2022-01-11 23:07:49

by David Howells

[permalink] [raw]
Subject: [GIT PULL] fscache, cachefiles: Rewrite

Hi Linus,

Could you pull this please? It's 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 patchset 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.

I think also that a conflict[10] spotted by Stephen Rothwell between my
series and some changes that went in since the branching point
shouldn't be an issue with this removed.


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.

Tested-by: Dominique Martinet <[email protected]> # 9p
Tested-by: [email protected] # afs
Tested-by: Jeff Layton <[email protected]> # ceph
Tested-by: Dave Wysochanski <[email protected]> # nfs
Tested-by: Daire Byrne <[email protected]> # nfs

David


Changes
=======
ver #5:
- Fixed three cookie handling bugs in cifs[9].
- Dropped cifs support for the moment.

ver #4:
- Dropped a pair of patches to try and cope with multipage folios in
afs_write_begin/end() - it should really be done in the caller[7].
- Fixed the use of sizeof with memset in cifs.
- Removed an extraneous kdoc param.
- Added a patch to add a tracepoint for fscache_use/unuse_cookie().
- In cifs, tcon->vol_create_time is __le64 so doesn't need cpu_to_le64().
- Add an expanded version of a patch to use current_is_kswapd() instead of
!gfpflags_allow_blocking()[8].
- Removed a couple of debugging print statements.

ver #3:
- Fixed a race in the cookie state machine between LRU discard and
relinquishment[4].
- Fixed up the hashing to make it portable[5].
- Fixed up some netfs coherency data to make it portable.
- Fixed some missing NFS_FSCACHE=n fallback functions in nfs[6].
- Added a patch to store volume coherency data in an xattr.
- Added a check that the cookie is unhashed before being freed.
- Fixed fscache to use remove_proc_subtree() to remove /proc/fs/fscache/.

ver #2:
- Fix an unused-var warning due to CONFIG_9P_FSCACHE=n.
- Use gfpflags_allow_blocking() rather than using flag directly.
- Fixed some error logging in a couple of cachefiles functions.
- Fixed an error check in the fscache volume allocation.
- Need to unmark an inode we've moved to the graveyard before unlocking.
- Upgraded to -rc4 to allow for upstream changes to cifs.
- Should only change to inval state if can get access to cache.
- 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.
- Added missing transition to LRU_DISCARDING state.
- Added two ceph patches from Jeff Layton[2].
- Remove NFS_INO_FSCACHE as it's no longer used.
- In NFS, need to unuse a cookie on file-release, not inode-clear.
- Filled in the NFS cache I/O routines, borrowing from the previously posted
fallback I/O code[3].


Link: https://lore.kernel.org/r/[email protected]/ [1]
Link: https://lore.kernel.org/r/[email protected]/ [2]
Link: https://lore.kernel.org/r/[email protected]org.uk/ [3]
Link: https://lore.kernel.org/r/[email protected]/ [4]
Link: https://lore.kernel.org/r/[email protected]om [5]
Link: https://lore.kernel.org/r/61b90f3d.H1IkoeQfEsGNhvq9%[email protected]/ [6]
Link: https://lore.kernel.org/r/[email protected]om/ [7]
Link: https://lore.kernel.org/r/[email protected]/ [8]
Link: https://lore.kernel.org/r/[email protected]/ [9]
Link: https://lore.kernel.org/r/[email protected]/ [10]


References
==========

These patches have been published for review before, firstly as part of a
larger set:

Link: https://lore.kernel.org/r/[email protected]g.uk/

Link: https://lore.kernel.org/r/[email protected]org.uk/
Link: https://lore.kernel.org/r/[email protected]org.uk/
Link: https://lore.kernel.org/r/[email protected]rg.uk/

Link: https://lore.kernel.org/r/[email protected]rg.uk/

Then as a cut-down set:

