History log of /linux-master/include/linux/fscache.h
Revision Date Author Comments
# c522e3ad 07-Jan-2022 David Howells <dhowells@redhat.com>

fscache: Add a comment explaining how page-release optimisation works

Add a comment into fscache_note_page_release() to explain how the
page-release optimisation logic works[1]. It's not entirely obvious as it
has nothing to do with whether or not the netfs file contains data.

FSCACHE_COOKIE_NO_DATA_TO_READ is set if we have no data in the cache yet
(ie. the backing file lookup was negative, the file is 0 length or the
cookie got invalidated). It means that we have no data in the cache, not
that the file is necessarily empty on the server.

FSCACHE_COOKIE_HAVE_DATA is set once we've stored data in the backing file.
From that point on, we have data we *could* read - however, it's covered by
pages in the netfs pagecache until at such time one of those covering pages
is released.

So if we've written data to the cache (HAVE_DATA) and there wasn't any data
in the cache when we started (NO_DATA_TO_READ), it may no longer be true
that we can skip reading from the cache.

Read skipping is done by cachefiles_prepare_read().

Note that tracking is not done on a per-page basis, but only on a per-file
basis.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/043a206f03929c2667a465314144e518070a9b2d.camel@kernel.org/ [1]
Link: https://lore.kernel.org/r/164251408479.3435901.9540165422908194636.stgit@warthog.procyon.org.uk/ # v1


# 16f2f4e6 27-Aug-2021 David Howells <dhowells@redhat.com>

nfs: Implement cache I/O by accessing the cache directly

Move NFS to using fscache DIO API instead of the old upstream I/O API as
that has been removed. This is a stopgap solution as the intention is that
at sometime in the future, the cache will move to using larger blocks and
won't be able to store individual pages in order to deal with the potential
for data corruption due to the backing filesystem being able insert/remove
bridging blocks of zeros into its extent list[1].

NFS then reads and writes cache pages synchronously and one page at a time.

The preferred change would be to use the netfs lib, but the new I/O API can
be used directly. It's just that as the cache now needs to track data for
itself, caching blocks may exceed page size...

This code is somewhat borrowed from my "fallback I/O" patchset[2].

Changes
=======
ver #3:
- Restore lost =n fallback for nfs_fscache_release_page()[2].

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Dave Wysochanski <dwysocha@redhat.com>
Acked-by: Jeff Layton <jlayton@kernel.org>
cc: Trond Myklebust <trond.myklebust@hammerspace.com>
cc: Anna Schumaker <anna.schumaker@netapp.com>
cc: linux-nfs@vger.kernel.org
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/YO17ZNOcq+9PajfQ@mit.edu [1]
Link: https://lore.kernel.org/r/202112100957.2oEDT20W-lkp@intel.com/ [2]
Link: https://lore.kernel.org/r/163189108292.2509237.12615909591150927232.stgit@warthog.procyon.org.uk/ [2]
Link: https://lore.kernel.org/r/163906981318.143852.17220018647843475985.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967184451.1823006.6450645559828329590.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021577632.640689.11069627070150063812.stgit@warthog.procyon.org.uk/ # v4


# 32e15003 14-Dec-2021 David Howells <dhowells@redhat.com>

fscache, cachefiles: Store the volume coherency data

Store the volume coherency data in an xattr and check it when we rebind the
volume. If it doesn't match the cache volume is moved to the graveyard and
rebuilt anew.

Changes
=======
ver #4:
- Remove a couple of debugging prints.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/163967164397.1823006.2950539849831291830.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021563138.640689.15851092065380543119.stgit@warthog.procyon.org.uk/ # v4


# 16a96bdf 20-Oct-2021 David Howells <dhowells@redhat.com>

fscache: Provide a function to resize a cookie

Provide a function to change the size of the storage attached to a cookie,
to match the size of the file being cached when it's changed by truncate or
fallocate:

void fscache_resize_cookie(struct fscache_cookie *cookie,
loff_t new_size);

This acts synchronously and is expected to run under the inode lock of the
caller.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819621839.215744.7895597119803515402.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906922387.143852.16394459879816147793.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967128998.1823006.10740669081985775576.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021527861.640689.3466382085497236267.stgit@warthog.procyon.org.uk/ # v4


# 1f67e6d0 20-Oct-2021 David Howells <dhowells@redhat.com>

fscache: Provide a function to note the release of a page

Provide a function to be called from a network filesystem's releasepage
method to indicate that a page has been released that might have been a
reflection of data upon the server - and now that data must be reloaded
from the server or the cache.

This is used to end an optimisation for empty files, in particular files
that have just been created locally, whereby we know there cannot yet be
any data that we would need to read from the server or the cache.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819617128.215744.4725572296135656508.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906920354.143852.7511819614661372008.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967128061.1823006.611781655060034988.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021525963.640689.9264556596205140044.stgit@warthog.procyon.org.uk/ # v4


# 08276bda 20-Oct-2021 David Howells <dhowells@redhat.com>

vfs, fscache: Implement pinning of cache usage for writeback

Cachefiles has a problem in that it needs to keep the backing file for a
cookie open whilst there are local modifications pending that need to be
written to it. However, we don't want to keep the file open indefinitely,
as that causes EMFILE/ENFILE/ENOMEM problems.

Reopening the cache file, however, is a problem if this is being done due
to writeback triggered by exit(). Some filesystems will oops if we try to
open a file in that context because they want to access current->fs or
other resources that have already been dismantled.

To get around this, I added the following:

(1) An inode flag, I_PINNING_FSCACHE_WB, to be set on a network filesystem
inode to indicate that we have a usage count on the cookie caching
that inode.

(2) A flag in struct writeback_control, unpinned_fscache_wb, that is set
when __writeback_single_inode() clears the last dirty page from
i_pages - at which point it clears I_PINNING_FSCACHE_WB and sets this
flag.

This has to be done here so that clearing I_PINNING_FSCACHE_WB can be
done atomically with the check of PAGECACHE_TAG_DIRTY that clears
I_DIRTY_PAGES.

(3) A function, fscache_set_page_dirty(), which if it is not set, sets
I_PINNING_FSCACHE_WB and calls fscache_use_cookie() to pin the cache
resources.

(4) A function, fscache_unpin_writeback(), to be called by ->write_inode()
to unuse the cookie.

(5) A function, fscache_clear_inode_writeback(), to be called when the
inode is evicted, before clear_inode() is called. This cleans up any
lingering I_PINNING_FSCACHE_WB.

The network filesystem can then use these tools to make sure that
fscache_write_to_cache() can write locally modified data to the cache as
well as to the server.

For the future, I'm working on write helpers for netfs lib that should
allow this facility to be removed by keeping track of the dirty regions
separately - but that's incomplete at the moment and is also going to be
affected by folios, one way or another, since it deals with pages

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819615157.215744.17623791756928043114.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906917856.143852.8224898306177154573.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967124567.1823006.14188359004568060298.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021524705.640689.17824932021727663017.stgit@warthog.procyon.org.uk/ # v4


# b6e16652 20-Oct-2021 David Howells <dhowells@redhat.com>

fscache: Implement higher-level write I/O interface

Provide a higher-level function than fscache_write() to perform a write
from an inode's pagecache to the cache, whilst fending off concurrent
writes by means of the PG_fscache mark on a page:

void fscache_write_to_cache(struct fscache_cookie *cookie,
struct address_space *mapping,
loff_t start,
size_t len,
loff_t i_size,
netfs_io_terminated_t term_func,
void *term_func_priv,
bool caching);

If caching is false, this function does nothing except call (*term_func)()
if given. It assumes that, in such a case, PG_fscache will not have been
set on the pages.

Otherwise, if caching is true, this function requires the source pages to
have had PG_fscache set on them before calling. start and len define the
region of the file to be modified and i_size indicates the new file size.
The source pages are extracted from the mapping.

term_func and term_func_priv work as for fscache_write(). The PG_fscache
marks will be cleared at the end of the operation, before term_func is
called or the function otherwise returns.

