2021-04-08 14:06:44

by David Howells

[permalink] [raw]
Subject: [PATCH v6 07/30] netfs: Documentation for helper library

Add interface documentation for the netfs helper library.

Signed-off-by: David Howells <[email protected]>
cc: [email protected]
cc: [email protected]
cc: [email protected]
cc: [email protected]
cc: [email protected]
cc: [email protected]
cc: [email protected]
cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]g.uk/ # v4
Link: https://lore.kernel.org/r/[email protected]rg.uk/ # v5
---

Documentation/filesystems/index.rst | 1
Documentation/filesystems/netfs_library.rst | 526 +++++++++++++++++++++++++++
2 files changed, 527 insertions(+)
create mode 100644 Documentation/filesystems/netfs_library.rst

diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index 1f76b1cb3348..d4853cb919d2 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -53,6 +53,7 @@ filesystem implementations.
journalling
fscrypt
fsverity
+ netfs_library

Filesystems
===========
diff --git a/Documentation/filesystems/netfs_library.rst b/Documentation/filesystems/netfs_library.rst
new file mode 100644
index 000000000000..57a641847818
--- /dev/null
+++ b/Documentation/filesystems/netfs_library.rst
@@ -0,0 +1,526 @@
+.. 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.