Link: https://lore.kernel.org/r/[email protected]org.uk/ # v1
Link: https://lore.kernel.org/r/[email protected]rg.uk/ # v2
Link: https://lore.kernel.org/r/[email protected]rg.uk/ # v3
Link: https://lore.kernel.org/r/[email protected]g.uk/ # v4
Link: https://lore.kernel.org/r/[email protected]org.uk/ # v5

I split out a set to just restructure the I/O, which got merged back in to
this one:

Link: https://lore.kernel.org/r/[email protected]org.uk/
Link: https://lore.kernel.org/r/[email protected]org.uk/ # v2
Link: https://lore.kernel.org/r/[email protected]org.uk/ # v3
Link: https://lore.kernel.org/r/[email protected]org.uk/ # v4

... and a larger set to do the conversion, also merged back into this one:

Link: https://lore.kernel.org/r/[email protected]org.uk/ # v1
Link: https://lore.kernel.org/r/[email protected]org.uk/ # v2

Older versions of this one:

Link: https://lore.kernel.org/r/[email protected].uk/ # v1
Link: https://lore.kernel.org/r/[email protected]g.uk/ # v2
Link: https://lore.kernel.org/r/[email protected]org.uk/ # v3
Link: https://lore.kernel.org/r/[email protected]rg.uk/ # v4

Proposals/information about the design have been published here:

Link: https://lore.kernel.org/r/[email protected]/
Link: https://lore.kernel.org/r/[email protected]/
Link: https://lore.kernel.org/r/[email protected]/
Link: https://lore.kernel.org/r/[email protected]/

And requests for information:

Link: https://lore.kernel.org/r/[email protected]/
Link: https://lore.kernel.org/r/[email protected]/
Link: https://lore.kernel.org/r/[email protected]/

I've posted partial patches to try and help 9p and cifs along:

Link: https://lore.kernel.org/r/[email protected]/
Link: https://lore.kernel.org/r/[email protected]/
Link: https://lore.kernel.org/r/[email protected]/
Link: https://lore.kernel.org/r/[email protected]/
---
The following changes since commit 3cfef1b612e15a0c2f5b1c9d3f3f31ad72d56fcd:

netfs: fix parameter of cleanup() (2021-12-07 15:47:09 +0000)

are available in the Git repository at:

git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git tags/fscache-rewrite-20220111

for you to fetch changes up to d7bdba1c81f7e7bad12c7c7ce55afa3c7b0821ef:

9p, afs, ceph, nfs: Use current_is_kswapd() rather than gfpflags_allow_blocking() (2022-01-11 22:27:42 +0000)

----------------------------------------------------------------
fscache rewrite

----------------------------------------------------------------
Dave Wysochanski (1):
nfs: Convert to new fscache volume/cookie API