There is an additonal helper function to clear the PG_fscache bits from a
range of pages:

void fscache_clear_page_bits(struct fscache_cookie *cookie,
struct address_space *mapping,
loff_t start, size_t len,
bool caching);

If caching is true, the pages to be managed are expected to be located on
mapping in the range defined by start and len. If caching is false, it
does nothing.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819614155.215744.5528123235123721230.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906916346.143852.15632773570362489926.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967123599.1823006.12946816026724657428.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021522672.640689.4381958316198807813.stgit@warthog.procyon.org.uk/ # v4


# 9af1c6c3 20-Oct-2021 David Howells <dhowells@redhat.com>

fscache: Implement raw I/O interface

Provide a pair of functions to perform raw I/O on the cache. The first
function allows an arbitrary asynchronous direct-IO read to be made against
a cache object, though the read should be aligned and sized appropriately
for the backing device:

int fscache_read(struct netfs_cache_resources *cres,
loff_t start_pos,
struct iov_iter *iter,
enum netfs_read_from_hole read_hole,
netfs_io_terminated_t term_func,
void *term_func_priv);

The cache resources must have been previously initialised by
fscache_begin_read_operation(). A read operation is sent to the backing
filesystem, starting at start_pos within the file. The size of the read is
specified by the iterator, as is the location of the output buffer.

If there is a hole in the data it can be ignored and left to the backing
filesystem to deal with (NETFS_READ_HOLE_IGNORE), a hole at the beginning
can be skipped over and the buffer padded with zeros
(NETFS_READ_HOLE_CLEAR) or -ENODATA can be given (NETFS_READ_HOLE_FAIL).

If term_func is not NULL, the operation may be performed asynchronously.
Upon completion, successful or otherwise, (*term_func)() will be called and
passed term_func_priv, along with an error or the amount of data
transferred. If the op is run asynchronously, fscache_read() will return
-EIOCBQUEUED.

The second function allows an arbitrary asynchronous direct-IO write to be
made against a cache object, though the write should be aligned and sized
appropriately for the backing device:

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

This works in very similar way to fscache_read(), except that there's no
need to deal with holes (they're just overwritten).

The caller is responsible for preventing concurrent overlapping writes.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819613224.215744.7877577215582621254.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906915386.143852.16936177636106480724.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967122632.1823006.7487049517698562172.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021521420.640689.12747258780542678309.stgit@warthog.procyon.org.uk/ # v4


# ed1235eb 20-Oct-2021 David Howells <dhowells@redhat.com>

fscache: Provide a function to let the netfs update its coherency data

Provide a function to let the netfs update its coherency data:

void fscache_update_cookie(struct fscache_cookie *cookie,
const void *aux_data,
const loff_t *object_size);

This will update the auxiliary data and/or the size of the object attached
to a cookie if either pointer is not-NULL and flag that the disk needs to
be updated.

Note that fscache_unuse_cookie() also allows this to be done.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819610438.215744.4223265964131424954.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906913530.143852.18150303220217653820.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967117795.1823006.7493373142653442595.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021518440.640689.6369952464473039268.stgit@warthog.procyon.org.uk/ # v4


# d64f4554 20-Oct-2021 David Howells <dhowells@redhat.com>

fscache: Provide a means to begin an operation

Provide a function to begin a read operation:

int fscache_begin_read_operation(
struct netfs_cache_resources *cres,
struct fscache_cookie *cookie)

This is primarily intended to be called by network filesystems on behalf of
netfslib, but may also be called to use the I/O access functions directly.
It attaches the resources required by the cache to cres struct from the
supplied cookie.

This holds access to the cache behind the cookie for the duration of the
operation and forces cache withdrawal and cookie invalidation to perform
synchronisation on the operation. cres->inval_counter is set from the
cookie at this point so that it can be compared at the end of the
operation.

Note that this does not guarantee that the cache state is fully set up and
able to perform I/O immediately; looking up and creation may be left in
progress in the background. The operations intended to be called by the
network filesystem, such as reading and writing, are expected to wait for
the cookie to move to the correct state.

This will, however, potentially sleep, waiting for a certain minimum state
to be set or for operations such as invalidate to advance far enough that
I/O can resume.


Also provide a function for the cache to call to wait for the cache object
to get to a state where it can be used for certain things:

bool fscache_wait_for_operation(struct netfs_cache_resources *cres,
enum fscache_want_stage stage);

This looks at the cache resources provided by the begin function and waits
for them to get to an appropriate stage. There's a choice of wanting just
some parameters (FSCACHE_WANT_PARAM) or the ability to do I/O
(FSCACHE_WANT_READ or FSCACHE_WANT_WRITE).

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819603692.215744.146724961588817028.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906910672.143852.13856103384424986357.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967110245.1823006.2239170567540431836.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021513617.640689.16627329360866150606.stgit@warthog.procyon.org.uk/ # v4


# d24af13e 20-Oct-2021 David Howells <dhowells@redhat.com>

fscache: Implement cookie invalidation

Add a function to invalidate the cache behind a cookie:

void fscache_invalidate(struct fscache_cookie *cookie,
const void *aux_data,
loff_t size,
unsigned int flags)

This causes any cached data for the specified cookie to be discarded. If
the cookie is marked as being in use, a new cache object will be created if
possible and future I/O will use that instead. In-flight I/O should be
abandoned (writes) or reconsidered (reads). Each time it is called
cookie->inval_counter is incremented and this can be used to detect
invalidation at the end of an I/O operation.

The coherency data attached to the cookie can be updated and the cookie
size should be reset. One flag is available, FSCACHE_INVAL_DIO_WRITE,
which should be used to indicate invalidation due to a DIO write on a
file. This will temporarily disable caching for this cookie.

Changes
=======
ver #2:
- Should only change to inval state if can get access to cache.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819602231.215744.11206598147269491575.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906909707.143852.18056070560477964891.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967107447.1823006.5945029409592119962.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021512640.640689.11418616313147754172.stgit@warthog.procyon.org.uk/ # v4


# 12bb21a2 20-Oct-2021 David Howells <dhowells@redhat.com>

fscache: Implement cookie user counting and resource pinning

Provide a pair of functions to count the number of users of a cookie (open
files, writeback, invalidation, resizing, reads, writes), to obtain and pin
resources for the cookie and to prevent culling for the whilst there are
users.

The first function marks a cookie as being in use:

void fscache_use_cookie(struct fscache_cookie *cookie,
bool will_modify);

The caller should indicate the cookie to use and whether or not the caller
is in a context that may modify the cookie (e.g. a file open O_RDWR).

If the cookie is not already resourced, fscache will ask the cache backend
in the background to do whatever it needs to look up, create or otherwise
obtain the resources necessary to access data. This is pinned to the
cookie and may not be culled, though it may be withdrawn if the cache as a
whole is withdrawn.

The second function removes the in-use mark from a cookie and, optionally,
updates the coherency data:

void fscache_unuse_cookie(struct fscache_cookie *cookie,
const void *aux_data,
const loff_t *object_size);

If non-NULL, the aux_data buffer and/or the object_size will be saved into
the cookie and will be set on the backing store when the object is
committed.

If this removes the last usage on a cookie, the cookie is placed onto an
LRU list from which it will be removed and closed after a couple of seconds
if it doesn't get reused. This prevents resource overload in the cache -
in particular it prevents it from holding too many files open.

Changes
=======
ver #2:
- Fix fscache_unuse_cookie() to use atomic_dec_and_lock() to avoid a
potential race if the cookie gets reused before it completes the
unusement.
- Added missing transition to LRU_DISCARDING state.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819600612.215744.13678350304176542741.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906907567.143852.16979631199380722019.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967106467.1823006.6790864931048582667.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021511674.640689.10084988363699111860.stgit@warthog.procyon.org.uk/ # v4


# 7f3283ab 20-Oct-2021 David Howells <dhowells@redhat.com>

fscache: Implement cookie registration

Add functions to the fscache API to allow data file cookies to be acquired
and relinquished by the network filesystem. It is intended that the
filesystem will create such cookies per-inode under a volume.

To request a cookie, the filesystem should call:

struct fscache_cookie *
fscache_acquire_cookie(struct fscache_volume *volume,
u8 advice,
const void *index_key,
size_t index_key_len,
const void *aux_data,
size_t aux_data_len,
loff_t object_size)


The filesystem must first have created a volume cookie, which is passed in
here. If it passes in NULL then the function will just return a NULL
cookie.

A binary key should be passed in index_key and is of size index_key_len.
This is saved in the cookie and is used to locate the associated data in
the cache.

