527 lines
18 KiB
ReStructuredText
527 lines
18 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=================================
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NETWORK FILESYSTEM HELPER LIBRARY
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=================================
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.. Contents:
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- Overview.
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- Buffered read helpers.
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- Read helper functions.
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- Read helper structures.
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- Read helper operations.
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- Read helper procedure.
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- Read helper cache API.
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Overview
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========
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The network filesystem helper library is a set of functions designed to aid a
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network filesystem in implementing VM/VFS operations. For the moment, that
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just includes turning various VM buffered read operations into requests to read
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from the server. The helper library, however, can also interpose other
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services, such as local caching or local data encryption.
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Note that the library module doesn't link against local caching directly, so
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access must be provided by the netfs.
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Buffered Read Helpers
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=====================
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The library provides a set of read helpers that handle the ->readpage(),
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->readahead() and much of the ->write_begin() VM operations and translate them
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into a common call framework.
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The following services are provided:
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* Handles transparent huge pages (THPs).
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* Insulates the netfs from VM interface changes.
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* Allows the netfs to arbitrarily split reads up into pieces, even ones that
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don't match page sizes or page alignments and that may cross pages.
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* Allows the netfs to expand a readahead request in both directions to meet
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its needs.
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* Allows the netfs to partially fulfil a read, which will then be resubmitted.
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* Handles local caching, allowing cached data and server-read data to be
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interleaved for a single request.
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* Handles clearing of bufferage that aren't on the server.
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* Handle retrying of reads that failed, switching reads from the cache to the
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server as necessary.
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* In the future, this is a place that other services can be performed, such as
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local encryption of data to be stored remotely or in the cache.
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From the network filesystem, the helpers require a table of operations. This
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includes a mandatory method to issue a read operation along with a number of
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optional methods.
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Read Helper Functions
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---------------------
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Three read helpers are provided::
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* void netfs_readahead(struct readahead_control *ractl,
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const struct netfs_read_request_ops *ops,
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void *netfs_priv);``
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* int netfs_readpage(struct file *file,
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struct page *page,
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const struct netfs_read_request_ops *ops,
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void *netfs_priv);
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* int netfs_write_begin(struct file *file,
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struct address_space *mapping,
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loff_t pos,
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unsigned int len,
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unsigned int flags,
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struct page **_page,
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void **_fsdata,
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const struct netfs_read_request_ops *ops,
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void *netfs_priv);
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Each corresponds to a VM operation, with the addition of a couple of parameters
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for the use of the read helpers:
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* ``ops``
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A table of operations through which the helpers can talk to the filesystem.
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* ``netfs_priv``
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Filesystem private data (can be NULL).
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Both of these values will be stored into the read request structure.
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For ->readahead() and ->readpage(), the network filesystem should just jump
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into the corresponding read helper; whereas for ->write_begin(), it may be a
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little more complicated as the network filesystem might want to flush
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conflicting writes or track dirty data and needs to put the acquired page if an
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error occurs after calling the helper.
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The helpers manage the read request, calling back into the network filesystem
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through the suppplied table of operations. Waits will be performed as
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necessary before returning for helpers that are meant to be synchronous.
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If an error occurs and netfs_priv is non-NULL, ops->cleanup() will be called to
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deal with it. If some parts of the request are in progress when an error
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occurs, the request will get partially completed if sufficient data is read.
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Additionally, there is::
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* void netfs_subreq_terminated(struct netfs_read_subrequest *subreq,
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ssize_t transferred_or_error,
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bool was_async);
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which should be called to complete a read subrequest. This is given the number
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of bytes transferred or a negative error code, plus a flag indicating whether
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the operation was asynchronous (ie. whether the follow-on processing can be
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done in the current context, given this may involve sleeping).
