[GIT PULL] fscache rewrite

David Howells
2020-08-03T16:30:40Z

Hi Linus,

Here's a set of patches that massively overhauls fscache and cachefiles.
It improves the code by:

(*) Ripping out the stuff that uses page cache snooping and kernel_write()
and using kiocb instead. This gives multiple wins: uses async DIO
rather than snooping for updated pages and then copying them, less VM
overhead.

(*) Object management is also simplified, getting rid of the state machine
that was managing things and using a much simplified thread pool
instead.

(*) Object invalidation creates a tmpfile and diverts new activity to that
so that it doesn't have to synchronise in-flight ADIO.

(*) Using a bitmap stored in an xattr rather than using bmap to find out if
a block is present in the cache.

Probing the backing filesystem's metadata to find out is not reliable
in modern extent-based filesystems as them may insert or remove blocks
of zeros. Even SEEK_HOLE/SEEK_DATA are problematic since they don't
distinguish transparently-inserted bridging.

The patchset includes a read helper that handles ->readpage, ->readpages,
and preparatory writes in ->write_begin. Matthew Wilcox is looking at
using this as a way to roll his new ->readahead op out into filesystems. A
good chunk of this will move into MM code.

Note that this patchset does not include documentation changes yet. I have
them (mostly) done, but they were based on the plain-text format that got
ReST-ified, and I haven't managed to get around to the conversion yet. I
can create a follow-up patchset for that if this is taken.

Further note: There's a last minute change due to a bit of debugging code
that got left in mm/filemap.c that needed removing.

However, there are reasons you might not want to take it yet:

(1) It starts off by disabling fscache support in all the filesystems that
use it: afs, nfs, cifs, ceph and 9p. I've taken care of afs, Dave
Wysochanski has patches for nfs:

https://lore.kernel.org/linux-nfs/1596031949-26793-1-git-send-email-dwysocha@redhat.com/

but Trond and Anna haven't said anything yet, and Jeff Layton has
patches for ceph:

https://marc.info/?l=ceph-devel&m=159541538914631&w=2

and I've briefly discussed cifs with Steve, but nothing has started
there yet. 9p I haven't looked at yet.

Are we okay for going a kernel release with 4/5 filesystems with
caching disabled and then pushing the changes for individual
filesystems through their respective trees? I floated this question
last week, but have no replies either way.

(2) The patched afs fs passed xfstests -g quick (unlike the upstream code
that oopses pretty quickly with caching enabled). Dave and Jeff's nfs
and ceph code is getting close, but not quite there yet.

(3) Al has objections to the ITER_MAPPING iov_iter type that I added

https://lore.kernel.org/linux-fsdevel/20200719014436.GG2786714@ZenIV.linux.org.uk/

but note that iov_iter_for_each_range() is not actually used by anything.

However, Willy likes it and would prefer to make it ITER_XARRAY instead
as he might be able to use it in other places, though there's an issue
where I'm calling find_get_pages_contig() which takes a mapping (though
all it does is then get the xarray out of it). Willy has made
suggestions as to how this may be achieved, but I haven't got round to
looking at them yet.

Instead I would have to use ITER_BVEC, which has quite a high overhead,
though it would mean that the RCU read lock wouldn't be necessary. This
would require 1K of memory for every 256K block the cache wants to read;
for any read >1M, I'd have to use vmalloc() instead.

I'd also prefer not to use ITER_BVEC because the offset and length are
superfluous here. If ITER_MAPPING is not good, would it be possible to
have an ITER_PAGEARRAY that just takes a page array instead? Or, even,
create a transient xarray?

(4) The way object culling is managed needs overhauling too, but that's a
separate patchset in its own right. We could wait till that's done
too, but its lack doesn't prevent what we have now from being used.

David
---
The following changes since commit 9ebcfadb0610322ac537dd7aa5d9cbc2b2894c68:

Linux 5.8-rc3 (2020-06-28 15:00:24 -0700)

are available in the Git repository at:

git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git tags/fscache-iter-20200803

for you to fetch changes up to 2f716a79439b100a3ded54b828f9c87582d13f86:

fscache: disable cookie when doing an invalidation for DIO write (2020-08-03 17:22:36 +0100)

----------------------------------------------------------------
Filesystem caching rewrite