A coherency data buffer of size aux_data_len will be allocated and
initialised from the buffer pointed to by aux_data. This is used to
validate cache objects when they're opened and is stored on disk with them
when they're committed. The data is stored in the cookie and will be
updateable by various functions in later patches.

The object_size must also be given. This is also used to perform a
coherency check and to size the backing storage appropriately.

This function disallows a cookie from being acquired twice in parallel,
though it will cause the second user to wait if the first is busy
relinquishing its cookie.


When a network filesystem has finished with a cookie, it should call:

void
fscache_relinquish_cookie(struct fscache_volume *volume,
bool retire)

If retire is true, any backing data will be discarded immediately.

Changes
=======
ver #3:
- fscache_hash()'s size parameter is now in bytes. Use __le32 as the unit
to round up to.
- When comparing cookies, simply see if the attributes are the same rather
than subtracting them to produce a strcmp-style return[1].
- Add a check to see if the cookie is still hashed at the point of
freeing.

ver #2:
- Don't hold n_accesses elevated whilst cache is bound to a cookie, but
rather add a flag that prevents the state machine from being queued when
n_accesses reaches 0.
- Remove the unused cookie pointer field from the fscache_acquire
tracepoint.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/CAHk-=whtkzB446+hX0zdLsdcUJsJ=8_-0S1mE_R+YurThfUbLA@mail.gmail.com/ [1]
Link: https://lore.kernel.org/r/163819590658.215744.14934902514281054323.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906891983.143852.6219772337558577395.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967088507.1823006.12659006350221417165.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021498432.640689.12743483856927722772.stgit@warthog.procyon.org.uk/ # v4


# 62ab6335 20-Oct-2021 David Howells <dhowells@redhat.com>

fscache: Implement volume registration

Add functions to the fscache API to allow volumes to be acquired and
relinquished by the network filesystem. A volume is an index of data
storage cache objects. A volume is represented by a volume cookie in the
API. A filesystem would typically create a volume for a superblock and
then create per-inode cookies within it.

To request a volume, the filesystem calls:

struct fscache_volume *
fscache_acquire_volume(const char *volume_key,
const char *cache_name,
const void *coherency_data,
size_t coherency_len)

The volume_key is a printable string used to match the volume in the cache.
It should not contain any '/' characters. For AFS, for example, this would
be "afs,<cellname>,<volume_id>", e.g. "afs,example.com,523001".

The cache_name can be NULL, but if not it should be a string indicating the
name of the cache to use if there's more than one available.

The coherency data, if given, is an arbitrarily-sized blob that's attached
to the volume and is compared when the volume is looked up. If it doesn't
match, the old volume is judged to be out of date and it and everything
within it is discarded.

Acquiring a volume twice concurrently is disallowed, though the function
will wait if an old volume cookie is being relinquishing.


When a network filesystem has finished with a volume, it should return the
volume cookie by calling:

void
fscache_relinquish_volume(struct fscache_volume *volume,
const void *coherency_data,
bool invalidate)

If invalidate is true, the entire volume will be discarded; if false, the
volume will be synced and the coherency data will be updated.

Changes
=======
ver #4:
- Removed an extraneous param from kdoc on fscache_relinquish_volume()[3].

ver #3:
- fscache_hash()'s size parameter is now in bytes. Use __le32 as the unit
to round up to.
- When comparing cookies, simply see if the attributes are the same rather
than subtracting them to produce a strcmp-style return[2].
- Make the coherency data an arbitrary blob rather than a u64, but don't
store it for the moment.

ver #2:
- Fix error check[1].
- Make a fscache_acquire_volume() return errors, including EBUSY if a
conflicting volume cookie already exists. No error is printed now -
that's left to the netfs.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/20211203095608.GC2480@kili/ [1]
Link: https://lore.kernel.org/r/CAHk-=whtkzB446+hX0zdLsdcUJsJ=8_-0S1mE_R+YurThfUbLA@mail.gmail.com/ [2]
Link: https://lore.kernel.org/r/20211220224646.30e8205c@canb.auug.org.au/ [3]
Link: https://lore.kernel.org/r/163819588944.215744.1629085755564865996.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906890630.143852.13972180614535611154.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967086836.1823006.8191672796841981763.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021495816.640689.4403156093668590217.stgit@warthog.procyon.org.uk/ # v4


# 1e1236b8 20-Oct-2021 David Howells <dhowells@redhat.com>

fscache: Introduce new driver

Introduce basic skeleton of the new, rewritten fscache driver.

Changes
=======
ver #3:
- Use remove_proc_subtree(), not remove_proc_entry() to remove a populated
dir.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819584034.215744.4290533472390439030.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906887770.143852.3577888294989185666.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967080039.1823006.5702921801104057922.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021491014.640689.4292699878317589512.stgit@warthog.procyon.org.uk/ # v4


# 2cee6fbb 25-Oct-2021 David Howells <dhowells@redhat.com>

fscache: Remove the contents of the fscache driver, pending rewrite

Remove the code that comprises the fscache driver as it's going to be
substantially rewritten, with the majority of the code being erased in the
rewrite.

A small piece of linux/fscache.h is left as that is #included by a bunch of
network filesystems.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819578724.215744.18210619052245724238.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906884814.143852.6727245089843862889.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967077097.1823006.1377665951499979089.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021485548.640689.13876080567388696162.stgit@warthog.procyon.org.uk/ # v4


# 0dc54bd4 03-Nov-2021 Dominique Martinet <asmadeus@codewreck.org>

fscache_cookie_enabled: check cookie is valid before accessing it

fscache_cookie_enabled() could be called on NULL cookies and cause a
null pointer dereference when accessing cookie flags: just make sure
the cookie is valid first

Suggested-by: David Howells <dhowells@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>


# 20ec197b 29-Mar-2021 David Howells <dhowells@redhat.com>

fscache: Use refcount_t for the cookie refcount instead of atomic_t

Use refcount_t for the fscache_cookie refcount instead of atomic_t and
rename the 'usage' member to 'ref' in such cases. The tracepoints that
reference it change from showing "u=%d" to "r=%d".

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/162431204358.2908479.8006938388213098079.stgit@warthog.procyon.org.uk/


# 884a7688 10-Feb-2020 David Howells <dhowells@redhat.com>

fscache: Procfile to display cookies

Add /proc/fs/fscache/cookies to display active cookies.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/158861211871.340223.7223853943667440807.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/159465771021.1376105.6933857529128238020.stgit@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/160588460994.3465195.16963417803501149328.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/162431194785.2908479.786917990782538164.stgit@warthog.procyon.org.uk/


# 2908f5e1 10-Feb-2020 David Howells <dhowells@redhat.com>

fscache: Add a cookie debug ID and use that in traces

Add a cookie debug ID and use that in traces and in procfiles rather than
displaying the (hashed) pointer to the cookie. This is easier to correlate
and we don't lose anything when interpreting oops output since that shows
unhashed addresses and registers that aren't comparable to the hashed
values.

Changes:

ver #2:
- Fix the fscache_op tracepoint to handle a NULL cookie pointer.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/158861210988.340223.11688464116498247790.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/159465769844.1376105.14119502774019865432.stgit@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/160588459097.3465195.1273313637721852165.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/162431193544.2908479.17556704572948300790.stgit@warthog.procyon.org.uk/


# 26aaeffc 22-Feb-2021 David Howells <dhowells@redhat.com>

fscache, cachefiles: Add alternate API to use kiocb for read/write to cache

Add an alternate API by which the cache can be accessed through a kiocb,
doing async DIO, rather than using the current API that tells the cache
where all the pages are.

The new API is intended to be used in conjunction with the netfs helper
library. A filesystem must pick one or the other and not mix them.

Filesystems wanting to use the new API must #define FSCACHE_USE_NEW_IO_API
before #including the header. This prevents them from continuing to use
the old API at the same time as there are incompatibilities in how the
PG_fscache page bit is used.