David Howells (64):
fscache, cachefiles: Disable configuration
cachefiles: Delete the cachefiles driver pending rewrite
fscache: Remove the contents of the fscache driver, pending rewrite
netfs: Display the netfs inode number in the netfs_read tracepoint
netfs: Pass a flag to ->prepare_write() to say if there's no alloc'd space
fscache: Introduce new driver
fscache: Implement a hash function
fscache: Implement cache registration
fscache: Implement volume registration
fscache: Implement cookie registration
fscache: Implement cache-level access helpers
fscache: Implement volume-level access helpers
fscache: Implement cookie-level access helpers
fscache: Implement functions add/remove a cache
fscache: Provide and use cache methods to lookup/create/free a volume
fscache: Add a function for a cache backend to note an I/O error
fscache: Implement simple cookie state machine
fscache: Implement cookie user counting and resource pinning
fscache: Implement cookie invalidation
fscache: Provide a means to begin an operation
fscache: Count data storage objects in a cache
fscache: Provide read/write stat counters for the cache
fscache: Provide a function to let the netfs update its coherency data
netfs: Pass more information on how to deal with a hole in the cache
fscache: Implement raw I/O interface
fscache: Implement higher-level write I/O interface
vfs, fscache: Implement pinning of cache usage for writeback
fscache: Provide a function to note the release of a page
fscache: Provide a function to resize a cookie
cachefiles: Introduce rewritten driver
cachefiles: Define structs
cachefiles: Add some error injection support
cachefiles: Add a couple of tracepoints for logging errors
cachefiles: Add cache error reporting macro
cachefiles: Add security derivation
cachefiles: Register a miscdev and parse commands over it
cachefiles: Provide a function to check how much space there is
vfs, cachefiles: Mark a backing file in use with an inode flag
cachefiles: Implement a function to get/create a directory in the cache
cachefiles: Implement cache registration and withdrawal
cachefiles: Implement volume support
cachefiles: Add tracepoints for calls to the VFS
cachefiles: Implement object lifecycle funcs
cachefiles: Implement key to filename encoding
cachefiles: Implement metadata/coherency data storage in xattrs
cachefiles: Mark a backing file in use with an inode flag
cachefiles: Implement culling daemon commands
cachefiles: Implement backing file wrangling
cachefiles: Implement begin and end I/O operation
cachefiles: Implement cookie resize for truncate
cachefiles: Implement the I/O routines
fscache, cachefiles: Store the volume coherency data
cachefiles: Allow cachefiles to actually function
fscache, cachefiles: Display stats of no-space events
fscache, cachefiles: Display stat of culling events
afs: Convert afs to use the new fscache API
afs: Copy local writes to the cache when writing to the server
afs: Skip truncation on the server of data we haven't written yet
9p: Use fscache indexing rewrite and reenable caching
9p: Copy local writes to the cache when writing to the server
nfs: Implement cache I/O by accessing the cache directly
fscache: Rewrite documentation
fscache: Add a tracepoint for cookie use/unuse
9p, afs, ceph, nfs: Use current_is_kswapd() rather than gfpflags_allow_blocking()

Jeff Layton (2):
ceph: conversion to new fscache API
ceph: add fscache writeback support

