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diff --git a/Documentation/filesystems/caching/operations.rst b/Documentation/filesystems/caching/operations.rst new file mode 100644 index 000000000000..f7ddcc028939 --- /dev/null +++ b/Documentation/filesystems/caching/operations.rst @@ -0,0 +1,210 @@ +.. SPDX-License-Identifier: GPL-2.0 + +================================ +Asynchronous Operations Handling +================================ + +By: David Howells <dhowells@redhat.com> + +.. Contents: + + (*) Overview. + + (*) Operation record initialisation. + + (*) Parameters. + + (*) Procedure. + + (*) Asynchronous callback. + + +Overview +======== + +FS-Cache has an asynchronous operations handling facility that it uses for its +data storage and retrieval routines. Its operations are represented by +fscache_operation structs, though these are usually embedded into some other +structure. + +This facility is available to and expected to be be used by the cache backends, +and FS-Cache will create operations and pass them off to the appropriate cache +backend for completion. + +To make use of this facility, <linux/fscache-cache.h> should be #included. + + +Operation Record Initialisation +=============================== + +An operation is recorded in an fscache_operation struct:: + + struct fscache_operation { + union { + struct work_struct fast_work; + struct slow_work slow_work; + }; + unsigned long flags; + fscache_operation_processor_t processor; + ... + }; + +Someone wanting to issue an operation should allocate something with this +struct embedded in it. They should initialise it by calling:: + + void fscache_operation_init(struct fscache_operation *op, + fscache_operation_release_t release); + +with the operation to be initialised and the release function to use. + +The op->flags parameter should be set to indicate the CPU time provision and +the exclusivity (see the Parameters section). + +The op->fast_work, op->slow_work and op->processor flags should be set as +appropriate for the CPU time provision (see the Parameters section). + +FSCACHE_OP_WAITING may be set in op->flags prior to each submission of the +operation and waited for afterwards. + + +Parameters +========== + +There are a number of parameters that can be set in the operation record's flag +parameter. There are three options for the provision of CPU time in these +operations: + + (1) The operation may be done synchronously (FSCACHE_OP_MYTHREAD). A thread + may decide it wants to handle an operation itself without deferring it to + another thread. + + This is, for example, used in read operations for calling readpages() on + the backing filesystem in CacheFiles. Although readpages() does an + asynchronous data fetch, the determination of whether pages exist is done + synchronously - and the netfs does not proceed until this has been + determined. + + If this option is to be used, FSCACHE_OP_WAITING must be set in op->flags + before submitting the operation, and the operating thread must wait for it + to be cleared before proceeding:: + + wait_on_bit(&op->flags, FSCACHE_OP_WAITING, + TASK_UNINTERRUPTIBLE); + + + (2) The operation may be fast asynchronous (FSCACHE_OP_FAST), in which case it + will be given to keventd to process. Such an operation is not permitted + to sleep on I/O. + + This is, for example, used by CacheFiles to copy data from a backing fs + page to a netfs page after the backing fs has read the page in. + + If this option is used, op->fast_work and op->processor must be + initialised before submitting the operation:: + + INIT_WORK(&op->fast_work, do_some_work); + + + (3) The operation may be slow asynchronous (FSCACHE_OP_SLOW), in which case it + will be given to the slow work facility to process. Such an operation is + permitted to sleep on I/O. + + This is, for example, used by FS-Cache to handle background writes of + pages that have just been fetched from a remote server. + + If this option is used, op->slow_work and op->processor must be + initialised before submitting the operation:: + + fscache_operation_init_slow(op, processor) + + +Furthermore, operations may be one of two types: + + (1) Exclusive (FSCACHE_OP_EXCLUSIVE). Operations of this type may not run in + conjunction with any other operation on the object being operated upon. + + An example of this is the attribute change operation, in which the file + being written to may need truncation. + + (2) Shareable. Operations of this type may be running simultaneously. It's + up to the operation implementation to prevent interference between other + operations running at the same time. + + +Procedure +========= + +Operations are used through the following procedure: + + (1) The submitting thread must allocate the operation and initialise it + itself. Normally this would be part of a more specific structure with the + generic op embedded within. + + (2) The submitting thread must then submit the operation for processing using + one of the following two functions:: + + int fscache_submit_op(struct fscache_object *object, + struct fscache_operation *op); + + int fscache_submit_exclusive_op(struct fscache_object *object, + struct fscache_operation *op); + + The first function should be used to submit non-exclusive ops and the + second to submit exclusive ones. The caller must still set the + FSCACHE_OP_EXCLUSIVE flag. + + If successful, both functions will assign the operation to the specified + object and return 0. -ENOBUFS will be returned if the object specified is + permanently unavailable. + + The operation manager will defer operations on an object that is still + undergoing lookup or creation. The operation will also be deferred if an + operation of conflicting exclusivity is in progress on the object. + + If the operation is asynchronous, the manager will retain a reference to + it, so the caller should put their reference to it by passing it to:: + + void fscache_put_operation(struct fscache_operation *op); + + (3) If the submitting thread wants to do the work itself, and has marked the + operation with FSCACHE_OP_MYTHREAD, then it should monitor + FSCACHE_OP_WAITING as described above and check the state of the object if + necessary (the object might have died while the thread was waiting). + + When it has finished doing its processing, it should call + fscache_op_complete() and fscache_put_operation() on it. + + (4) The operation holds an effective lock upon the object, preventing other + exclusive ops conflicting until it is released. The operation can be + enqueued for further immediate asynchronous processing by adjusting the + CPU time provisioning option if necessary, eg:: + + op->flags &= ~FSCACHE_OP_TYPE; + op->flags |= ~FSCACHE_OP_FAST; + + and calling:: + + void fscache_enqueue_operation(struct fscache_operation *op) + + This can be used to allow other things to have use of the worker thread + pools. + + +Asynchronous Callback +===================== + +When used in asynchronous mode, the worker thread pool will invoke the +processor method with a pointer to the operation. This should then get at the +container struct by using container_of():: + + static void fscache_write_op(struct fscache_operation *_op) + { + struct fscache_storage *op = + container_of(_op, struct fscache_storage, op); + ... + } + +The caller holds a reference on the operation, and will invoke +fscache_put_operation() when the processor function returns. The processor +function is at liberty to call fscache_enqueue_operation() or to take extra +references. |