[PATCH v4 00/39] netfs, afs, 9p: Delegate high-level I/O to netfslib

David Howells
2023-12-13T15:24:01Z

Hi Jeff, Steve, Dominique,

I have been working on my netfslib helpers to the point that I can run
xfstests on AFS to completion (both with write-back buffering and, with a
small patch, write-through buffering in the pagecache). I have a patch for
9P, but am currently unable to test it.

The patches remove a little over 800 lines from AFS, 300 from 9P, albeit with
around 3000 lines added to netfs. Hopefully, I will be able to remove a bunch
of lines from Ceph too.

I've split the CIFS patches out to a separate branch, cifs-netfs, where a
further 2000+ lines are removed. I can run a certain amount of xfstests on
CIFS, though I'm running into ksmbd issues and not all the tests work
correctly because of issues between fallocate and what the SMB protocol
actually supports.

I've also dropped the content-crypto patches out for the moment as they're
only usable by the ceph changes which I'm still working on.

The patch to use PG_writeback instead of PG_fscache for writing to the
cache has also been deferred, pending 9p, afs, ceph and cifs all being
converted.

The main aims of these patches are to get high-level I/O and knowledge of
the pagecache out of the filesystem drivers as much as possible and to get
rid, as much of possible, of the knowledge that pages/folios exist.

Further, I would like to see ->write_begin, ->write_end and ->launder_folio
go away.

Features that are added by these patches to that which is already there in
netfslib:

(1) NFS-style (and Ceph-style) locking around DIO vs buffered I/O calls to
prevent these from happening at the same time. mmap'd I/O can, of
necessity, happen at any time ignoring these locks.

(2) Support for unbuffered I/O. The data is kept in the bounce buffer and
the pagecache is not used. This can be turned on with an inode flag.

(3) Support for direct I/O. This is basically unbuffered I/O with some
extra restrictions and no RMW.

(4) Support for using a bounce buffer in an operation. The bounce buffer
may be bigger than the target data/buffer, allowing for crypto
rounding.

(5) ->write_begin() and ->write_end() are ignored in favour of merging all
of that into one function, netfs_perform_write(), thereby avoiding the
function pointer traversals.

(6) Support for write-through caching in the pagecache.
netfs_perform_write() adds the pages is modifies to an I/O operation
as it goes and directly marks them writeback rather than dirty. When
writing back from write-through, it limits the range written back.
This should allow CIFS to deal with byte-range mandatory locks
correctly.

(7) O_*SYNC and RWF_*SYNC writes use write-through rather than writing to
the pagecache and then flushing afterwards. An AIO O_*SYNC write will
notify of completion when the sub-writes all complete.

(8) Support for write-streaming where modifed data is held in !uptodate
folios, with a private struct attached indicating the range that is
valid.

(9) Support for write grouping, multiplexing a pointer to a group in the
folio private data with the write-streaming data. The writepages
algorithm only writes stuff back that's in the nominated group. This
is intended for use by Ceph to write is snaps in order.

(10) Skipping reads for which we know the server could only supply zeros or
EOF (for instance if we've done a local write that leaves a hole in
the file and extends the local inode size).

General notes:

(1) The fscache module is merged into the netfslib module to avoid cyclic
exported symbol usage that prevents either module from being loaded.

(2) Some helpers from fscache are reassigned to netfslib by name.

(3) netfslib now makes use of folio->private, which means the filesystem
can't use it.

(4) The filesystem provides wrappers to call the write helpers, allowing
it to do pre-validation, oplock/capability fetching and the passing in
of write group info.

(5) I want to try flushing the data when tearing down an inode before
invalidating it to try and render launder_folio unnecessary.

(6) Write-through caching will generate and dispatch write subrequests as
it gathers enough data to hit wsize and has whole pages that at least
span that size. This needs to be a bit more flexible, allowing for a
filesystem such as CIFS to have a variable wsize.

