[PATCH v5 00/32] netfs: Read performance improvements and "single-blob" support

David Howells
2024-12-16T20:41:47Z

Hi Christian, Steve, Willy,

This set of patches is primarily about two things: improving read
performance and supporting monolithic single-blob objects that have to be
read/written as such (e.g. AFS directory contents). The implementation of
the two parts is interwoven as each makes the other possible.

READ PERFORMANCE
================

The read performance improvements are intended to speed up some loss of
performance detected in cifs and to a lesser extend in afs. The problem is
that we queue too many work items during the collection of read results:
each individual subrequest is collected by its own work item, and then they
have to interact with each other when a series of subrequests don't exactly
align with the pattern of folios that are being read by the overall
request.

Whilst the processing of the pages covered by individual subrequests as
they complete potentially allows folios to be woken in parallel and with
minimum delay, it can shuffle wakeups for sequential reads out of order -
and that is the most common I/O pattern.

The final assessment and cleanup of an operation is then held up until the
last I/O completes - and for a synchronous sequential operation, this means
the bouncing around of work items just adds latency.

Two changes have been made to make this work:

(1) All collection is now done in a single "work item" that works
progressively through the subrequests as they complete (and also
dispatches retries as necessary).

(2) For readahead and AIO, this work item be done on a workqueue and can
run in parallel with the ultimate consumer of the data; for
synchronous direct or unbuffered reads, the collection is run in the
application thread and not offloaded.

Functions such as smb2_readv_callback() then just tell netfslib that the
subrequest has terminated; netfslib does a minimal bit of processing on the
spot - stat counting and tracing mostly - and then queues/wakes up the
worker. This simplifies the logic as the collector just walks sequentially
through the subrequests as they complete and walks through the folios, if
buffered, unlocking them as it goes. It also keeps to a minimum the amount
of latency injected into the filesystem's low-level I/O handling

The way netfs supports filesystems using the deprecated PG_private_2 flag
is changed: folios are flagged and added to a write request as they
complete and that takes care of scheduling the writes to the cache. The
originating read request can then just unlock the pages whatever happens.

SINGLE-BLOB OBJECT SUPPORT
==========================

Single-blob objects are files for which the content of the file must be
read from or written to the server in a single operation because reading
them in parts may yield inconsistent results. AFS directories are an
example of this as there exists the possibility that the contents are
generated on the fly and would differ between reads or might change due to
third party interference.

Such objects will be written to and retrieved from the cache if one is
present, though we allow/may need to propose multiple subrequests to do so.
The important part is that read from/write to the *server* is monolithic.

Single blob reading is, for the moment, fully synchronous and does result
collection in the application thread and, also for the moment, the API is
supplied the buffer in the form of a folio_queue chain rather than using
the pagecache.

AFS CHANGES
===========

This series makes a number of changes to the kafs filesystem, primarily in
the area of directory handling:

(1) AFS's FetchData RPC reply processing is made partially asynchronous
which allows the netfs_io_request's outstanding operation counter to
be removed as part of reducing the collection to a single work item.

(2) Directory and symlink reading are plumbed through netfslib using the
single-blob object API and are now cacheable with fscache. This also
allows the afs_read struct to be eliminated and netfs_io_subrequest to
be used directly instead.

(3) Directory and symlink content are now stored in a folio_queue buffer
rather than in the pagecache. This means we don't require the RCU
read lock and xarray iteration to access it, and folios won't randomly
disappear under us because the VM wants them back.

There are some downsides to this, though: the storage folios are no
longer known to the VM, drop_caches can't flush them, the folios are
not migrateable. The inode must also be marked dirty manually to get
the data written to the cache in the background.

(4) The vnode operation lock is changed from a mutex struct to a private
lock implementation. The problem is that the lock now needs to be
dropped in a separate thread and mutexes don't permit that.

