WSL2-Linux-Kernel/Documentation/filesystems/netfs_library.rst

.. SPDX-License-Identifier: GPL-2.0

=================================
NETWORK FILESYSTEM HELPER LIBRARY
=================================

.. Contents:

 - Overview.
 - Buffered read helpers.
   - Read helper functions.
   - Read helper structures.
   - Read helper operations.
   - Read helper procedure.
   - Read helper cache API.


Overview
========

The network filesystem helper library is a set of functions designed to aid a
network filesystem in implementing VM/VFS operations.  For the moment, that
just includes turning various VM buffered read operations into requests to read
from the server.  The helper library, however, can also interpose other
services, such as local caching or local data encryption.

Note that the library module doesn't link against local caching directly, so
access must be provided by the netfs.


Buffered Read Helpers
=====================

The library provides a set of read helpers that handle the ->readpage(),
->readahead() and much of the ->write_begin() VM operations and translate them
into a common call framework.

The following services are provided:

 * Handles transparent huge pages (THPs).

 * Insulates the netfs from VM interface changes.

 * Allows the netfs to arbitrarily split reads up into pieces, even ones that
   don't match page sizes or page alignments and that may cross pages.

 * Allows the netfs to expand a readahead request in both directions to meet
   its needs.

 * Allows the netfs to partially fulfil a read, which will then be resubmitted.

 * Handles local caching, allowing cached data and server-read data to be
   interleaved for a single request.

 * Handles clearing of bufferage that aren't on the server.

 * Handle retrying of reads that failed, switching reads from the cache to the
   server as necessary.

 * In the future, this is a place that other services can be performed, such as
   local encryption of data to be stored remotely or in the cache.

From the network filesystem, the helpers require a table of operations.  This
includes a mandatory method to issue a read operation along with a number of
optional methods.


Read Helper Functions
---------------------

Three read helpers are provided::

 * void netfs_readahead(struct readahead_control *ractl,
			const struct netfs_read_request_ops *ops,
			void *netfs_priv);``
 * int netfs_readpage(struct file *file,
		      struct page *page,
		      const struct netfs_read_request_ops *ops,
		      void *netfs_priv);
 * int netfs_write_begin(struct file *file,
			 struct address_space *mapping,
			 loff_t pos,
			 unsigned int len,
			 unsigned int flags,
			 struct page **_page,
			 void **_fsdata,
			 const struct netfs_read_request_ops *ops,
			 void *netfs_priv);

Each corresponds to a VM operation, with the addition of a couple of parameters
for the use of the read helpers:

 * ``ops``

   A table of operations through which the helpers can talk to the filesystem.

 * ``netfs_priv``

   Filesystem private data (can be NULL).

Both of these values will be stored into the read request structure.

For ->readahead() and ->readpage(), the network filesystem should just jump
into the corresponding read helper; whereas for ->write_begin(), it may be a
little more complicated as the network filesystem might want to flush
conflicting writes or track dirty data and needs to put the acquired page if an
error occurs after calling the helper.

The helpers manage the read request, calling back into the network filesystem
through the suppplied table of operations.  Waits will be performed as
necessary before returning for helpers that are meant to be synchronous.

If an error occurs and netfs_priv is non-NULL, ops->cleanup() will be called to
deal with it.  If some parts of the request are in progress when an error
occurs, the request will get partially completed if sufficient data is read.

Additionally, there is::

  * void netfs_subreq_terminated(struct netfs_read_subrequest *subreq,
				 ssize_t transferred_or_error,
				 bool was_async);

which should be called to complete a read subrequest.  This is given the number
of bytes transferred or a negative error code, plus a flag indicating whether
the operation was asynchronous (ie. whether the follow-on processing can be
done in the current context, given this may involve sleeping).