Changes:
v6:
- Provide a routine to shape a write so that the start and length can be
aligned for DIO[3].

v4:
- Use the vfs_iocb_iter_read/write() helpers[1]
- Move initial definition of fscache_begin_read_operation() here.
- Remove a commented-out line[2]
- Combine ki->term_func calls in cachefiles_read_complete()[2].
- Remove explicit NULL initialiser[2].
- Remove extern on func decl[2].
- Put in param names on func decl[2].
- Remove redundant else[2].
- Fill out the kdoc comment for fscache_begin_read_operation().
- Rename fs/fscache/page2.c to io.c to match later patches.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-and-tested-by: Jeff Layton <jlayton@kernel.org>
Tested-by: Dave Wysochanski <dwysocha@redhat.com>
Tested-By: Marc Dionne <marc.dionne@auristor.com>
cc: Christoph Hellwig <hch@lst.de>
cc: linux-cachefs@redhat.com
cc: linux-afs@lists.infradead.org
cc: linux-nfs@vger.kernel.org
cc: linux-cifs@vger.kernel.org
cc: ceph-devel@vger.kernel.org
cc: v9fs-developer@lists.sourceforge.net
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20210216102614.GA27555@lst.de/ [1]
Link: https://lore.kernel.org/r/20210216084230.GA23669@lst.de/ [2]
Link: https://lore.kernel.org/r/161781047695.463527.7463536103593997492.stgit@warthog.procyon.org.uk/ [3]
Link: https://lore.kernel.org/r/161118142558.1232039.17993829899588971439.stgit@warthog.procyon.org.uk/ # rfc
Link: https://lore.kernel.org/r/161161037850.2537118.8819808229350326503.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/161340402057.1303470.8038373593844486698.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/161539545919.286939.14573472672781434757.stgit@warthog.procyon.org.uk/ # v4
Link: https://lore.kernel.org/r/161653801477.2770958.10543270629064934227.stgit@warthog.procyon.org.uk/ # v5
Link: https://lore.kernel.org/r/161789084517.6155.12799689829859169640.stgit@warthog.procyon.org.uk/ # v6


# b533a83f 15-Feb-2021 David Howells <dhowells@redhat.com>

netfs, mm: Move PG_fscache helper funcs to linux/netfs.h

Move the PG_fscache related helper funcs (such as SetPageFsCache()) to
linux/netfs.h rather than linux/fscache.h as the intention is to move to a
model where they're used by the network filesystem and the helper library,
but not by fscache/cachefiles itself.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Jeff Layton <jlayton@kernel.org>
Tested-by: Dave Wysochanski <dwysocha@redhat.com>
Tested-By: Marc Dionne <marc.dionne@auristor.com>
cc: Matthew Wilcox <willy@infradead.org>
cc: linux-mm@kvack.org
cc: linux-cachefs@redhat.com
cc: linux-afs@lists.infradead.org
cc: linux-nfs@vger.kernel.org
cc: linux-cifs@vger.kernel.org
cc: ceph-devel@vger.kernel.org
cc: v9fs-developer@lists.sourceforge.net
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/161340392347.1303470.18065131603507621762.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/161539534516.286939.6265142985563005000.stgit@warthog.procyon.org.uk/ # v4
Link: https://lore.kernel.org/r/161653792959.2770958.5386546945273988117.stgit@warthog.procyon.org.uk/ # v5
Link: https://lore.kernel.org/r/161789073997.6155.18442271115255650614.stgit@warthog.procyon.org.uk/ # v6


# efc930fa 27-Apr-2020 Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

docs: filesystems: caching/netfs-api.txt: convert it to ReST

- Add a SPDX header;
- Adjust document and section titles;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add it to filesystems/caching/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/cfe4cb1bf8e1f0093d44c30801ec42e74721e543.1588021877.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>


# 2874c5fd 27-May-2019 Thomas Gleixner <tglx@linutronix.de>

treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 152

Based on 1 normalized pattern(s):

this program is free software you can redistribute it and or modify
it under the terms of the gnu general public license as published by
the free software foundation either version 2 of the license or at
your option any later version

extracted by the scancode license scanner the SPDX license identifier

GPL-2.0-or-later

has been chosen to replace the boilerplate/reference in 3029 file(s).

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Allison Randal <allison@lohutok.net>
Cc: linux-spdx@vger.kernel.org
Link: https://lkml.kernel.org/r/20190527070032.746973796@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>


# ec0328e4 04-Apr-2018 David Howells <dhowells@redhat.com>

fscache: Maintain a catalogue of allocated cookies

Maintain a catalogue of allocated cookies so that cookie collisions can be
handled properly. For the moment, this just involves printing a warning
and returning a NULL cookie to the caller of fscache_acquire_cookie(), but
in future it might make sense to wait for the old cookie to finish being
cleaned up.

This requires the cookie key to be stored attached to the cookie so that we
still have the key available if the netfs relinquishes the cookie. This is
done by an earlier patch.

The catalogue also renders redundant fscache_netfs_list (used for checking
for duplicates), so that can be removed.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Anna Schumaker <anna.schumaker@netapp.com>
Tested-by: Steve Dickson <steved@redhat.com>


# ee1235a9 04-Apr-2018 David Howells <dhowells@redhat.com>

fscache: Pass object size in rather than calling back for it

Pass the object size in to fscache_acquire_cookie() and
fscache_write_page() rather than the netfs providing a callback by which it
can be received. This makes it easier to update the size of the object
when a new page is written that extends the object.

The current object size is also passed by fscache to the check_aux
function, obviating the need to store it in the aux data.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Anna Schumaker <anna.schumaker@netapp.com>
Tested-by: Steve Dickson <steved@redhat.com>


# 402cb8dd 04-Apr-2018 David Howells <dhowells@redhat.com>

fscache: Attach the index key and aux data to the cookie

Attach copies of the index key and auxiliary data to the fscache cookie so
that:

(1) The callbacks to the netfs for this stuff can be eliminated. This
can simplify things in the cache as the information is still
available, even after the cache has relinquished the cookie.

(2) Simplifies the locking requirements of accessing the information as we
don't have to worry about the netfs object going away on us.

(3) The cache can do lazy updating of the coherency information on disk.
As long as the cache is flushed before reboot/poweroff, there's no
need to update the coherency info on disk every time it changes.

(4) Cookies can be hashed or put in a tree as the index key is easily
available. This allows:

(a) Checks for duplicate cookies can be made at the top fscache layer
rather than down in the bowels of the cache backend.

(b) Caching can be added to a netfs object that has a cookie if the
cache is brought online after the netfs object is allocated.

A certain amount of space is made in the cookie for inline copies of the
data, but if it won't fit there, extra memory will be allocated for it.

The downside of this is that live cache operation requires more memory.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Anna Schumaker <anna.schumaker@netapp.com>
Tested-by: Steve Dickson <steved@redhat.com>


# 98801506 02-Jan-2018 David Howells <dhowells@redhat.com>

fscache: Fix the default for fscache_maybe_release_page()

Fix the default for fscache_maybe_release_page() for when the cookie isn't
valid or the page isn't cached. It mustn't return false as that indicates
the page cannot yet be freed.

The problem with the default is that if, say, there's no cache, but a
network filesystem's pages are using up almost all the available memory, a
system can OOM because the filesystem ->releasepage() op will not allow
them to be released as fscache_maybe_release_page() incorrectly prevents
it.

This can be tested by writing a sequence of 512MiB files to an AFS mount.
It does not affect NFS or CIFS because both of those wrap the call in a
check of PG_fscache and it shouldn't bother Ceph as that only has
PG_private set whilst writeback is in progress. This might be an issue for
9P, however.

Note that the pages aren't entirely stuck. Removing a file or unmounting
will clear things because that uses ->invalidatepage() instead.

Fixes: 201a15428bd5 ("FS-Cache: Handle pages pending storage that get evicted under OOM conditions")
Reported-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Marc Dionne <marc.dionne@auristor.com>
cc: stable@vger.kernel.org # 2.6.32+


# 26b433d0 06-Sep-2017 Jan Kara <jack@suse.cz>

fscache: remove unused ->now_uncached callback

Patch series "Ranged pagevec lookup", v2.

In this series I make pagevec_lookup() update the index (to be
consistent with pagevec_lookup_tag() and also as a preparation for
ranged lookups), provide ranged variant of pagevec_lookup() and use it
in places where it makes sense. This not only removes some common code
but is also a measurable performance win for some use cases (see patch
4/10) where radix tree is sparse and searching & grabing of a page after
the end of the range has measurable overhead.

This patch (of 10):

The callback doesn't ever get called. Remove it.

