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