(7) The filesystem driver is just given read and write calls with an
iov_iter describing the data/buffer to use. Ideally, they don't see
pages or folios at all. A function, extract_iter_to_sg(), is already
available to decant part of an iterator into a scatterlist for crypto
purposes.

9P notes:

(1) I haven't managed to test this as I haven't been able to get Ganesha
to work correctly with 9P.

(2) Writes should now occur in larger-than-page-sized chunks.

(3) It should be possible to turn on multipage folio support in 9P now.

Changes
=======
ver #4)
- Slimmed down the branch:
- Split the cifs-related patches off to a separate branch (cifs-netfs)
- Deferred the content-encryption to the in-progress ceph changes.
- Deferred the use-PG_writeback rather than PG_fscache patch
- Rebased on a later linux-next with afs-rotation patches.

ver #3)
- Moved the fscache module into netfslib to avoid export cycles.
- Fixed a bunch of bugs.
- Got CIFS to pass as much of xfstests as possible.
- Added a patch to make 9P use all the helpers.
- Added a patch to stop using PG_fscache, but rather dirty pages on
reading and have writepages write to the cache.

ver #2)
- Folded the addition of NETFS_RREQ_NONBLOCK/BLOCKED into first patch that
uses them.
- Folded addition of rsize member into first user.
- Don't set rsize in ceph (yet) and set it in kafs to 256KiB. cifs sets
it dynamically.
- Moved direct_bv next to direct_bv_count in struct netfs_io_request and
labelled it with a __counted_by().
- Passed flags into netfs_xa_store_and_mark() rather than two bools.
- Removed netfs_set_up_buffer() as it wasn't used.

David

Link: https://lore.kernel.org/r/20231013160423.2218093-1-dhowells@redhat.com/ # v1
Link: https://lore.kernel.org/r/20231117211544.1740466-1-dhowells@redhat.com/ # v2

David Howells (39):
netfs, fscache: Move fs/fscache/* into fs/netfs/
netfs, fscache: Combine fscache with netfs
netfs, fscache: Remove ->begin_cache_operation
netfs, fscache: Move /proc/fs/fscache to /proc/fs/netfs and put in a
symlink
netfs: Move pinning-for-writeback from fscache to netfs
netfs: Add a procfile to list in-progress requests
netfs: Allow the netfs to make the io (sub)request alloc larger
netfs: Add a ->free_subrequest() op
afs: Don't use folio->private to record partial modification
netfs: Provide invalidate_folio and release_folio calls
netfs: Implement unbuffered/DIO vs buffered I/O locking
netfs: Add iov_iters to (sub)requests to describe various buffers
netfs: Add support for DIO buffering
netfs: Provide tools to create a buffer in an xarray
netfs: Add bounce buffering support
netfs: Add func to calculate pagecount/size-limited span of an
iterator
netfs: Limit subrequest by size or number of segments
netfs: Export netfs_put_subrequest() and some tracepoints
netfs: Extend the netfs_io_*request structs to handle writes
netfs: Add a hook to allow tell the netfs to update its i_size
netfs: Make netfs_put_request() handle a NULL pointer
netfs: Make the refcounting of netfs_begin_read() easier to use
netfs: Prep to use folio->private for write grouping and streaming
write
netfs: Dispatch write requests to process a writeback slice
netfs: Provide func to copy data to pagecache for buffered write
netfs: Make netfs_read_folio() handle streaming-write pages
netfs: Allocate multipage folios in the writepath
netfs: Implement support for unbuffered/DIO read
netfs: Implement unbuffered/DIO write support
netfs: Implement buffered write API
netfs: Allow buffered shared-writeable mmap through
netfs_page_mkwrite()
netfs: Provide netfs_file_read_iter()
netfs, cachefiles: Pass upper bound length to allow expansion
netfs: Provide a writepages implementation
netfs: Provide a launder_folio implementation
netfs: Implement a write-through caching option
netfs: Optimise away reads above the point at which there can be no
data
afs: Use the netfs write helpers
9p: Use netfslib read/write_iter