Read Helper Structures
----------------------

The read helpers make use of a couple of structures to maintain the state of
the read.  The first is a structure that manages a read request as a whole::

	struct netfs_read_request {
		struct inode		*inode;
		struct address_space	*mapping;
		struct netfs_cache_resources cache_resources;
		void			*netfs_priv;
		loff_t			start;
		size_t			len;
		loff_t			i_size;
		const struct netfs_read_request_ops *netfs_ops;
		unsigned int		debug_id;
		...
	};

The above fields are the ones the netfs can use.  They are:

 * ``inode``
 * ``mapping``

   The inode and the address space of the file being read from.  The mapping
   may or may not point to inode->i_data.

 * ``cache_resources``

   Resources for the local cache to use, if present.

 * ``netfs_priv``

   The network filesystem's private data.  The value for this can be passed in
   to the helper functions or set during the request.  The ->cleanup() op will
   be called if this is non-NULL at the end.

 * ``start``
 * ``len``

   The file position of the start of the read request and the length.  These
   may be altered by the ->expand_readahead() op.

 * ``i_size``

   The size of the file at the start of the request.

 * ``netfs_ops``

   A pointer to the operation table.  The value for this is passed into the
   helper functions.

 * ``debug_id``

   A number allocated to this operation that can be displayed in trace lines
   for reference.


The second structure is used to manage individual slices of the overall read
request::

	struct netfs_read_subrequest {
		struct netfs_read_request *rreq;
		loff_t			start;
		size_t			len;
		size_t			transferred;
		unsigned long		flags;
		unsigned short		debug_index;
		...
	};

Each subrequest is expected to access a single source, though the helpers will
handle falling back from one source type to another.  The members are:

 * ``rreq``

   A pointer to the read request.

 * ``start``
 * ``len``

   The file position of the start of this slice of the read request and the
   length.

 * ``transferred``

   The amount of data transferred so far of the length of this slice.  The
   network filesystem or cache should start the operation this far into the
   slice.  If a short read occurs, the helpers will call again, having updated
   this to reflect the amount read so far.

 * ``flags``

   Flags pertaining to the read.  There are two of interest to the filesystem
   or cache:

   * ``NETFS_SREQ_CLEAR_TAIL``

     This can be set to indicate that the remainder of the slice, from
     transferred to len, should be cleared.

   * ``NETFS_SREQ_SEEK_DATA_READ``

     This is a hint to the cache that it might want to try skipping ahead to
     the next data (ie. using SEEK_DATA).

 * ``debug_index``

   A number allocated to this slice that can be displayed in trace lines for
   reference.


Read Helper Operations
----------------------

The network filesystem must provide the read helpers with a table of operations
through which it can issue requests and negotiate::

	struct netfs_read_request_ops {
		void (*init_rreq)(struct netfs_read_request *rreq, struct file *file);
		bool (*is_cache_enabled)(struct inode *inode);
		int (*begin_cache_operation)(struct netfs_read_request *rreq);
		void (*expand_readahead)(struct netfs_read_request *rreq);
		bool (*clamp_length)(struct netfs_read_subrequest *subreq);
		void (*issue_op)(struct netfs_read_subrequest *subreq);
		bool (*is_still_valid)(struct netfs_read_request *rreq);
		int (*check_write_begin)(struct file *file, loff_t pos, unsigned len,
					 struct page *page, void **_fsdata);
		void (*done)(struct netfs_read_request *rreq);
		void (*cleanup)(struct address_space *mapping, void *netfs_priv);
	};

The operations are as follows:

 * ``init_rreq()``

   [Optional] This is called to initialise the request structure.  It is given
   the file for reference and can modify the ->netfs_priv value.

 * ``is_cache_enabled()``

   [Required] This is called by netfs_write_begin() to ask if the file is being
   cached.  It should return true if it is being cached and false otherwise.

 * ``begin_cache_operation()``

   [Optional] This is called to ask the network filesystem to call into the
   cache (if present) to initialise the caching state for this read.  The netfs
   library module cannot access the cache directly, so the cache should call
   something like fscache_begin_read_operation() to do this.