Documentation/filesystems/caching/backend-api.rst | 850 +++++-------
Documentation/filesystems/caching/cachefiles.rst | 6 +-
Documentation/filesystems/caching/fscache.rst | 525 +++-----
Documentation/filesystems/caching/index.rst | 4 +-
Documentation/filesystems/caching/netfs-api.rst | 1136 +++++-----------
Documentation/filesystems/caching/object.rst | 313 -----
Documentation/filesystems/caching/operations.rst | 210 ---
Documentation/filesystems/netfs_library.rst | 16 +-
fs/9p/cache.c | 195 +--
fs/9p/cache.h | 25 +-
fs/9p/v9fs.c | 17 +-
fs/9p/v9fs.h | 13 +-
fs/9p/vfs_addr.c | 57 +-
fs/9p/vfs_dir.c | 13 +
fs/9p/vfs_file.c | 3 +-
fs/9p/vfs_inode.c | 26 +-
fs/9p/vfs_inode_dotl.c | 3 +-
fs/9p/vfs_super.c | 3 +
fs/afs/Makefile | 3 -
fs/afs/cache.c | 68 -
fs/afs/cell.c | 12 -
fs/afs/file.c | 38 +-
fs/afs/inode.c | 101 +-
fs/afs/internal.h | 37 +-
fs/afs/main.c | 14 -
fs/afs/super.c | 1 +
fs/afs/volume.c | 29 +-
fs/afs/write.c | 88 +-
fs/cachefiles/Kconfig | 7 +
fs/cachefiles/Makefile | 6 +-
fs/cachefiles/bind.c | 278 ----
fs/cachefiles/cache.c | 378 ++++++
fs/cachefiles/daemon.c | 180 ++-
fs/cachefiles/error_inject.c | 46 +
fs/cachefiles/interface.c | 747 +++++------
fs/cachefiles/internal.h | 270 ++--
fs/cachefiles/io.c | 330 +++--
fs/cachefiles/key.c | 201 ++-
fs/cachefiles/main.c | 22 +-
fs/cachefiles/namei.c | 1223 ++++++++---------
fs/cachefiles/rdwr.c | 972 --------------
fs/cachefiles/security.c | 2 +-
fs/cachefiles/volume.c | 139 ++
fs/cachefiles/xattr.c | 421 +++---
fs/ceph/addr.c | 102 +-
fs/ceph/cache.c | 218 +---
fs/ceph/cache.h | 97 +-
fs/ceph/caps.c | 3 +-
fs/ceph/file.c | 13 +-
fs/ceph/inode.c | 22 +-
fs/ceph/super.c | 10 +-
fs/ceph/super.h | 3 +-
fs/cifs/Kconfig | 2 +-
fs/fs-writeback.c | 8 +
fs/fscache/Kconfig | 3 +
fs/fscache/Makefile | 6 +-
fs/fscache/cache.c | 618 ++++-----
fs/fscache/cookie.c | 1448 +++++++++++----------
fs/fscache/fsdef.c | 98 --
fs/fscache/internal.h | 317 +----
fs/fscache/io.c | 376 ++++--
fs/fscache/main.c | 147 +--
fs/fscache/netfs.c | 74 --
fs/fscache/object.c | 1125 ----------------
fs/fscache/operation.c | 633 ---------
fs/fscache/page.c | 1242 ------------------
fs/fscache/proc.c | 47 +-
fs/fscache/stats.c | 293 +----
fs/fscache/volume.c | 517 ++++++++
fs/namei.c | 3 +-
fs/netfs/read_helper.c | 10 +-
fs/nfs/Makefile | 2 +-
fs/nfs/client.c | 4 -
fs/nfs/direct.c | 2 +
fs/nfs/file.c | 13 +-
fs/nfs/fscache-index.c | 140 --
fs/nfs/fscache.c | 490 +++----
fs/nfs/fscache.h | 180 +--
fs/nfs/inode.c | 11 +-
fs/nfs/nfstrace.h | 1 -
fs/nfs/read.c | 25 +-
fs/nfs/super.c | 28 +-
fs/nfs/write.c | 8 +-
include/linux/fs.h | 4 +
include/linux/fscache-cache.h | 614 ++-------
include/linux/fscache.h | 1021 ++++++---------
include/linux/netfs.h | 15 +-
include/linux/nfs_fs.h | 1 -
include/linux/nfs_fs_sb.h | 9 +-
include/linux/writeback.h | 1 +
include/trace/events/cachefiles.h | 527 ++++++--
include/trace/events/fscache.h | 642 +++++----
include/trace/events/netfs.h | 5 +-
93 files changed, 7205 insertions(+), 13001 deletions(-)
delete mode 100644 Documentation/filesystems/caching/object.rst
delete mode 100644 Documentation/filesystems/caching/operations.rst
delete mode 100644 fs/afs/cache.c
delete mode 100644 fs/cachefiles/bind.c
create mode 100644 fs/cachefiles/cache.c
create mode 100644 fs/cachefiles/error_inject.c
delete mode 100644 fs/cachefiles/rdwr.c
create mode 100644 fs/cachefiles/volume.c
delete mode 100644 fs/fscache/fsdef.c
delete mode 100644 fs/fscache/netfs.c
delete mode 100644 fs/fscache/object.c
delete mode 100644 fs/fscache/operation.c
delete mode 100644 fs/fscache/page.c
create mode 100644 fs/fscache/volume.c
delete mode 100644 fs/nfs/fscache-index.c



2022-01-11 23:32:05

by Stephen Rothwell

[permalink] [raw]
Subject: Re: [GIT PULL] fscache, cachefiles: Rewrite

Hi David,

On Tue, 11 Jan 2022 23:07:24 +0000 David Howells <[email protected]> wrote:
>
> I think also that a conflict[10] spotted by Stephen Rothwell between my
> series and some changes that went in since the branching point
> shouldn't be an issue with this removed.

There is also this conflict against the pifdf tree (which may or may
not be merged before this):

https://lore.kernel.org/all/[email protected]/

--
Cheers,
Stephen Rothwell


Attachments:
(No filename) (488.00 B)
OpenPGP digital signature