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diff --git a/Documentation/filesystems/caching/backend-api.txt b/Documentation/filesystems/caching/backend-api.txt deleted file mode 100644 index c418280c915f..000000000000 --- a/Documentation/filesystems/caching/backend-api.txt +++ /dev/null @@ -1,726 +0,0 @@ - ========================== - 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 - - -===================== -UNREGISTERING A CACHE -===================== - -A cache can be withdrawn from the system by calling this function with a -pointer to the cache definition: - - void fscache_withdraw_cache(struct fscache_cache *cache); - -In CacheFS's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. - - - (*) Check coherency state of an object [mandatory]: - - int (*check_consistency)(struct fscache_object *object) - - This method is called to have the cache check the saved auxiliary data of - the object against the netfs's idea of the state. 0 should be returned - if they're consistent and -ESTALE otherwise. -ENOMEM and -ERESTARTSYS - may also be returned. - - (*) 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(). - - - (*) Invalidate data object [mandatory]: - - int (*invalidate_object)(struct fscache_operation *op) - - This is called to invalidate a data object (as pointed to by op->object). - All the data stored for this object should be discarded and an - attr_changed operation should be performed. The caller will follow up - with an object update operation. - - fscache_op_complete() must be called on op before returning. - - - (*) 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. - - fscache_put_retrieval() should be called after a page or pages are dealt - with. This will complete the operation when all pages are dealt with. - - - (*) 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. - - - (*) Record that one or more pages being retrieved or allocated have been dealt - with: - - void fscache_retrieval_complete(struct fscache_retrieval *op, - int n_pages); - - This is called to record the fact that one or more pages have been dealt - with and are no longer the concern of this operation. When the number of - pages remaining in the operation reaches 0, the operation will be - completed. - - - (*) Record operation completion: - - void fscache_op_complete(struct fscache_operation *op); - - This is called to record the completion of an operation. This deducts - this operation from the parent object's run state, potentially permitting - one or more pending operations to start running. - - - (*) 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. - - - (*) Indicate that a stale object was found and discarded: - - void fscache_object_retrying_stale(struct fscache_object *object); - - This is called to indicate that the lookup procedure found an object in - the cache that the netfs decided was stale. The object has been - discarded from the cache and the lookup will be performed again. - - - (*) Indicate that the caching backend killed an object: - - void fscache_object_mark_killed(struct fscache_object *object, - enum fscache_why_object_killed why); - - This is called to indicate that the cache backend preemptively killed an - object. The why parameter should be set to indicate the reason: - - FSCACHE_OBJECT_IS_STALE - the object was stale and needs discarding. - FSCACHE_OBJECT_NO_SPACE - there was insufficient cache space - FSCACHE_OBJECT_WAS_RETIRED - the object was retired when relinquished. - FSCACHE_OBJECT_WAS_CULLED - the object was culled to make space. - - - (*) 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 while 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. |