Merge tag 'fscache-rewrite-20220111' of git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs

Pull fscache rewrite from David Howells:
 "This is a set of patches that rewrites the fscache driver and the
  cachefiles driver, significantly simplifying the code compared to
  what's upstream, removing the complex operation scheduling and object
  state machine in favour of something much smaller and simpler.

  The series is structured such that the first few patches disable
  fscache use by the network filesystems using it, remove the cachefiles
  driver entirely and as much of the fscache driver as can be got away
  with without causing build failures in the network filesystems.

  The patches after that recreate fscache and then cachefiles,
  attempting to add the pieces in a logical order. Finally, the
  filesystems are reenabled and then the very last patch changes the
  documentation.

  [!] Note: I have dropped the cifs patch for the moment, leaving local
      caching in cifs disabled. I've been having trouble getting that
      working. I think I have it done, but it needs more testing (there
      seem to be some test failures occurring with v5.16 also from
      xfstests), so I propose deferring that patch to the end of the
      merge window.

  WHY REWRITE?
  ============

  Fscache's operation scheduling API was intended to handle sequencing
  of cache operations, which were all required (where possible) to run
  asynchronously in parallel with the operations being done by the
  network filesystem, whilst allowing the cache to be brought online and
  offline and to interrupt service for invalidation.

  With the advent of the tmpfile capacity in the VFS, however, an
  opportunity arises to do invalidation much more simply, without having
  to wait for I/O that's actually in progress: Cachefiles can simply
  create a tmpfile, cut over the file pointer for the backing object
  attached to a cookie and abandon the in-progress I/O, dismissing it
  upon completion.

  Future work here would involve using Omar Sandoval's vfs_link() with
  AT_LINK_REPLACE[1] to allow an extant file to be displaced by a new
  hard link from a tmpfile as currently I have to unlink the old file
  first.

  These patches can also simplify the object state handling as I/O
  operations to the cache don't all have to be brought to a stop in
  order to invalidate a file. To that end, and with an eye on to writing
  a new backing cache model in the future, I've taken the opportunity to
  simplify the indexing structure.

  I've separated the index cookie concept from the file cookie concept
  by C type now. The former is now called a "volume cookie" (struct
  fscache_volume) and there is a container of file cookies. There are
  then just the two levels. All the index cookie levels are collapsed
  into a single volume cookie, and this has a single printable string as
  a key. For instance, an AFS volume would have a key of something like
  "afs,example.com,1000555", combining the filesystem name, cell name
  and volume ID. This is freeform, but must not have '/' chars in it.

  I've also eliminated all pointers back from fscache into the network
  filesystem. This required the duplication of a little bit of data in
  the cookie (cookie key, coherency data and file size), but it's not
  actually that much. This gets rid of problems with making sure we keep
  netfs data structures around so that the cache can access them.

  These patches mean that most of the code that was in the drivers
  before is simply gone and those drivers are now almost entirely new
  code. That being the case, there doesn't seem any particular reason to
  try and maintain bisectability across it. Further, there has to be a
  point in the middle where things are cut over as there's a single
  point everything has to go through (ie. /dev/cachefiles) and it can't
  be in use by two drivers at once.

  ISSUES YET OUTSTANDING
  ======================

  There are some issues still outstanding, unaddressed by this patchset,
  that will need fixing in future patchsets, but that don't stop this
  series from being usable:

  (1) The cachefiles driver needs to stop using the backing filesystem's
      metadata to store information about what parts of the cache are
      populated. This is not reliable with modern extent-based
      filesystems.

      Fixing this is deferred to a separate patchset as it involves
      negotiation with the network filesystem and the VM as to how much
      data to download to fulfil a read - which brings me on to (2)...

  (2) NFS (and CIFS with the dropped patch) do not take account of how
      the cache would like I/O to be structured to meet its granularity
      requirements. Previously, the cache used page granularity, which
      was fine as the network filesystems also dealt in page
      granularity, and the backing filesystem (ext4, xfs or whatever)
      did whatever it did out of sight. However, we now have folios to
      deal with and the cache will now have to store its own metadata to
      track its contents.

      The change I'm looking at making for cachefiles is to store
      content bitmaps in one or more xattrs and making a bit in the map
      correspond to something like a 256KiB block. However, the size of
      an xattr and the fact that they have to be read/updated in one go
      means that I'm looking at covering 1GiB of data per 512-byte map
      and storing each map in an xattr. Cachefiles has the potential to
      grow into a fully fledged filesystem of its very own if I'm not
      careful.

      However, I'm also looking at changing things even more radically
      and going to a different model of how the cache is arranged and
      managed - one that's more akin to the way, say, openafs does
      things - which brings me on to (3)...

  (3) The way cachefilesd does culling is very inefficient for large
      caches and it would be better to move it into the kernel if I can
      as cachefilesd has to keep asking the kernel if it can cull a
      file. Changing the way the backend works would allow this to be
      addressed.

