS-Cache imposes on the highest byte it's willing to
- try and store for a netfs. Any page over this limit is automatically
- rejected by fscache_read_alloc_page() and co with -ENOBUFS.
-
-
- (*) Mark pages as being cached:
-
- void fscache_mark_pages_cached(struct fscache_retrieval *op,
- struct pagevec *pagevec);
-
- This marks a set of pages as being cached. After this has been called,
- the netfs must call fscache_uncache_page() to unmark the pages.
-
-
- (*) Perform coherency check on an object:
-
- enum fscache_checkaux fscache_check_aux(struct fscache_object *object,
- const void *data,
- uint16_t datalen);
-
- This asks the netfs to perform a coherency check on an object that has
- just been looked up. The cookie attached to the object will determine the
- netfs to use. data and datalen should specify where the auxiliary data
- retrieved from the cache can be found.
-
- One of three values will be returned:
-
- (*) FSCACHE_CHECKAUX_OKAY
-
- The coherency data indicates the object is valid as is.
-
- (*) FSCACHE_CHECKAUX_NEEDS_UPDATE
-
- The coherency data needs updating, but otherwise the object is
- valid.
-
- (*) FSCACHE_CHECKAUX_OBSOLETE
-
- The coherency data indicates that the object is obsolete and should
- be discarded.
-
-
- (*) Initialise a freshly allocated object:
-
- void fscache_object_init(struct fscache_object *object);
-
- This initialises all the fields in an object representation.
-
-
- (*) Indicate the destruction of an object:
-
- void fscache_object_destroyed(struct fscache_cache *cache);
-
- This must be called to inform FS-Cache that an object that belonged to a
- cache has been destroyed and deallocated. This will allow continuation
- of the cache withdrawal process when it is stopped pending destruction of
- all the objects.
-
-
- (*) Indicate negative lookup on an object:
-
- void fscache_object_lookup_negative(struct fscache_object *object);
-
- This is called to indicate to FS-Cache that a lookup process for an object
- found a negative result.
-
- This changes the state of an object to permit reads pending on lookup
- completion to go off and start fetching data from the netfs server as it's
- known at this point that there can't be any data in the cache.
-
- This may be called multiple times on an object. Only the first call is
- significant - all subsequent calls are ignored.
-
-
- (*) Indicate an object has been obtained:
-
- void fscache_obtained_object(struct fscache_object *object);
-
- This is called to indicate to FS-Cache that a lookup process for an object
- produced a positive result, or that an object was created. This should
- only be called once for any particular object.
-
- This changes the state of an object to indicate:
-
- (1) if no call to fscache_object_lookup_negative() has been made on
- this object, that there may be data available, and that reads can
- now go and look for it; and
-
- (2) that writes may now proceed against this object.
-
-
- (*) Indicate that object lookup failed:
-
- void fscache_object_lookup_error(struct fscache_object *object);
-
- This marks an object as having encountered a fatal error (usually EIO)
- and causes it to move into a state whereby it will be withdrawn as soon
- as possible.
-
-
- (*) Get and release references on a retrieval record:
-
- void fscache_get_retrieval(struct fscache_retrieval *op);
- void fscache_put_retrieval(struct fscache_retrieval *op);
-
- These two functions are used to retain a retrieval record whilst doing
- asynchronous data retrieval and block allocation.
-
-
- (*) Enqueue a retrieval record for processing.
-
- void fscache_enqueue_retrieval(struct fscache_retrieval *op);
-
- This enqueues a retrieval record for processing by the FS-Cache thread
- pool. One of the threads in the pool will invoke the retrieval record's
- op->op.processor callback function. This function may be called from
- within the callback function.
-
-
- (*) List of object state names:
-
- const char *fscache_object_states[];
-
- For debugging purposes, this may be used to turn the state that an object
- is in into a text string for display purposes.