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Read Helper Structures
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----------------------
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The read helpers make use of a couple of structures to maintain the state of
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the read. The first is a structure that manages a read request as a whole::
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struct netfs_read_request {
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struct inode *inode;
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struct address_space *mapping;
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struct netfs_cache_resources cache_resources;
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void *netfs_priv;
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loff_t start;
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size_t len;
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loff_t i_size;
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const struct netfs_read_request_ops *netfs_ops;
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unsigned int debug_id;
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...
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};
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The above fields are the ones the netfs can use. They are:
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* ``inode``
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* ``mapping``
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The inode and the address space of the file being read from. The mapping
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may or may not point to inode->i_data.
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* ``cache_resources``
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Resources for the local cache to use, if present.
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* ``netfs_priv``
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The network filesystem's private data. The value for this can be passed in
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to the helper functions or set during the request. The ->cleanup() op will
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be called if this is non-NULL at the end.
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* ``start``
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* ``len``
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The file position of the start of the read request and the length. These
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may be altered by the ->expand_readahead() op.
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* ``i_size``
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The size of the file at the start of the request.
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* ``netfs_ops``
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A pointer to the operation table. The value for this is passed into the
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helper functions.
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* ``debug_id``
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A number allocated to this operation that can be displayed in trace lines
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for reference.
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The second structure is used to manage individual slices of the overall read
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request::
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struct netfs_read_subrequest {
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struct netfs_read_request *rreq;
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loff_t start;
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size_t len;
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size_t transferred;
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unsigned long flags;
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unsigned short debug_index;
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...
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};
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Each subrequest is expected to access a single source, though the helpers will
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handle falling back from one source type to another. The members are:
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* ``rreq``
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A pointer to the read request.
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* ``start``
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* ``len``
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The file position of the start of this slice of the read request and the
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length.
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* ``transferred``
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The amount of data transferred so far of the length of this slice. The
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network filesystem or cache should start the operation this far into the
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slice. If a short read occurs, the helpers will call again, having updated
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this to reflect the amount read so far.
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* ``flags``
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Flags pertaining to the read. There are two of interest to the filesystem
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or cache:
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* ``NETFS_SREQ_CLEAR_TAIL``
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This can be set to indicate that the remainder of the slice, from
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transferred to len, should be cleared.
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* ``NETFS_SREQ_SEEK_DATA_READ``
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This is a hint to the cache that it might want to try skipping ahead to
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the next data (ie. using SEEK_DATA).
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* ``debug_index``
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A number allocated to this slice that can be displayed in trace lines for
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reference.
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Read Helper Operations
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----------------------
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The network filesystem must provide the read helpers with a table of operations
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through which it can issue requests and negotiate::
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struct netfs_read_request_ops {
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void (*init_rreq)(struct netfs_read_request *rreq, struct file *file);
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bool (*is_cache_enabled)(struct inode *inode);
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int (*begin_cache_operation)(struct netfs_read_request *rreq);
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void (*expand_readahead)(struct netfs_read_request *rreq);
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bool (*clamp_length)(struct netfs_read_subrequest *subreq);
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void (*issue_op)(struct netfs_read_subrequest *subreq);
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bool (*is_still_valid)(struct netfs_read_request *rreq);
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int (*check_write_begin)(struct file *file, loff_t pos, unsigned len,
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struct page *page, void **_fsdata);
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void (*done)(struct netfs_read_request *rreq);
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void (*cleanup)(struct address_space *mapping, void *netfs_priv);
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};
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The operations are as follows:
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* ``init_rreq()``
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[Optional] This is called to initialise the request structure. It is given
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the file for reference and can modify the ->netfs_priv value.
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* ``is_cache_enabled()``
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[Required] This is called by netfs_write_begin() to ask if the file is being
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cached. It should return true if it is being cached and false otherwise.
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* ``begin_cache_operation()``
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[Optional] This is called to ask the network filesystem to call into the
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cache (if present) to initialise the caching state for this read. The netfs
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library module cannot access the cache directly, so the cache should call
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something like fscache_begin_read_operation() to do this.