----------------------------------------------------------------
David Howells (59):
nfs, cifs, ceph, 9p: Disable use of fscache prior to its rewrite
afs: Disable use of the fscache I/O routines
fscache: Add a cookie debug ID and use that in traces
fscache: Procfile to display cookies
fscache: Remove the old I/O API
fscache: Remove the netfs data from the cookie
fscache: Remove struct fscache_cookie_def
fscache: Remove store_limit* from struct fscache_object
fscache: Remove fscache_check_consistency()
fscache: Remove fscache_attr_changed()
fscache: Remove obsolete stats
fscache: Remove old I/O tracepoints
fscache: Temporarily disable fscache_invalidate()
fscache: Remove the I/O operation manager
iov_iter: Add ITER_MAPPING
vm: Add wait/unlock functions for PG_fscache
vfs: Export rw_verify_area() for use by cachefiles
vfs: Provide S_CACHE_FILE inode flag
mm: Provide lru_to_last_page() to get last of a page list
cachefiles: Remove tree of active files and use S_CACHE_FILE inode flag
fscache: Provide a simple thread pool for running ops asynchronously
fscache: Replace the object management state machine
fscache: Rewrite the I/O API based on iov_iter
fscache: Keep track of size of a file last set independently on the server
fscache, cachefiles: Fix disabled histogram warnings
fscache: Recast assertion in terms of cookie not being an index
cachefiles: Remove some redundant checks on unsigned values
cachefiles: trace: Log coherency checks
cachefiles: Split cachefiles_drop_object() up a bit
cachefiles: Implement new fscache I/O backend API
cachefiles: Merge object->backer into object->dentry
cachefiles: Implement a content-present indicator and bitmap
cachefiles: Shape read requests
cachefiles: Round the cachefile size up to DIO block size
cachefiles: Implement read and write parts of new I/O API
cachefiles: Add I/O tracepoints
fscache: Add read helper
fscache: Display cache-specific data in /proc/fs/fscache/objects
fscache: Remove more obsolete stats
fscache: New stats
fscache, cachefiles: Rewrite invalidation
fscache: Implement "will_modify" parameter on fscache_use_cookie()
fscache: Provide resize operation
fscache: Remove the update operation
cachefiles: Shape write requests
afs: Fix interruption of operations
afs: Move key to afs_read struct
afs: Don't truncate iter during data fetch
afs: Log remote unmarshalling errors
afs: Set up the iov_iter before calling afs_extract_data()
afs: Use ITER_MAPPING for writing
afs: Interpose struct fscache_io_request into struct afs_read
afs: Note the amount transferred in fetch-data delivery
afs: Wait on PG_fscache before modifying/releasing a page
afs: Use new fscache I/O API
afs: Copy local writes to the cache when writing to the server
afs: Invoke fscache_resize_cookie() when handling ATTR_SIZE for setattr
afs: Add O_DIRECT read support
afs: Skip truncation on the server of data we haven't written yet

Jeff Layton (1):
fscache: disable cookie when doing an invalidation for DIO write

fs/9p/Kconfig | 2 +-
fs/Makefile | 2 +-
fs/afs/Kconfig | 4 +-
fs/afs/cache.c | 54 --
fs/afs/cell.c | 9 +-
fs/afs/dir.c | 242 +++++--
fs/afs/file.c | 577 +++++++--------
fs/afs/fs_operation.c | 4 +-
fs/afs/fsclient.c | 154 ++--
fs/afs/inode.c | 104 ++-
fs/afs/internal.h | 58 +-
fs/afs/rxrpc.c | 150 ++--
fs/afs/volume.c | 9 +-
fs/afs/write.c | 435 +++++++----
fs/afs/yfsclient.c | 113 ++-
fs/cachefiles/Makefile | 3 +-
fs/cachefiles/bind.c | 11 +-
fs/cachefiles/content-map.c | 499 +++++++++++++
fs/cachefiles/daemon.c | 10 +-
fs/cachefiles/interface.c | 580 ++++++++-------
fs/cachefiles/internal.h | 142 ++--
fs/cachefiles/io.c | 325 +++++++++
fs/cachefiles/main.c | 12 +-
fs/cachefiles/namei.c | 508 +++++--------
fs/cachefiles/rdwr.c | 974 -------------------------
fs/cachefiles/xattr.c | 263 +++----
fs/ceph/Kconfig | 2 +-
fs/cifs/Kconfig | 2 +-
fs/fscache/Kconfig | 24 +-
fs/fscache/Makefile | 15 +-
fs/fscache/cache.c | 145 ++--
fs/fscache/cookie.c | 898 ++++++++++-------------
fs/fscache/dispatcher.c | 150 ++++
fs/fscache/fsdef.c | 56 +-
fs/fscache/histogram.c | 2 +-
fs/fscache/internal.h | 264 +++----
fs/fscache/io.c | 206 ++++++
fs/fscache/main.c | 35 +-
fs/fscache/netfs.c | 10 +-
fs/fscache/obj.c | 366 ++++++++++
fs/fscache/object-list.c | 129 +---
fs/fscache/object.c | 1133 -----------------------------
fs/fscache/object_bits.c | 120 +++
fs/fscache/operation.c | 633 ----------------
fs/fscache/page.c | 1248 --------------------------------
fs/fscache/proc.c | 13 +-
fs/fscache/read_helper.c | 701 ++++++++++++++++++
fs/fscache/stats.c | 269 +++----
fs/internal.h | 5 -
fs/nfs/Kconfig | 2 +-
fs/nfs/fscache-index.c | 4 +-
fs/read_write.c | 1 +
include/linux/fs.h | 2 +
include/linux/fscache-cache.h | 508 +++----------
include/linux/fscache-obsolete.h | 13 +
include/linux/fscache.h | 834 +++++++++------------
include/linux/mm.h | 1 +
include/linux/pagemap.h | 14 +
include/linux/uio.h | 11 +
include/net/af_rxrpc.h | 2 +-
include/trace/events/afs.h | 51 +-
include/trace/events/cachefiles.h | 285 ++++++--
include/trace/events/fscache.h | 428 ++---------
include/trace/events/fscache_support.h | 97 +++
lib/iov_iter.c | 286 +++++++-
mm/filemap.c | 18 +
net/rxrpc/recvmsg.c | 9 +-
67 files changed, 5941 insertions(+), 8295 deletions(-)
create mode 100644 fs/cachefiles/content-map.c
create mode 100644 fs/cachefiles/io.c
delete mode 100644 fs/cachefiles/rdwr.c
create mode 100644 fs/fscache/dispatcher.c
create mode 100644 fs/fscache/io.c
create mode 100644 fs/fscache/obj.c
delete mode 100644 fs/fscache/object.c
create mode 100644 fs/fscache/object_bits.c
delete mode 100644 fs/fscache/operation.c
delete mode 100644 fs/fscache/page.c
create mode 100644 fs/fscache/read_helper.c
create mode 100644 include/linux/fscache-obsolete.h
create mode 100644 include/trace/events/fscache_support.h