   The cache gets to store its state in ->cache_resources and must set a table
   of operations of its own there (though of a different type).

   This should return 0 on success and an error code otherwise.  If an error is
   reported, the operation may proceed anyway, just without local caching (only
   out of memory and interruption errors cause failure here).

 * ``expand_readahead()``

   [Optional] This is called to allow the filesystem to expand the size of a
   readahead read request.  The filesystem gets to expand the request in both
   directions, though it's not permitted to reduce it as the numbers may
   represent an allocation already made.  If local caching is enabled, it gets
   to expand the request first.

   Expansion is communicated by changing ->start and ->len in the request
   structure.  Note that if any change is made, ->len must be increased by at
   least as much as ->start is reduced.

 * ``clamp_length()``

   [Optional] This is called to allow the filesystem to reduce the size of a
   subrequest.  The filesystem can use this, for example, to chop up a request
   that has to be split across multiple servers or to put multiple reads in
   flight.

   This should return 0 on success and an error code on error.

 * ``issue_op()``

   [Required] The helpers use this to dispatch a subrequest to the server for
   reading.  In the subrequest, ->start, ->len and ->transferred indicate what
   data should be read from the server.

   There is no return value; the netfs_subreq_terminated() function should be
   called to indicate whether or not the operation succeeded and how much data
   it transferred.  The filesystem also should not deal with setting pages
   uptodate, unlocking them or dropping their refs - the helpers need to deal
   with this as they have to coordinate with copying to the local cache.

   Note that the helpers have the pages locked, but not pinned.  It is possible
   to use the ITER_XARRAY iov iterator to refer to the range of the inode that
   is being operated upon without the need to allocate large bvec tables.

 * ``is_still_valid()``

   [Optional] This is called to find out if the data just read from the local
   cache is still valid.  It should return true if it is still valid and false
   if not.  If it's not still valid, it will be reread from the server.

 * ``check_write_begin()``

   [Optional] This is called from the netfs_write_begin() helper once it has
   allocated/grabbed the page to be modified to allow the filesystem to flush
   conflicting state before allowing it to be modified.

   It should return 0 if everything is now fine, -EAGAIN if the page should be
   regrabbed and any other error code to abort the operation.

 * ``done``

   [Optional] This is called after the pages in the request have all been
   unlocked (and marked uptodate if applicable).

 * ``cleanup``

   [Optional] This is called as the request is being deallocated so that the
   filesystem can clean up ->netfs_priv.



Read Helper Procedure
---------------------

The read helpers work by the following general procedure:

 * Set up the request.

 * For readahead, allow the local cache and then the network filesystem to
   propose expansions to the read request.  This is then proposed to the VM.
   If the VM cannot fully perform the expansion, a partially expanded read will
   be performed, though this may not get written to the cache in its entirety.

 * Loop around slicing chunks off of the request to form subrequests:

   * If a local cache is present, it gets to do the slicing, otherwise the
     helpers just try to generate maximal slices.

   * The network filesystem gets to clamp the size of each slice if it is to be
     the source.  This allows rsize and chunking to be implemented.

   * The helpers issue a read from the cache or a read from the server or just
     clears the slice as appropriate.

   * The next slice begins at the end of the last one.

   * As slices finish being read, they terminate.

 * When all the subrequests have terminated, the subrequests are assessed and
   any that are short or have failed are reissued:

   * Failed cache requests are issued against the server instead.

   * Failed server requests just fail.

   * Short reads against either source will be reissued against that source
     provided they have transferred some more data:

     * The cache may need to skip holes that it can't do DIO from.

     * If NETFS_SREQ_CLEAR_TAIL was set, a short read will be cleared to the
       end of the slice instead of reissuing.

 * Once the data is read, the pages that have been fully read/cleared:

   * Will be marked uptodate.

   * If a cache is present, will be marked with PG_fscache.

   * Unlocked

 * Any pages that need writing to the cache will then have DIO writes issued.