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The cache gets to store its state in ->cache_resources and must set a table
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of operations of its own there (though of a different type).
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This should return 0 on success and an error code otherwise. If an error is
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reported, the operation may proceed anyway, just without local caching (only
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out of memory and interruption errors cause failure here).
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* ``expand_readahead()``
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[Optional] This is called to allow the filesystem to expand the size of a
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readahead read request. The filesystem gets to expand the request in both
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directions, though it's not permitted to reduce it as the numbers may
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represent an allocation already made. If local caching is enabled, it gets
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to expand the request first.
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Expansion is communicated by changing ->start and ->len in the request
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structure. Note that if any change is made, ->len must be increased by at
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least as much as ->start is reduced.
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* ``clamp_length()``
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[Optional] This is called to allow the filesystem to reduce the size of a
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subrequest. The filesystem can use this, for example, to chop up a request
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that has to be split across multiple servers or to put multiple reads in
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flight.
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This should return 0 on success and an error code on error.
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* ``issue_op()``
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[Required] The helpers use this to dispatch a subrequest to the server for
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reading. In the subrequest, ->start, ->len and ->transferred indicate what
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data should be read from the server.
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There is no return value; the netfs_subreq_terminated() function should be
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called to indicate whether or not the operation succeeded and how much data
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it transferred. The filesystem also should not deal with setting pages
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uptodate, unlocking them or dropping their refs - the helpers need to deal
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with this as they have to coordinate with copying to the local cache.
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Note that the helpers have the pages locked, but not pinned. It is possible
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to use the ITER_XARRAY iov iterator to refer to the range of the inode that
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is being operated upon without the need to allocate large bvec tables.
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* ``is_still_valid()``
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[Optional] This is called to find out if the data just read from the local
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cache is still valid. It should return true if it is still valid and false
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if not. If it's not still valid, it will be reread from the server.
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* ``check_write_begin()``
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[Optional] This is called from the netfs_write_begin() helper once it has
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allocated/grabbed the page to be modified to allow the filesystem to flush
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conflicting state before allowing it to be modified.
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It should return 0 if everything is now fine, -EAGAIN if the page should be
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regrabbed and any other error code to abort the operation.
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* ``done``
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[Optional] This is called after the pages in the request have all been
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unlocked (and marked uptodate if applicable).
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* ``cleanup``
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[Optional] This is called as the request is being deallocated so that the
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filesystem can clean up ->netfs_priv.
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Read Helper Procedure
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---------------------
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The read helpers work by the following general procedure:
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* Set up the request.
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* For readahead, allow the local cache and then the network filesystem to
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propose expansions to the read request. This is then proposed to the VM.
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If the VM cannot fully perform the expansion, a partially expanded read will
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be performed, though this may not get written to the cache in its entirety.
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* Loop around slicing chunks off of the request to form subrequests:
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* If a local cache is present, it gets to do the slicing, otherwise the
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helpers just try to generate maximal slices.
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* The network filesystem gets to clamp the size of each slice if it is to be
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the source. This allows rsize and chunking to be implemented.
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* The helpers issue a read from the cache or a read from the server or just
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clears the slice as appropriate.
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* The next slice begins at the end of the last one.
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* As slices finish being read, they terminate.
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* When all the subrequests have terminated, the subrequests are assessed and
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any that are short or have failed are reissued:
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* Failed cache requests are issued against the server instead.
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* Failed server requests just fail.
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* Short reads against either source will be reissued against that source
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provided they have transferred some more data:
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* The cache may need to skip holes that it can't do DIO from.
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* If NETFS_SREQ_CLEAR_TAIL was set, a short read will be cleared to the
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end of the slice instead of reissuing.
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* Once the data is read, the pages that have been fully read/cleared:
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* Will be marked uptodate.
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* If a cache is present, will be marked with PG_fscache.
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* Unlocked
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* Any pages that need writing to the cache will then have DIO writes issued.