Link: http://lkml.kernel.org/r/20170726114704.7626-2-jack@suse.cz
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>


# 94d30ae9 20-Sep-2013 David Howells <dhowells@redhat.com>

FS-Cache: Provide the ability to enable/disable cookies

Provide the ability to enable and disable fscache cookies. A disabled cookie
will reject or ignore further requests to:

Acquire a child cookie
Invalidate and update backing objects
Check the consistency of a backing object
Allocate storage for backing page
Read backing pages
Write to backing pages

but still allows:

Checks/waits on the completion of already in-progress objects
Uncaching of pages
Relinquishment of cookies

Two new operations are provided:

(1) Disable a cookie:

void fscache_disable_cookie(struct fscache_cookie *cookie,
bool invalidate);

If the cookie is not already disabled, this locks the cookie against other
dis/enablement ops, marks the cookie as being disabled, discards or
invalidates any backing objects and waits for cessation of activity on any
associated object.

This is a wrapper around a chunk split out of fscache_relinquish_cookie(),
but it reinitialises the cookie such that it can be reenabled.

All possible failures are handled internally. The caller should consider
calling fscache_uncache_all_inode_pages() afterwards to make sure all page
markings are cleared up.

(2) Enable a cookie:

void fscache_enable_cookie(struct fscache_cookie *cookie,
bool (*can_enable)(void *data),
void *data)

If the cookie is not already enabled, this locks the cookie against other
dis/enablement ops, invokes can_enable() and, if the cookie is not an
index cookie, will begin the procedure of acquiring backing objects.

The optional can_enable() function is passed the data argument and returns
a ruling as to whether or not enablement should actually be permitted to
begin.

All possible failures are handled internally. The cookie will only be
marked as enabled if provisional backing objects are allocated.

A later patch will introduce these to NFS. Cookie enablement during nfs_open()
is then contingent on i_writecount <= 0. can_enable() checks for a race
between open(O_RDONLY) and open(O_WRONLY/O_RDWR). This simplifies NFS's cookie
handling and allows us to get rid of open(O_RDONLY) accidentally introducing
caching to an inode that's open for writing already.

One operation has its API modified:

(3) Acquire a cookie.

struct fscache_cookie *fscache_acquire_cookie(
struct fscache_cookie *parent,
const struct fscache_cookie_def *def,
void *netfs_data,
bool enable);

This now has an additional argument that indicates whether the requested
cookie should be enabled by default. It doesn't need the can_enable()
function because the caller must prevent multiple calls for the same netfs
object and it doesn't need to take the enablement lock because no one else
can get at the cookie before this returns.

Signed-off-by: David Howells <dhowells@redhat.com


# 5a6f282a 21-Aug-2013 Milosz Tanski <milosz@adfin.com>

fscache: Netfs function for cleanup post readpages

Currently the fscache code expect the netfs to call fscache_readpages_or_alloc
inside the aops readpages callback. It marks all the pages in the list
provided by readahead with PG_private_2. In the cases that the netfs fails to
read all the pages (which is legal) it ends up returning to the readahead and
triggering a BUG. This happens because the page list still contains marked
pages.

This patch implements a simple fscache_readpages_cancel function that the netfs
should call before returning from readpages. It will revoke the pages from the
underlying cache backend and unmark them.

The problem was originally worked out in the Ceph devel tree, but it also
occurs in CIFS. It appears that NFS, AFS and 9P are okay as read_cache_pages()
will clean up the unprocessed pages in the case of an error.

This can be used to address the following oops:

[12410647.597278] BUG: Bad page state in process petabucket pfn:3d504e
[12410647.597292] page:ffffea000f541380 count:0 mapcount:0 mapping:
(null) index:0x0
[12410647.597298] page flags: 0x200000000001000(private_2)

...

[12410647.597334] Call Trace:
[12410647.597345] [<ffffffff815523f2>] dump_stack+0x19/0x1b
[12410647.597356] [<ffffffff8111def7>] bad_page+0xc7/0x120
[12410647.597359] [<ffffffff8111e49e>] free_pages_prepare+0x10e/0x120
[12410647.597361] [<ffffffff8111fc80>] free_hot_cold_page+0x40/0x170
[12410647.597363] [<ffffffff81123507>] __put_single_page+0x27/0x30
[12410647.597365] [<ffffffff81123df5>] put_page+0x25/0x40
[12410647.597376] [<ffffffffa02bdcf9>] ceph_readpages+0x2e9/0x6e0 [ceph]
[12410647.597379] [<ffffffff81122a8f>] __do_page_cache_readahead+0x1af/0x260
[12410647.597382] [<ffffffff81122ea1>] ra_submit+0x21/0x30
[12410647.597384] [<ffffffff81118f64>] filemap_fault+0x254/0x490
[12410647.597387] [<ffffffff8113a74f>] __do_fault+0x6f/0x4e0
[12410647.597391] [<ffffffff810125bd>] ? __switch_to+0x16d/0x4a0
[12410647.597395] [<ffffffff810865ba>] ? finish_task_switch+0x5a/0xc0
[12410647.597398] [<ffffffff8113d856>] handle_pte_fault+0xf6/0x930
[12410647.597401] [<ffffffff81008c33>] ? pte_mfn_to_pfn+0x93/0x110
[12410647.597403] [<ffffffff81008cce>] ? xen_pmd_val+0xe/0x10
[12410647.597405] [<ffffffff81005469>] ? __raw_callee_save_xen_pmd_val+0x11/0x1e
[12410647.597407] [<ffffffff8113f361>] handle_mm_fault+0x251/0x370
[12410647.597411] [<ffffffff812b0ac4>] ? call_rwsem_down_read_failed+0x14/0x30
[12410647.597414] [<ffffffff8155bffa>] __do_page_fault+0x1aa/0x550
[12410647.597418] [<ffffffff8108011d>] ? up_write+0x1d/0x20
[12410647.597422] [<ffffffff8113141c>] ? vm_mmap_pgoff+0xbc/0xe0
[12410647.597425] [<ffffffff81143bb8>] ? SyS_mmap_pgoff+0xd8/0x240
[12410647.597427] [<ffffffff8155c3ae>] do_page_fault+0xe/0x10
[12410647.597431] [<ffffffff81558818>] page_fault+0x28/0x30

Signed-off-by: Milosz Tanski <milosz@adfin.com>
Signed-off-by: David Howells <dhowells@redhat.com>


# da9803bc 21-Aug-2013 David Howells <dhowells@redhat.com>

FS-Cache: Add interface to check consistency of a cached object

Extend the fscache netfs API so that the netfs can ask as to whether a cache
object is up to date with respect to its corresponding netfs object:

int fscache_check_consistency(struct fscache_cookie *cookie)

This will call back to the netfs to check whether the auxiliary data associated
with a cookie is correct. It returns 0 if it is and -ESTALE if it isn't; it
may also return -ENOMEM and -ERESTARTSYS.

The backends now have to implement a mandatory operation pointer:

int (*check_consistency)(struct fscache_object *object)

that corresponds to the above API call. FS-Cache takes care of pinning the
object and the cookie in memory and managing this call with respect to the
object state.

Original-author: Hongyi Jia <jiayisuse@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Hongyi Jia <jiayisuse@gmail.com>
cc: Milosz Tanski <milosz@adfin.com>


# ef778e7a 20-Dec-2012 David Howells <dhowells@redhat.com>

FS-Cache: Provide proper invalidation

Provide a proper invalidation method rather than relying on the netfs retiring
the cookie it has and getting a new one. The problem with this is that isn't
easy for the netfs to make sure that it has completed/cancelled all its
outstanding storage and retrieval operations on the cookie it is retiring.

Instead, have the cache provide an invalidation method that will cancel or wait
for all currently outstanding operations before invalidating the cache, and
will cause new operations to queue up behind that. Whilst invalidation is in
progress, some requests will be rejected until the cache can stack a barrier on
the operation queue to cause new operations to be deferred behind it.

Signed-off-by: David Howells <dhowells@redhat.com>


# c4d6d8db 20-Dec-2012 David Howells <dhowells@redhat.com>

CacheFiles: Fix the marking of cached pages

Under some circumstances CacheFiles defers the marking of pages with PG_fscache
so that it can take advantage of pagevecs to reduce the number of calls to
fscache_mark_pages_cached() and the netfs's hook to keep track of this.