 * Synchronous operations will wait for reading to be complete.

 * Writes to the cache will proceed asynchronously and the pages will have the
   PG_fscache mark removed when that completes.

 * The request structures will be cleaned up when everything has completed.


Read Helper Cache API
---------------------

When implementing a local cache to be used by the read helpers, two things are
required: some way for the network filesystem to initialise the caching for a
read request and a table of operations for the helpers to call.

The network filesystem's ->begin_cache_operation() method is called to set up a
cache and this must call into the cache to do the work.  If using fscache, for
example, the cache would call::

	int fscache_begin_read_operation(struct netfs_read_request *rreq,
					 struct fscache_cookie *cookie);

passing in the request pointer and the cookie corresponding to the file.

The netfs_read_request object contains a place for the cache to hang its
state::

	struct netfs_cache_resources {
		const struct netfs_cache_ops	*ops;
		void				*cache_priv;
		void				*cache_priv2;
	};

This contains an operations table pointer and two private pointers.  The
operation table looks like the following::

	struct netfs_cache_ops {
		void (*end_operation)(struct netfs_cache_resources *cres);

		void (*expand_readahead)(struct netfs_cache_resources *cres,
					 loff_t *_start, size_t *_len, loff_t i_size);

		enum netfs_read_source (*prepare_read)(struct netfs_read_subrequest *subreq,
						       loff_t i_size);

		int (*read)(struct netfs_cache_resources *cres,
			    loff_t start_pos,
			    struct iov_iter *iter,
			    bool seek_data,
			    netfs_io_terminated_t term_func,
			    void *term_func_priv);

		int (*write)(struct netfs_cache_resources *cres,
			     loff_t start_pos,
			     struct iov_iter *iter,
			     netfs_io_terminated_t term_func,
			     void *term_func_priv);
	};

With a termination handler function pointer::

	typedef void (*netfs_io_terminated_t)(void *priv,
					      ssize_t transferred_or_error,
					      bool was_async);

The methods defined in the table are:

 * ``end_operation()``

   [Required] Called to clean up the resources at the end of the read request.

 * ``expand_readahead()``

   [Optional] Called at the beginning of a netfs_readahead() operation to allow
   the cache to expand a request in either direction.  This allows the cache to
   size the request appropriately for the cache granularity.

   The function is passed poiners to the start and length in its parameters,
   plus the size of the file for reference, and adjusts the start and length
   appropriately.  It should return one of:

   * ``NETFS_FILL_WITH_ZEROES``
   * ``NETFS_DOWNLOAD_FROM_SERVER``
   * ``NETFS_READ_FROM_CACHE``
   * ``NETFS_INVALID_READ``

   to indicate whether the slice should just be cleared or whether it should be
   downloaded from the server or read from the cache - or whether slicing
   should be given up at the current point.

 * ``prepare_read()``

   [Required] Called to configure the next slice of a request.  ->start and
   ->len in the subrequest indicate where and how big the next slice can be;
   the cache gets to reduce the length to match its granularity requirements.

 * ``read()``

   [Required] Called to read from the cache.  The start file offset is given
   along with an iterator to read to, which gives the length also.  It can be
   given a hint requesting that it seek forward from that start position for
   data.

   Also provided is a pointer to a termination handler function and private
   data to pass to that function.  The termination function should be called
   with the number of bytes transferred or an error code, plus a flag
   indicating whether the termination is definitely happening in the caller's
   context.

 * ``write()``

   [Required] Called to write to the cache.  The start file offset is given
   along with an iterator to write from, which gives the length also.

   Also provided is a pointer to a termination handler function and private
   data to pass to that function.  The termination function should be called
   with the number of bytes transferred or an error code, plus a flag
   indicating whether the termination is definitely happening in the caller's
   context.

Note that these methods are passed a pointer to the cache resource structure,
not the read request structure as they could be used in other situations where
there isn't a read request structure as well, such as writing dirty data to the
cache.