6abbaa5b01
Match the page writeback functions by adding folio_start_fscache(), folio_end_fscache(), folio_wait_fscache() and folio_wait_fscache_killable(). Remove set_page_private_2(). Also rewrite the kernel-doc to describe when to use the function rather than what the function does, and include the kernel-doc in the appropriate rst file. Saves 31 bytes of text in netfs_rreq_unlock() due to set_page_fscache() calling page_folio() once instead of three times. Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org> Acked-by: Vlastimil Babka <vbabka@suse.cz> Reviewed-by: William Kucharski <william.kucharski@oracle.com> Reviewed-by: Christoph Hellwig <hch@lst.de> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Acked-by: Mike Rapoport <rppt@linux.ibm.com> Reviewed-by: David Howells <dhowells@redhat.com>
529 lines
18 KiB
ReStructuredText
529 lines
18 KiB
ReStructuredText
.. 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.
|
|
|
|
.. kernel-doc:: include/linux/netfs.h
|