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* Synchronous operations will wait for reading to be complete.
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* Writes to the cache will proceed asynchronously and the pages will have the
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PG_fscache mark removed when that completes.
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* The request structures will be cleaned up when everything has completed.
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Read Helper Cache API
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---------------------
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When implementing a local cache to be used by the read helpers, two things are
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required: some way for the network filesystem to initialise the caching for a
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read request and a table of operations for the helpers to call.
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The network filesystem's ->begin_cache_operation() method is called to set up a
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cache and this must call into the cache to do the work. If using fscache, for
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example, the cache would call::
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int fscache_begin_read_operation(struct netfs_read_request *rreq,
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struct fscache_cookie *cookie);
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passing in the request pointer and the cookie corresponding to the file.
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The netfs_read_request object contains a place for the cache to hang its
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state::
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struct netfs_cache_resources {
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const struct netfs_cache_ops *ops;
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void *cache_priv;
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void *cache_priv2;
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};
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This contains an operations table pointer and two private pointers. The
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operation table looks like the following::
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struct netfs_cache_ops {
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void (*end_operation)(struct netfs_cache_resources *cres);
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void (*expand_readahead)(struct netfs_cache_resources *cres,
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loff_t *_start, size_t *_len, loff_t i_size);
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enum netfs_read_source (*prepare_read)(struct netfs_read_subrequest *subreq,
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loff_t i_size);
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int (*read)(struct netfs_cache_resources *cres,
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loff_t start_pos,
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struct iov_iter *iter,
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bool seek_data,
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netfs_io_terminated_t term_func,
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void *term_func_priv);
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int (*write)(struct netfs_cache_resources *cres,
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loff_t start_pos,
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struct iov_iter *iter,
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netfs_io_terminated_t term_func,
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void *term_func_priv);
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};
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With a termination handler function pointer::
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typedef void (*netfs_io_terminated_t)(void *priv,
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ssize_t transferred_or_error,
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bool was_async);
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The methods defined in the table are:
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* ``end_operation()``
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[Required] Called to clean up the resources at the end of the read request.
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* ``expand_readahead()``
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[Optional] Called at the beginning of a netfs_readahead() operation to allow
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the cache to expand a request in either direction. This allows the cache to
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size the request appropriately for the cache granularity.
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The function is passed poiners to the start and length in its parameters,
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plus the size of the file for reference, and adjusts the start and length
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appropriately. It should return one of:
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* ``NETFS_FILL_WITH_ZEROES``
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* ``NETFS_DOWNLOAD_FROM_SERVER``
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* ``NETFS_READ_FROM_CACHE``
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* ``NETFS_INVALID_READ``
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to indicate whether the slice should just be cleared or whether it should be
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downloaded from the server or read from the cache - or whether slicing
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should be given up at the current point.
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* ``prepare_read()``
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[Required] Called to configure the next slice of a request. ->start and
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->len in the subrequest indicate where and how big the next slice can be;
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the cache gets to reduce the length to match its granularity requirements.
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* ``read()``
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[Required] Called to read from the cache. The start file offset is given
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along with an iterator to read to, which gives the length also. It can be
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given a hint requesting that it seek forward from that start position for
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data.
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Also provided is a pointer to a termination handler function and private
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data to pass to that function. The termination function should be called
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with the number of bytes transferred or an error code, plus a flag
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indicating whether the termination is definitely happening in the caller's
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context.
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* ``write()``
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[Required] Called to write to the cache. The start file offset is given
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along with an iterator to write from, which gives the length also.
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Also provided is a pointer to a termination handler function and private
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data to pass to that function. The termination function should be called
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with the number of bytes transferred or an error code, plus a flag
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indicating whether the termination is definitely happening in the caller's
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context.
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Note that these methods are passed a pointer to the cache resource structure,
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not the read request structure as they could be used in other situations where
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there isn't a read request structure as well, such as writing dirty data to the
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cache.
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