  BITS THAT MAY BE CONTROVERSIAL
  ==============================

  There are some bits I've added that may be controversial:

  (1) I've provided a flag, S_KERNEL_FILE, that cachefiles uses to check
      if a files is already being used by some other kernel service
      (e.g. a duplicate cachefiles cache in the same directory) and
      reject it if it is. This isn't entirely necessary, but it helps
      prevent accidental data corruption.

      I don't want to use S_SWAPFILE as that has other effects, but
      quite possibly swapon() should set S_KERNEL_FILE too.

      Note that it doesn't prevent userspace from interfering, though
      perhaps it should. (I have made it prevent a marked directory from
      being rmdir-able).

  (2) Cachefiles wants to keep the backing file for a cookie open whilst
      we might need to write to it from network filesystem writeback.
      The problem is that the network filesystem unuses its cookie when
      its file is closed, and so we have nothing pinning the cachefiles
      file open and it will get closed automatically after a short time
      to avoid EMFILE/ENFILE 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 suchlike.

      To get around this, I added the following:

      (A) 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.

      (B) 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.

      (C) 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.

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

      (E) 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"

Link: https://lore.kernel.org/all/510611.1641942444@warthog.procyon.org.uk/
Tested-by: Dominique Martinet <asmadeus@codewreck.org> # 9p
Tested-by: kafs-testing@auristor.com # afs
Tested-by: Jeff Layton <jlayton@kernel.org> # ceph
Tested-by: Dave Wysochanski <dwysocha@redhat.com> # nfs
Tested-by: Daire Byrne <daire@dneg.com> # nfs

* tag 'fscache-rewrite-20220111' of git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs: (67 commits)
  9p, afs, ceph, nfs: Use current_is_kswapd() rather than gfpflags_allow_blocking()
  fscache: Add a tracepoint for cookie use/unuse
  fscache: Rewrite documentation
  ceph: add fscache writeback support
  ceph: conversion to new fscache API
  nfs: Implement cache I/O by accessing the cache directly
  nfs: Convert to new fscache volume/cookie API
  9p: Copy local writes to the cache when writing to the server
  9p: Use fscache indexing rewrite and reenable caching
  afs: Skip truncation on the server of data we haven't written yet
  afs: Copy local writes to the cache when writing to the server
  afs: Convert afs to use the new fscache API
  fscache, cachefiles: Display stat of culling events
  fscache, cachefiles: Display stats of no-space events
  cachefiles: Allow cachefiles to actually function
  fscache, cachefiles: Store the volume coherency data
  cachefiles: Implement the I/O routines
  cachefiles: Implement cookie resize for truncate
  cachefiles: Implement begin and end I/O operation
  cachefiles: Implement backing file wrangling
  ...