(5) When a new directory or symlink is created, we now initialise it
locally and mark it valid rather than downloading it (we know what
it's likely to look like).

(6) We now use the in-directory hashtable to reduce the number of entries
we need to scan when doing a lookup. The edit routines have to
maintain the hash chains.

(7) Cancellation (e.g. by signal) of an async call after the rxrpc_call
has been set up is now offloaded to the worker thread as there will be
a notification from rxrpc upon completion. This avoids a double
cleanup.

SUPPORTING CHANGES
==================

To support the above some other changes are also made:

(1) A "rolling buffer" implementation is created to abstract out the two
separate folio_queue chaining implementations I had (one for read and
one for write).

(2) Functions are provided to create/extend a buffer in a folio_queue
chain and tear it down again. This is used to handle AFS directories,
but could also be used to create bounce buffers for content crypto and
transport crypto.

(3) The was_async argument is dropped from netfs_read_subreq_terminated().
Instead we wake the read collection work item by either queuing it or
waking up the app thread.

(4) We don't need to use BH-excluding locks when communicating between the
issuing thread and the collection thread as neither of them now run in
BH context.

MISCELLANY
==========

Also included are a number of new tracepoints; a split of the netfslib
write collection code to put retrying into its own file (it gets more
complicated with content encryption).

There are also some minor fixes AFS included, including fixing the AFS
directory format struct layout, reducing some directory over-invalidation
and making afs_mkdir() translate EEXIST to ENOTEMPY (which is not available
on all systems the servers support).

Finally, there's a patch to try and detect entry into the folio unlock
function with no folio_queue structs in the buffer (which isn't allowed in
the cases that can get there). This is a debugging patch, but should be
minimal overhead.

The patches can also be found here:

https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=netfs-writeback

CHANGES
=======

ver #5)
- Rebase on top of v6.13-rc3 plus my netfs-fixes branch.
- Fixed the read-result collector changes to make ceph work.

ver #4)
- Fix abandonment of folios that are contributed to by a failed read
subreq (such as from an interrupted op).
- In afs, fix handling of failure between the directory/symlink content
buffer being allocated and reading into it (thereby making it appear
valid).

ver #3)
- In afs, fix a number of issues in asynchronous FetchData handling.
- In afs, fix a signedness issue in an error check dir editing.
- To afs, add a patch to fix handling of interruption by a signal whilst
an async call is being set up.

ver #2)
- Handle that we might be in RCU pathwalk in afs_get_link().
- Fix double-call of afs_put_operation() in async FetchData.
- Don't set ->mapping on directory and symlink folios as they're not in
the pagecache.
- Add an afs patch to search the directory's hash table on lookup.
- Add an afs patch to preset the contents of a new symlink on creation.
- Add an afs patch to add a tracepoint in the async FetchData response
processing.
- Add a patch to report if a NULL folio_queue pointer is seen in
netfs_writeback_unlock_folios().

Thanks,
David

David Howells (32):
netfs: Clean up some whitespace in trace header
cachefiles: Clean up some whitespace in trace header
netfs: Use a folio_queue allocation and free functions
netfs: Add a tracepoint to log the lifespan of folio_queue structs
netfs: Abstract out a rolling folio buffer implementation
netfs: Make netfs_advance_write() return size_t
netfs: Split retry code out of fs/netfs/write_collect.c
netfs: Drop the error arg from netfs_read_subreq_terminated()
netfs: Drop the was_async arg from netfs_read_subreq_terminated()
netfs: Don't use bh spinlock
afs: Don't use mutex for I/O operation lock
afs: Fix EEXIST error returned from afs_rmdir() to be ENOTEMPTY
afs: Fix directory format encoding struct
netfs: Remove some extraneous directory invalidations
cachefiles: Add some subrequest tracepoints
cachefiles: Add auxiliary data trace
afs: Add more tracepoints to do with tracking validity
netfs: Add functions to build/clean a buffer in a folio_queue
netfs: Add support for caching single monolithic objects such as AFS
dirs
afs: Make afs_init_request() get a key if not given a file
afs: Use netfslib for directories
afs: Use netfslib for symlinks, allowing them to be cached
afs: Eliminate afs_read
afs: Fix cleanup of immediately failed async calls
afs: Make {Y,}FS.FetchData an asynchronous operation
Display waited-on page index after 1min of waiting
netfs: Change the read result collector to only use one work item
afs: Make afs_mkdir() locally initialise a new directory's content
afs: Use the contained hashtable to search a directory
afs: Locally initialise the contents of a new symlink on creation
afs: Add a tracepoint for afs_read_receive()
netfs: Report on NULL folioq in netfs_writeback_unlock_folios()

