Re: [RFC][PATCH 00/25] Network fs helper library & fscache kiocb API

[J. Bruce Fields]

On Wed, Jan 20, 2021 at 10:21:24PM +0000, David Howells wrote:
>      Note that this uses SEEK_HOLE/SEEK_DATA to locate the data available
>      to be read from the cache.  Whilst this is an improvement from the
>      bmap interface, it still has a problem with regard to a modern
>      extent-based filesystem inserting or removing bridging blocks of
>      zeros.

What are the consequences from the point of view of a user?

--b.

> 
> This is a step towards overhauling the fscache API.  The change is opt-in
> on the part of the network filesystem.  A netfs should not try to mix the
> old and the new API because of conflicting ways of handling pages and the
> PG_fscache page flag and because it would be mixing DIO with buffered I/O.
> Further, the helper library can't be used with the old API.
> 
> This does not change any of the fscache cookie handling APIs or the way
> invalidation is done.
> 
> In the near term, I intend to deprecate and remove the old I/O API
> (fscache_allocate_page{,s}(), fscache_read_or_alloc_page{,s}(),
> fscache_write_page() and fscache_uncache_page()) and eventually replace
> most of fscache/cachefiles with something simpler and easier to follow.
> 
> The patchset contains four parts:
> 
>  (1) Some helper patches, including provision of an ITER_XARRAY iov
>      iterator and a function to do readahead expansion.
> 
>  (2) Patches to add the netfs helper library.
> 
>  (3) A patch to add the fscache/cachefiles kiocb API
> 
>  (4) Patches to add support in AFS for this.
> 
> With this, AFS without a cache passes all expected xfstests; with a cache,
> there's an extra failure, but that's also there before these patches.
> Fixing that probably requires a greater overhaul.
> 
> These patches can be found also on:
> 
> 	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=fscache-netfs-lib
> 
> David
> ---
> David Howells (24):
>       iov_iter: Add ITER_XARRAY
>       vm: Add wait/unlock functions for PG_fscache
>       mm: Implement readahead_control pageset expansion
>       vfs: Export rw_verify_area() for use by cachefiles
>       netfs: Make a netfs helper module
>       netfs: Provide readahead and readpage netfs helpers
>       netfs: Add tracepoints
>       netfs: Gather stats
>       netfs: Add write_begin helper
>       netfs: Define an interface to talk to a cache
>       fscache, cachefiles: Add alternate API to use kiocb for read/write to cache
>       afs: Disable use of the fscache I/O routines
>       afs: Pass page into dirty region helpers to provide THP size
>       afs: Print the operation debug_id when logging an unexpected data version
>       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_XARRAY for writing
>       afs: Wait on PG_fscache before modifying/releasing a page
>       afs: Extract writeback extension into its own function
>       afs: Prepare for use of THPs
>       afs: Use the fs operation ops to handle FetchData completion
>       afs: Use new fscache read helper API
> 
> Takashi Iwai (1):
>       cachefiles: Drop superfluous readpages aops NULL check
> 
> 
>  fs/Kconfig                    |    1 +
>  fs/Makefile                   |    1 +
>  fs/afs/Kconfig                |    1 +
>  fs/afs/dir.c                  |  225 ++++---
>  fs/afs/file.c                 |  472 ++++----------
>  fs/afs/fs_operation.c         |    4 +-
>  fs/afs/fsclient.c             |  108 ++--
>  fs/afs/inode.c                |    7 +-
>  fs/afs/internal.h             |   57 +-
>  fs/afs/rxrpc.c                |  150 ++---
>  fs/afs/write.c                |  610 ++++++++++--------
>  fs/afs/yfsclient.c            |   82 +--
>  fs/cachefiles/Makefile        |    1 +
>  fs/cachefiles/interface.c     |    5 +-
>  fs/cachefiles/internal.h      |    9 +
>  fs/cachefiles/rdwr.c          |    2 -
>  fs/cachefiles/rdwr2.c         |  406 ++++++++++++
>  fs/fscache/Makefile           |    3 +-
>  fs/fscache/internal.h         |    3 +
>  fs/fscache/page.c             |    2 +-
>  fs/fscache/page2.c            |  116 ++++
>  fs/fscache/stats.c            |    1 +
>  fs/internal.h                 |    5 -
>  fs/netfs/Kconfig              |   23 +
>  fs/netfs/Makefile             |    5 +
>  fs/netfs/internal.h           |   97 +++
>  fs/netfs/read_helper.c        | 1142 +++++++++++++++++++++++++++++++++
>  fs/netfs/stats.c              |   57 ++
>  fs/read_write.c               |    1 +
>  include/linux/fs.h            |    1 +
>  include/linux/fscache-cache.h |    4 +
>  include/linux/fscache.h       |   28 +-
>  include/linux/netfs.h         |  167 +++++
>  include/linux/pagemap.h       |   16 +
>  include/net/af_rxrpc.h        |    2 +-
>  include/trace/events/afs.h    |   74 +--
>  include/trace/events/netfs.h  |  201 ++++++
>  mm/filemap.c                  |   18 +
>  mm/readahead.c                |   70 ++
>  net/rxrpc/recvmsg.c           |    9 +-
>  40 files changed, 3171 insertions(+), 1015 deletions(-)
>  create mode 100644 fs/cachefiles/rdwr2.c
>  create mode 100644 fs/fscache/page2.c
>  create mode 100644 fs/netfs/Kconfig
>  create mode 100644 fs/netfs/Makefile
>  create mode 100644 fs/netfs/internal.h
>  create mode 100644 fs/netfs/read_helper.c
>  create mode 100644 fs/netfs/stats.c
>  create mode 100644 include/linux/netfs.h
>  create mode 100644 include/trace/events/netfs.h
>