1 files changed, 0 insertions, 210 deletions

diff --git a/Documentation/filesystems/caching/operations.rst b/Documentation/filesystems/caching/operations.rst
deleted file mode 100644
index 9983e1675447..000000000000
--- a/Documentation/filesystems/caching/operations.rst
+++ /dev/null
@@ -1,210 +0,0 @@
-.. SPDX-License-Identifier: GPL-2.0
-
-================================
-Asynchronous Operations Handling
-================================
-
-By: David Howells <dhowells@redhat.com>
-
-.. Contents:
-
- (*) Overview.
-
- (*) Operation record initialisation.
-
- (*) Parameters.
-
- (*) Procedure.
-
- (*) Asynchronous callback.
-
-
-Overview
-========
-
-FS-Cache has an asynchronous operations handling facility that it uses for its
-data storage and retrieval routines.  Its operations are represented by
-fscache_operation structs, though these are usually embedded into some other
-structure.
-
-This facility is available to and expected to be used by the cache backends,
-and FS-Cache will create operations and pass them off to the appropriate cache
-backend for completion.
-
-To make use of this facility, <linux/fscache-cache.h> should be #included.
-
-
-Operation Record Initialisation
-===============================
-
-An operation is recorded in an fscache_operation struct::
-
-	struct fscache_operation {
-		union {
-			struct work_struct fast_work;
-			struct slow_work slow_work;
-		};
-		unsigned long		flags;
-		fscache_operation_processor_t processor;
-		...
-	};
-
-Someone wanting to issue an operation should allocate something with this
-struct embedded in it.  They should initialise it by calling::
-
-	void fscache_operation_init(struct fscache_operation *op,
-				    fscache_operation_release_t release);
-
-with the operation to be initialised and the release function to use.
-
-The op->flags parameter should be set to indicate the CPU time provision and
-the exclusivity (see the Parameters section).
-
-The op->fast_work, op->slow_work and op->processor flags should be set as
-appropriate for the CPU time provision (see the Parameters section).
-
-FSCACHE_OP_WAITING may be set in op->flags prior to each submission of the
-operation and waited for afterwards.
-
-
-Parameters
-==========
-
-There are a number of parameters that can be set in the operation record's flag
-parameter.  There are three options for the provision of CPU time in these
-operations:
-
- (1) The operation may be done synchronously (FSCACHE_OP_MYTHREAD).  A thread
-     may decide it wants to handle an operation itself without deferring it to
-     another thread.
-
-     This is, for example, used in read operations for calling readpages() on
-     the backing filesystem in CacheFiles.  Although readpages() does an
-     asynchronous data fetch, the determination of whether pages exist is done
-     synchronously - and the netfs does not proceed until this has been
-     determined.
-
-     If this option is to be used, FSCACHE_OP_WAITING must be set in op->flags
-     before submitting the operation, and the operating thread must wait for it
-     to be cleared before proceeding::
-
-		wait_on_bit(&op->flags, FSCACHE_OP_WAITING,
-			    TASK_UNINTERRUPTIBLE);
-
-
- (2) The operation may be fast asynchronous (FSCACHE_OP_FAST), in which case it
-     will be given to keventd to process.  Such an operation is not permitted
-     to sleep on I/O.
-
-     This is, for example, used by CacheFiles to copy data from a backing fs
-     page to a netfs page after the backing fs has read the page in.
-
-     If this option is used, op->fast_work and op->processor must be
-     initialised before submitting the operation::
-
-		INIT_WORK(&op->fast_work, do_some_work);
-
-
- (3) The operation may be slow asynchronous (FSCACHE_OP_SLOW), in which case it
-     will be given to the slow work facility to process.  Such an operation is
-     permitted to sleep on I/O.
-
-     This is, for example, used by FS-Cache to handle background writes of
-     pages that have just been fetched from a remote server.
-
-     If this option is used, op->slow_work and op->processor must be
-     initialised before submitting the operation::
-
-		fscache_operation_init_slow(op, processor)
-
-
-Furthermore, operations may be one of two types:
-
- (1) Exclusive (FSCACHE_OP_EXCLUSIVE).  Operations of this type may not run in
-     conjunction with any other operation on the object being operated upon.
-
-     An example of this is the attribute change operation, in which the file
-     being written to may need truncation.
-
- (2) Shareable.  Operations of this type may be running simultaneously.  It's
-     up to the operation implementation to prevent interference between other
-     operations running at the same time.
-
-
-Procedure
-=========
-
-Operations are used through the following procedure:
-
- (1) The submitting thread must allocate the operation and initialise it
-     itself.  Normally this would be part of a more specific structure with the
-     generic op embedded within.
-
- (2) The submitting thread must then submit the operation for processing using
-     one of the following two functions::
-
-	int fscache_submit_op(struct fscache_object *object,
-			      struct fscache_operation *op);
-
-	int fscache_submit_exclusive_op(struct fscache_object *object,
-					struct fscache_operation *op);
-
-     The first function should be used to submit non-exclusive ops and the
-     second to submit exclusive ones.  The caller must still set the
-     FSCACHE_OP_EXCLUSIVE flag.
-
-     If successful, both functions will assign the operation to the specified
-     object and return 0.  -ENOBUFS will be returned if the object specified is
-     permanently unavailable.
-
-     The operation manager will defer operations on an object that is still
-     undergoing lookup or creation.  The operation will also be deferred if an
-     operation of conflicting exclusivity is in progress on the object.
-
-     If the operation is asynchronous, the manager will retain a reference to
-     it, so the caller should put their reference to it by passing it to::
-
-	void fscache_put_operation(struct fscache_operation *op);
-
- (3) If the submitting thread wants to do the work itself, and has marked the
-     operation with FSCACHE_OP_MYTHREAD, then it should monitor
-     FSCACHE_OP_WAITING as described above and check the state of the object if
-     necessary (the object might have died while the thread was waiting).
-
-     When it has finished doing its processing, it should call
-     fscache_op_complete() and fscache_put_operation() on it.
-
- (4) The operation holds an effective lock upon the object, preventing other
-     exclusive ops conflicting until it is released.  The operation can be
-     enqueued for further immediate asynchronous processing by adjusting the
-     CPU time provisioning option if necessary, eg::
-
-	op->flags &= ~FSCACHE_OP_TYPE;
-	op->flags |= ~FSCACHE_OP_FAST;
-
-     and calling::
-
-	void fscache_enqueue_operation(struct fscache_operation *op)
-
-     This can be used to allow other things to have use of the worker thread
-     pools.
-
-
-Asynchronous Callback
-=====================
-
-When used in asynchronous mode, the worker thread pool will invoke the
-processor method with a pointer to the operation.  This should then get at the
-container struct by using container_of()::
-
-	static void fscache_write_op(struct fscache_operation *_op)
-	{
-		struct fscache_storage *op =
-			container_of(_op, struct fscache_storage, op);
-	...
-	}
-
-The caller holds a reference on the operation, and will invoke
-fscache_put_operation() when the processor function returns.  The processor
-function is at liberty to call fscache_enqueue_operation() or to take extra
-references.