There are, however, two problems with this:

(1) It can lead to the PG_fscache mark being applied _after_ the page is set
PG_uptodate and unlocked (by the call to fscache_end_io()).

(2) CacheFiles's ref on the page is dropped immediately following
fscache_end_io() - and so may not still be held when the mark is applied.
This can lead to the page being passed back to the allocator before the
mark is applied.

Fix this by, where appropriate, marking the page before calling
fscache_end_io() and releasing the page. This means that we can't take
advantage of pagevecs and have to make a separate call for each page to the
marking routines.

The symptoms of this are Bad Page state errors cropping up under memory
pressure, for example:

BUG: Bad page state in process tar pfn:002da
page:ffffea0000009fb0 count:0 mapcount:0 mapping: (null) index:0x1447
page flags: 0x1000(private_2)
Pid: 4574, comm: tar Tainted: G W 3.1.0-rc4-fsdevel+ #1064
Call Trace:
[<ffffffff8109583c>] ? dump_page+0xb9/0xbe
[<ffffffff81095916>] bad_page+0xd5/0xea
[<ffffffff81095d82>] get_page_from_freelist+0x35b/0x46a
[<ffffffff810961f3>] __alloc_pages_nodemask+0x362/0x662
[<ffffffff810989da>] __do_page_cache_readahead+0x13a/0x267
[<ffffffff81098942>] ? __do_page_cache_readahead+0xa2/0x267
[<ffffffff81098d7b>] ra_submit+0x1c/0x20
[<ffffffff8109900a>] ondemand_readahead+0x28b/0x29a
[<ffffffff81098ee2>] ? ondemand_readahead+0x163/0x29a
[<ffffffff810990ce>] page_cache_sync_readahead+0x38/0x3a
[<ffffffff81091d8a>] generic_file_aio_read+0x2ab/0x67e
[<ffffffffa008cfbe>] nfs_file_read+0xa4/0xc9 [nfs]
[<ffffffff810c22c4>] do_sync_read+0xba/0xfa
[<ffffffff81177a47>] ? security_file_permission+0x7b/0x84
[<ffffffff810c25dd>] ? rw_verify_area+0xab/0xc8
[<ffffffff810c29a4>] vfs_read+0xaa/0x13a
[<ffffffff810c2a79>] sys_read+0x45/0x6c
[<ffffffff813ac37b>] system_call_fastpath+0x16/0x1b

As can be seen, PG_private_2 (== PG_fscache) is set in the page flags.

Instrumenting fscache_mark_pages_cached() to verify whether page->mapping was
set appropriately showed that sometimes it wasn't. This led to the discovery
that sometimes the page has apparently been reclaimed by the time the marker
got to see it.

Reported-by: M. Stevens <m@tippett.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>


# c902ce1b 06-Jul-2011 David Howells <dhowells@redhat.com>

FS-Cache: Add a helper to bulk uncache pages on an inode

Add an FS-Cache helper to bulk uncache pages on an inode. This will
only work for the circumstance where the pages in the cache correspond
1:1 with the pages attached to an inode's page cache.

This is required for CIFS and NFS: When disabling inode cookie, we were
returning the cookie and setting cifsi->fscache to NULL but failed to
invalidate any previously mapped pages. This resulted in "Bad page
state" errors and manifested in other kind of errors when running
fsstress. Fix it by uncaching mapped pages when we disable the inode
cookie.

This patch should fix the following oops and "Bad page state" errors
seen during fsstress testing.

------------[ cut here ]------------
kernel BUG at fs/cachefiles/namei.c:201!
invalid opcode: 0000 [#1] SMP
Pid: 5, comm: kworker/u:0 Not tainted 2.6.38.7-30.fc15.x86_64 #1 Bochs Bochs
RIP: 0010: cachefiles_walk_to_object+0x436/0x745 [cachefiles]
RSP: 0018:ffff88002ce6dd00 EFLAGS: 00010282
RAX: ffff88002ef165f0 RBX: ffff88001811f500 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000100 RDI: 0000000000000282
RBP: ffff88002ce6dda0 R08: 0000000000000100 R09: ffffffff81b3a300
R10: 0000ffff00066c0a R11: 0000000000000003 R12: ffff88002ae54840
R13: ffff88002ae54840 R14: ffff880029c29c00 R15: ffff88001811f4b0
FS: 00007f394dd32720(0000) GS:ffff88002ef00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 00007fffcb62ddf8 CR3: 000000001825f000 CR4: 00000000000006e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process kworker/u:0 (pid: 5, threadinfo ffff88002ce6c000, task ffff88002ce55cc0)
Stack:
0000000000000246 ffff88002ce55cc0 ffff88002ce6dd58 ffff88001815dc00
ffff8800185246c0 ffff88001811f618 ffff880029c29d18 ffff88001811f380
ffff88002ce6dd50 ffffffff814757e4 ffff88002ce6dda0 ffffffff8106ac56
Call Trace:
cachefiles_lookup_object+0x78/0xd4 [cachefiles]
fscache_lookup_object+0x131/0x16d [fscache]
fscache_object_work_func+0x1bc/0x669 [fscache]
process_one_work+0x186/0x298
worker_thread+0xda/0x15d
kthread+0x84/0x8c
kernel_thread_helper+0x4/0x10
RIP cachefiles_walk_to_object+0x436/0x745 [cachefiles]
---[ end trace 1d481c9af1804caa ]---

I tested the uncaching by the following means:

(1) Create a big file on my NFS server (104857600 bytes).

(2) Read the file into the cache with md5sum on the NFS client. Look in
/proc/fs/fscache/stats:

Pages : mrk=25601 unc=0

(3) Open the file for read/write ("bash 5<>/warthog/bigfile"). Look in proc
again:

Pages : mrk=25601 unc=25601

Reported-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-and-Tested-by: Suresh Jayaraman <sjayaraman@suse.de>
cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>


# 25985edc 30-Mar-2011 Lucas De Marchi <lucas.demarchi@profusion.mobi>

Fix common misspellings

Fixes generated by 'codespell' and manually reviewed.

Signed-off-by: Lucas De Marchi <lucas.demarchi@profusion.mobi>


# 49a3df80 06-Jul-2010 Suresh Jayaraman <sjayaraman@suse.de>

fscache: fix missing kerneldoc annotation

.. and make kerneldoc scripts happy.

Signed-off-by: Suresh Jayaraman <sjayaraman@suse.de>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>


# ab0cfb92 06-Jul-2010 Suresh Jayaraman <sjayaraman@suse.de>

fscache: fix a trivial typo in the comment

Signed-off-by: Suresh Jayaraman <sjayaraman@suse.de>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>


# 201a1542 19-Nov-2009 David Howells <dhowells@redhat.com>

FS-Cache: Handle pages pending storage that get evicted under OOM conditions

Handle netfs pages that the vmscan algorithm wants to evict from the pagecache
under OOM conditions, but that are waiting for write to the cache. Under these
conditions, vmscan calls the releasepage() function of the netfs, asking if a
page can be discarded.

The problem is typified by the following trace of a stuck process:

kslowd005 D 0000000000000000 0 4253 2 0x00000080
ffff88001b14f370 0000000000000046 ffff880020d0d000 0000000000000007
0000000000000006 0000000000000001 ffff88001b14ffd8 ffff880020d0d2a8
000000000000ddf0 00000000000118c0 00000000000118c0 ffff880020d0d2a8
Call Trace:
[<ffffffffa00782d8>] __fscache_wait_on_page_write+0x8b/0xa7 [fscache]
[<ffffffff8104c0f1>] ? autoremove_wake_function+0x0/0x34
[<ffffffffa0078240>] ? __fscache_check_page_write+0x63/0x70 [fscache]
[<ffffffffa00b671d>] nfs_fscache_release_page+0x4e/0xc4 [nfs]
[<ffffffffa00927f0>] nfs_release_page+0x3c/0x41 [nfs]
[<ffffffff810885d3>] try_to_release_page+0x32/0x3b
[<ffffffff81093203>] shrink_page_list+0x316/0x4ac
[<ffffffff8109372b>] shrink_inactive_list+0x392/0x67c
[<ffffffff813532fa>] ? __mutex_unlock_slowpath+0x100/0x10b
[<ffffffff81058df0>] ? trace_hardirqs_on_caller+0x10c/0x130
[<ffffffff8135330e>] ? mutex_unlock+0x9/0xb
[<ffffffff81093aa2>] shrink_list+0x8d/0x8f
[<ffffffff81093d1c>] shrink_zone+0x278/0x33c
[<ffffffff81052d6c>] ? ktime_get_ts+0xad/0xba
[<ffffffff81094b13>] try_to_free_pages+0x22e/0x392
[<ffffffff81091e24>] ? isolate_pages_global+0x0/0x212
[<ffffffff8108e743>] __alloc_pages_nodemask+0x3dc/0x5cf
[<ffffffff81089529>] grab_cache_page_write_begin+0x65/0xaa
[<ffffffff8110f8c0>] ext3_write_begin+0x78/0x1eb
[<ffffffff81089ec5>] generic_file_buffered_write+0x109/0x28c
[<ffffffff8103cb69>] ? current_fs_time+0x22/0x29
[<ffffffff8108a509>] __generic_file_aio_write+0x350/0x385
[<ffffffff8108a588>] ? generic_file_aio_write+0x4a/0xae
[<ffffffff8108a59e>] generic_file_aio_write+0x60/0xae
[<ffffffff810b2e82>] do_sync_write+0xe3/0x120
[<ffffffff8104c0f1>] ? autoremove_wake_function+0x0/0x34
[<ffffffff810b18e1>] ? __dentry_open+0x1a5/0x2b8
[<ffffffff810b1a76>] ? dentry_open+0x82/0x89
[<ffffffffa00e693c>] cachefiles_write_page+0x298/0x335 [cachefiles]
[<ffffffffa0077147>] fscache_write_op+0x178/0x2c2 [fscache]
[<ffffffffa0075656>] fscache_op_execute+0x7a/0xd1 [fscache]
[<ffffffff81082093>] slow_work_execute+0x18f/0x2d1
[<ffffffff8108239a>] slow_work_thread+0x1c5/0x308
[<ffffffff8104c0f1>] ? autoremove_wake_function+0x0/0x34
[<ffffffff810821d5>] ? slow_work_thread+0x0/0x308
[<ffffffff8104be91>] kthread+0x7a/0x82
[<ffffffff8100beda>] child_rip+0xa/0x20
[<ffffffff8100b87c>] ? restore_args+0x0/0x30
[<ffffffff8102ef83>] ? tg_shares_up+0x171/0x227
[<ffffffff8104be17>] ? kthread+0x0/0x82
[<ffffffff8100bed0>] ? child_rip+0x0/0x20

In the above backtrace, the following is happening:

(1) A page storage operation is being executed by a slow-work thread
(fscache_write_op()).

(2) FS-Cache farms the operation out to the cache to perform
(cachefiles_write_page()).

(3) CacheFiles is then calling Ext3 to perform the actual write, using Ext3's
standard write (do_sync_write()) under KERNEL_DS directly from the netfs
page.

(4) However, for Ext3 to perform the write, it must allocate some memory, in
particular, it must allocate at least one page cache page into which it
can copy the data from the netfs page.

(5) Under OOM conditions, the memory allocator can't immediately come up with
a page, so it uses vmscan to find something to discard
(try_to_free_pages()).

(6) vmscan finds a clean netfs page it might be able to discard (possibly the
one it's trying to write out).

(7) The netfs is called to throw the page away (nfs_release_page()) - but it's
called with __GFP_WAIT, so the netfs decides to wait for the store to
complete (__fscache_wait_on_page_write()).

(8) This blocks a slow-work processing thread - possibly against itself.

The system ends up stuck because it can't write out any netfs pages to the
cache without allocating more memory.

To avoid this, we make FS-Cache cancel some writes that aren't in the middle of
actually being performed. This means that some data won't make it into the
cache this time. To support this, a new FS-Cache function is added
fscache_maybe_release_page() that replaces what the netfs releasepage()
functions used to do with respect to the cache.

The decisions fscache_maybe_release_page() makes are counted and displayed
through /proc/fs/fscache/stats on a line labelled "VmScan". There are four
counters provided: "nos=N" - pages that weren't pending storage; "gon=N" -
pages that were pending storage when we first looked, but weren't by the time
we got the object lock; "bsy=N" - pages that we ignored as they were actively
being written when we looked; and "can=N" - pages that we cancelled the storage
of.

What I'd really like to do is alter the behaviour of the cancellation
heuristics, depending on how necessary it is to expel pages. If there are
plenty of other pages that aren't waiting to be written to the cache that
could be ejected first, then it would be nice to hold up on immediate
cancellation of cache writes - but I don't see a way of doing that.

Signed-off-by: David Howells <dhowells@redhat.com>


# b5108822 03-Apr-2009 David Howells <dhowells@redhat.com>

FS-Cache: Implement data I/O part of netfs API

Implement the data I/O part of the FS-Cache netfs API. The documentation and
API header file were added in a previous patch.

This patch implements the following functions for the netfs to call:

(*) fscache_attr_changed().

Indicate that the object has changed its attributes. The only attribute
currently recorded is the file size. Only pages within the set file size
will be stored in the cache.

This operation is submitted for asynchronous processing, and will return
immediately. It will return -ENOMEM if an out of memory error is
encountered, -ENOBUFS if the object is not actually cached, or 0 if the
operation is successfully queued.

(*) fscache_read_or_alloc_page().
(*) fscache_read_or_alloc_pages().

Request data be fetched from the disk, and allocate internal metadata to
track the netfs pages and reserve disk space for unknown pages.

These operations perform semi-asynchronous data reads. Upon returning
they will indicate which pages they think can be retrieved from disk, and
will have set in progress attempts to retrieve those pages.

These will return, in order of preference, -ENOMEM on memory allocation
error, -ERESTARTSYS if a signal interrupted proceedings, -ENODATA if one
or more requested pages are not yet cached, -ENOBUFS if the object is not
actually cached or if there isn't space for future pages to be cached on
this object, or 0 if successful.

In the case of the multipage function, the pages for which reads are set
in progress will be removed from the list and the page count decreased
appropriately.

If any read operations should fail, the completion function will be given
an error, and will also be passed contextual information to allow the
netfs to fall back to querying the server for the absent pages.

For each successful read, the page completion function will also be
called.

Any pages subsequently tracked by the cache will have PG_fscache set upon
them on return. fscache_uncache_page() must be called for such pages.

If supplied by the netfs, the mark_pages_cached() cookie op will be
invoked for any pages now tracked.

(*) fscache_alloc_page().

Allocate internal metadata to track a netfs page and reserve disk space.

This will return -ENOMEM on memory allocation error, -ERESTARTSYS on
signal, -ENOBUFS if the object isn't cached, or there isn't enough space
in the cache, or 0 if successful.

Any pages subsequently tracked by the cache will have PG_fscache set upon
them on return. fscache_uncache_page() must be called for such pages.

If supplied by the netfs, the mark_pages_cached() cookie op will be
invoked for any pages now tracked.

(*) fscache_write_page().

Request data be stored to disk. This may only be called on pages that
have been read or alloc'd by the above three functions and have not yet
been uncached.

This will return -ENOMEM on memory allocation error, -ERESTARTSYS on
signal, -ENOBUFS if the object isn't cached, or there isn't immediately
enough space in the cache, or 0 if successful.

On a successful return, this operation will have queued the page for
asynchronous writing to the cache. The page will be returned with
PG_fscache_write set until the write completes one way or another. The
caller will not be notified if the write fails due to an I/O error. If
that happens, the object will become available and all pending writes will
be aborted.

Note that the cache may batch up page writes, and so it may take a while
to get around to writing them out.

The caller must assume that until PG_fscache_write is cleared the page is
use by the cache. Any changes made to the page may be reflected on disk.
The page may even be under DMA.

(*) fscache_uncache_page().

Indicate that the cache should stop tracking a page previously read or
alloc'd from the cache. If the page was alloc'd only, but unwritten, it
will not appear on disk.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Daire Byrne <Daire.Byrne@framestore.com>


# ccc4fc3d 03-Apr-2009 David Howells <dhowells@redhat.com>

FS-Cache: Implement the cookie management part of the netfs API

Implement the cookie management part of the FS-Cache netfs client API. The
documentation and API header file were added in a previous patch.

This patch implements the following three functions:

(1) fscache_acquire_cookie().