Documentation/filesystems/netfs_library.rst | 23 +-
MAINTAINERS | 2 +-
fs/9p/vfs_addr.c | 352 +----
fs/9p/vfs_file.c | 89 +-
fs/9p/vfs_inode.c | 5 +-
fs/9p/vfs_super.c | 14 +-
fs/Kconfig | 1 -
fs/Makefile | 1 -
fs/afs/file.c | 213 +--
fs/afs/inode.c | 26 +-
fs/afs/internal.h | 72 +-
fs/afs/super.c | 2 +-
fs/afs/write.c | 826 +----------
fs/cachefiles/internal.h | 2 +-
fs/cachefiles/io.c | 10 +-
fs/cachefiles/ondemand.c | 2 +-
fs/ceph/addr.c | 25 +-
fs/ceph/cache.h | 35 +-
fs/ceph/inode.c | 2 +-
fs/fs-writeback.c | 10 +-
fs/fscache/Kconfig | 40 -
fs/fscache/Makefile | 16 -
fs/fscache/internal.h | 277 ----
fs/netfs/Kconfig | 39 +
fs/netfs/Makefile | 22 +-
fs/netfs/buffered_read.c | 229 ++-
fs/netfs/buffered_write.c | 1247 +++++++++++++++++
fs/netfs/direct_read.c | 252 ++++
fs/netfs/direct_write.c | 170 +++
fs/{fscache/cache.c => netfs/fscache_cache.c} | 0
.../cookie.c => netfs/fscache_cookie.c} | 0
fs/netfs/fscache_internal.h | 14 +
fs/{fscache/io.c => netfs/fscache_io.c} | 42 +-
fs/{fscache/main.c => netfs/fscache_main.c} | 25 +-
fs/{fscache/proc.c => netfs/fscache_proc.c} | 23 +-
fs/{fscache/stats.c => netfs/fscache_stats.c} | 4 +-
.../volume.c => netfs/fscache_volume.c} | 0
fs/netfs/internal.h | 288 ++++
fs/netfs/io.c | 214 ++-
fs/netfs/iterator.c | 97 ++
fs/netfs/locking.c | 215 +++
fs/netfs/main.c | 110 ++
fs/netfs/misc.c | 260 ++++
fs/netfs/objects.c | 63 +-
fs/netfs/output.c | 478 +++++++
fs/netfs/stats.c | 31 +-
fs/nfs/Kconfig | 4 +-
fs/nfs/fscache.c | 7 -
fs/smb/client/cifsfs.c | 9 +-
fs/smb/client/file.c | 18 +-
fs/smb/client/fscache.c | 2 +-
include/linux/fs.h | 2 +-
include/linux/fscache.h | 45 -
include/linux/netfs.h | 176 ++-
include/linux/writeback.h | 2 +-
include/trace/events/afs.h | 31 -
include/trace/events/netfs.h | 155 +-
mm/filemap.c | 1 +
58 files changed, 4197 insertions(+), 2123 deletions(-)
delete mode 100644 fs/fscache/Kconfig
delete mode 100644 fs/fscache/Makefile
delete mode 100644 fs/fscache/internal.h
create mode 100644 fs/netfs/buffered_write.c
create mode 100644 fs/netfs/direct_read.c
create mode 100644 fs/netfs/direct_write.c
rename fs/{fscache/cache.c => netfs/fscache_cache.c} (100%)
rename fs/{fscache/cookie.c => netfs/fscache_cookie.c} (100%)
create mode 100644 fs/netfs/fscache_internal.h
rename fs/{fscache/io.c => netfs/fscache_io.c} (86%)
rename fs/{fscache/main.c => netfs/fscache_main.c} (84%)
rename fs/{fscache/proc.c => netfs/fscache_proc.c} (58%)
rename fs/{fscache/stats.c => netfs/fscache_stats.c} (97%)
rename fs/{fscache/volume.c => netfs/fscache_volume.c} (100%)
create mode 100644 fs/netfs/locking.c
create mode 100644 fs/netfs/misc.c
create mode 100644 fs/netfs/output.c