fs/9p/vfs_addr.c | 6 +-
fs/afs/Makefile | 1 +
fs/afs/callback.c | 4 +-
fs/afs/dir.c | 809 +++++++++++++++---------------
fs/afs/dir_edit.c | 383 ++++++++------
fs/afs/dir_search.c | 227 +++++++++
fs/afs/file.c | 260 +++++-----
fs/afs/fs_operation.c | 113 ++++-
fs/afs/fsclient.c | 62 +--
fs/afs/inode.c | 140 +++++-
fs/afs/internal.h | 143 ++++--
fs/afs/main.c | 2 +-
fs/afs/mntpt.c | 22 +-
fs/afs/rotate.c | 4 +-
fs/afs/rxrpc.c | 37 +-
fs/afs/super.c | 4 +-
fs/afs/validation.c | 31 +-
fs/afs/vlclient.c | 1 +
fs/afs/write.c | 16 +-
fs/afs/xdr_fs.h | 2 +-
fs/afs/yfsclient.c | 49 +-
fs/cachefiles/io.c | 4 +
fs/cachefiles/xattr.c | 9 +-
fs/ceph/addr.c | 22 +-
fs/netfs/Makefile | 5 +-
fs/netfs/buffered_read.c | 290 ++++-------
fs/netfs/direct_read.c | 78 +--
fs/netfs/direct_write.c | 10 +-
fs/netfs/internal.h | 41 +-
fs/netfs/main.c | 6 +-
fs/netfs/misc.c | 164 +++---
fs/netfs/objects.c | 21 +-
fs/netfs/read_collect.c | 761 +++++++++++++++++-----------
fs/netfs/read_pgpriv2.c | 207 +++-----
fs/netfs/read_retry.c | 209 ++++----
fs/netfs/read_single.c | 195 +++++++
fs/netfs/rolling_buffer.c | 226 +++++++++
fs/netfs/stats.c | 4 +-
fs/netfs/write_collect.c | 281 ++---------
fs/netfs/write_issue.c | 241 ++++++++-
fs/netfs/write_retry.c | 232 +++++++++
fs/nfs/fscache.c | 6 +-
fs/nfs/fscache.h | 3 +-
fs/smb/client/cifssmb.c | 12 +-
fs/smb/client/file.c | 3 +-
fs/smb/client/smb2ops.c | 2 +-
fs/smb/client/smb2pdu.c | 15 +-
include/linux/folio_queue.h | 12 +-
include/linux/netfs.h | 54 +-
include/linux/rolling_buffer.h | 61 +++
include/trace/events/afs.h | 210 +++++++-
include/trace/events/cachefiles.h | 185 +++----
include/trace/events/netfs.h | 229 ++++-----
lib/kunit_iov_iter.c | 4 +-
mm/filemap.c | 11 +-
55 files changed, 3921 insertions(+), 2208 deletions(-)
create mode 100644 fs/afs/dir_search.c
create mode 100644 fs/netfs/read_single.c
create mode 100644 fs/netfs/rolling_buffer.c
create mode 100644 fs/netfs/write_retry.c
create mode 100644 include/linux/rolling_buffer.h