Acquire a cookie to represent an object to the netfs. If the object in
question is a non-index object, then that object and its parent indices
will be created on disk at this point if they don't already exist. Index
creation is deferred because an index may reside in multiple caches.

(2) fscache_relinquish_cookie().

Retire or release a cookie previously acquired. At this point, the
object on disk may be destroyed.

(3) fscache_update_cookie().

Update the in-cache representation of a cookie. This is used to update
the auxiliary data for coherency management purposes.

With this patch it is possible to have a netfs instruct a cache backend to
look up, validate and create metadata on disk and to destroy it again.
The ability to actually store and retrieve data in the objects so created is
added in later patches.

Note that these functions will never return an error. _All_ errors are
handled internally to FS-Cache.

The worst that can happen is that fscache_acquire_cookie() may return a NULL
pointer - which is considered a negative cookie pointer and can be passed back
to any function that takes a cookie without harm. A negative cookie pointer
merely suppresses caching at that level.

The stub in linux/fscache.h will detect inline the negative cookie pointer and
abort the operation as fast as possible. This means that the compiler doesn't
have to set up for a call in that case.

See the documentation in Documentation/filesystems/caching/netfs-api.txt for
more information.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Daire Byrne <Daire.Byrne@framestore.com>


# 726dd7ff 03-Apr-2009 David Howells <dhowells@redhat.com>

FS-Cache: Add netfs registration

Add functions to register and unregister a network filesystem or other client
of the FS-Cache service. This allocates and releases the cookie representing
the top-level index for a netfs, and makes it available to the netfs.

If the FS-Cache facility is disabled, then the calls are optimised away at
compile time.

Note that whilst this patch may appear to work with FS-Cache enabled and a
netfs attempting to use it, it will leak the cookie it allocates for the netfs
as fscache_relinquish_cookie() is implemented in a later patch. This will
cause the slab code to emit a warning when the module is removed.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Daire Byrne <Daire.Byrne@framestore.com>


# 0e04d4ce 03-Apr-2009 David Howells <dhowells@redhat.com>

FS-Cache: Add cache tag handling

Implement two features of FS-Cache:

(1) The ability to request and release cache tags - names by which a cache may
be known to a netfs, and thus selected for use.

(2) An internal function by which a cache is selected by consulting the netfs,
if the netfs wishes to be consulted.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Daire Byrne <Daire.Byrne@framestore.com>


# 2d6fff63 03-Apr-2009 David Howells <dhowells@redhat.com>

FS-Cache: Add the FS-Cache netfs API and documentation

Add the API for a generic facility (FS-Cache) by which filesystems (such as AFS
or NFS) may call on local caching capabilities without having to know anything
about how the cache works, or even if there is a cache:

+---------+
| | +--------------+
| NFS |--+ | |
| | | +-->| CacheFS |
+---------+ | +----------+ | | /dev/hda5 |
| | | | +--------------+
+---------+ +-->| | |
| | | |--+
| AFS |----->| FS-Cache |
| | | |--+
+---------+ +-->| | |
| | | | +--------------+
+---------+ | +----------+ | | |
| | | +-->| CacheFiles |
| ISOFS |--+ | /var/cache |
| | +--------------+
+---------+

General documentation and documentation of the netfs specific API are provided
in addition to the header files.

As this patch stands, it is possible to build a filesystem against the facility
and attempt to use it. All that will happen is that all requests will be
immediately denied as if no cache is present.

Further patches will implement the core of the facility. The facility will
transfer requests from networking filesystems to appropriate caches if
possible, or else gracefully deny them.

If this facility is disabled in the kernel configuration, then all its
operations will trivially reduce to nothing during compilation.

WHY NOT I_MAPPING?
==================

I have added my own API to implement caching rather than using i_mapping to do
this for a number of reasons. These have been discussed a lot on the LKML and
CacheFS mailing lists, but to summarise the basics:

(1) Most filesystems don't do hole reportage. Holes in files are treated as
blocks of zeros and can't be distinguished otherwise, making it difficult
to distinguish blocks that have been read from the network and cached from
those that haven't.

(2) The backing inode must be fully populated before being exposed to
userspace through the main inode because the VM/VFS goes directly to the
backing inode and does not interrogate the front inode's VM ops.

Therefore:

(a) The backing inode must fit entirely within the cache.

(b) All backed files currently open must fit entirely within the cache at
the same time.

(c) A working set of files in total larger than the cache may not be
cached.

(d) A file may not grow larger than the available space in the cache.

(e) A file that's open and cached, and remotely grows larger than the
cache is potentially stuffed.

(3) Writes go to the backing filesystem, and can only be transferred to the
network when the file is closed.

(4) There's no record of what changes have been made, so the whole file must
be written back.

(5) The pages belong to the backing filesystem, and all metadata associated
with that page are relevant only to the backing filesystem, and not
anything stacked atop it.

OVERVIEW
========

FS-Cache provides (or will provide) the following facilities:

(1) Caches can be added / removed at any time, even whilst in use.

(2) Adds a facility by which tags can be used to refer to caches, even if
they're not available yet.

(3) More than one cache can be used at once. Caches can be selected
explicitly by use of tags.

(4) The netfs is provided with an interface that allows either party to
withdraw caching facilities from a file (required for (1)).

(5) A netfs may annotate cache objects that belongs to it. This permits the
storage of coherency maintenance data.

(6) Cache objects will be pinnable and space reservations will be possible.

(7) The interface to the netfs returns as few errors as possible, preferring
rather to let the netfs remain oblivious.

(8) Cookies are used to represent indices, files and other objects to the
netfs. The simplest cookie is just a NULL pointer - indicating nothing
cached there.

(9) The netfs is allowed to propose - dynamically - any index hierarchy it
desires, though it must be aware that the index search function is
recursive, stack space is limited, and indices can only be children of
indices.

(10) Indices can be used to group files together to reduce key size and to make
group invalidation easier. The use of indices may make lookup quicker,
but that's cache dependent.

(11) Data I/O is effectively done directly to and from the netfs's pages. The
netfs indicates that page A is at index B of the data-file represented by
cookie C, and that it should be read or written. The cache backend may or
may not start I/O on that page, but if it does, a netfs callback will be
invoked to indicate completion. The I/O may be either synchronous or
asynchronous.

(12) Cookies can be "retired" upon release. At this point FS-Cache will mark
them as obsolete and the index hierarchy rooted at that point will get
recycled.

(13) The netfs provides a "match" function for index searches. In addition to
saying whether a match was made or not, this can also specify that an
entry should be updated or deleted.

FS-Cache maintains a virtual index tree in which all indices, files, objects
and pages are kept. Bits of this tree may actually reside in one or more
caches.

FSDEF
|
+------------------------------------+
| |
NFS AFS
| |
+--------------------------+ +-----------+
| | | |
homedir mirror afs.org redhat.com
| | |
+------------+ +---------------+ +----------+
| | | | | |
00001 00002 00007 00125 vol00001 vol00002
| | | | |
+---+---+ +-----+ +---+ +------+------+ +-----+----+
| | | | | | | | | | | | |
PG0 PG1 PG2 PG0 XATTR PG0 PG1 DIRENT DIRENT DIRENT R/W R/O Bak
| |
PG0 +-------+
| |
00001 00003
|
+---+---+
| | |
PG0 PG1 PG2

In the example above, two netfs's can be seen to be backed: NFS and AFS. These
have different index hierarchies:

(*) The NFS primary index will probably contain per-server indices. Each
server index is indexed by NFS file handles to get data file objects.
Each data file objects can have an array of pages, but may also have
further child objects, such as extended attributes and directory entries.
Extended attribute objects themselves have page-array contents.

(*) The AFS primary index contains per-cell indices. Each cell index contains
per-logical-volume indices. Each of volume index contains up to three
indices for the read-write, read-only and backup mirrors of those volumes.
Each of these contains vnode data file objects, each of which contains an
array of pages.

The very top index is the FS-Cache master index in which individual netfs's
have entries.

Any index object may reside in more than one cache, provided it only has index
children. Any index with non-index object children will be assumed to only
reside in one cache.

The FS-Cache overview can be found in:

Documentation/filesystems/caching/fscache.txt

The netfs API to FS-Cache can be found in:

Documentation/filesystems/caching/netfs-api.txt

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Daire Byrne <Daire.Byrne@framestore.com>