Here's a set of patches that removes the old fscache I/O API by the following
means:
(1) A simple fallback API is added that can read or write a single page
synchronously. The functions for this have "fallback" in their names
as they have to be removed at some point.
(2) An implementation of this is provided in cachefiles. It creates a kiocb
to use DIO to the backing file rather than calling readpage on the
backing filesystem page and then snooping the page wait queue.
(3) NFS is switched to use the fallback API.
(4) CIFS is switched to use the fallback API also for the moment.
(5) 9P is switched to using netfslib.
(6) The old I/O API is removed from fscache and the page snooping
implementation is removed from cachefiles.
The reasons for doing this are:
(A) Using a kiocb to do asynchronous DIO from/to the pages of the backing
file is now a possibility that didn't exist when cachefiles was created.
This is much simpler than the snooping mechanism with a proper callback
path and it also requires fewer copies and less memory.
(B) We have to stop using bmap() or SEEK_DATA/SEEK_HOLE to work out what
blocks are present in the backing file is dangerous and can lead to data
corruption if the backing filesystem can insert or remove blocks of zeros
arbitrarily in order to optimise its extent list[1].
Whilst this patchset doesn't fix that yet, it does simplify the code and
the fix for that can be made in a subsequent patchset.
(C) In order to fix (B), the cache will need to keep track itself of what
data is present. To make this easier to manage, the intention is to
increase the cache block granularity to, say, 256KiB - importantly, a
size that will span multiple pages - which means the single-page
interface will have to go away. netfslib is designed to deal with
that on behalf of a filesystem, though a filesystem could use raw
cache calls instead and manage things itself.
These patches can be found also on:
https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=fscache-remove-old-io
David
Changes
=======
ver #3:
- Added a patch to make fscache_cookie_enabled() handle a NULL cookie.
- Added a patch to make cachefiles_prepare_read() indicate that the
caller should fill a block with zeros if it's beyond EOF rather than
risking the read get marked invalid and trigger a warning.
- Merged fixes from Dave Wysochanski for the NFS patch[4].
ver #2:
- Changed "deprecated" to "fallback" in the new function names[2].
- Cleaned up some kernel test robot warnings[3].
- Made the netfs read helpers use NETFS_READ_HOLE_* flags.
References
==========
Link: https://lore.kernel.org/r/YO17ZNOcq+9PajfQ@xxxxxxx [1]
Link: https://lore.kernel.org/r/CAHk-=wiVK+1CyEjW8u71zVPK8msea=qPpznX35gnX+s8sXnJTg@xxxxxxxxxxxxxx/ [2]
Link: https://lore.kernel.org/r/202109150420.QX7dDzSE-lkp@xxxxxxxxx/ [3]
Link: https://listman.redhat.com/archives/linux-cachefs/2021-October/msg00011.html [4]
Older postings
==============
Link: https://lore.kernel.org/r/163162767601.438332.9017034724960075707.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # rfc v1
Link: https://lore.kernel.org/r/163189104510.2509237.10805032055807259087.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # rfc v2
Note that some of this was seen in previous patchsets too:
# [RFC PATCH 00/61] fscache, cachefiles: Rewrite the I/O interface in terms of kiocb/iov_iter
Link: https://lore.kernel.org/r/158861203563.340223.7585359869938129395.stgit@xxxxxxxxxxxxxxxxxxxxxx/
# [PATCH 00/14] fscache: Rewrite 1: Disable and clean in preparation for rewrite
Link: https://lore.kernel.org/r/159465766378.1376105.11619976251039287525.stgit@xxxxxxxxxxxxxxxxxxxxxx/
# [RFC PATCH 00/76] fscache: Modernisation
Link: https://lore.kernel.org/r/160588455242.3465195.3214733858273019178.stgit@xxxxxxxxxxxxxxxxxxxxxx/
---
David Howells (10):
fscache: Generalise the ->begin_read_operation method
fscache: Fix fscache_cookie_enabled() to handle NULL cookie
cachefiles: Always indicate we should fill a post-EOF page with zeros
fscache: Implement a fallback I/O interface to replace the old API
nfs: Move to using the alternate fallback fscache I/O API
9p: Convert to using the netfs helper lib to do reads and caching
cifs: (untested) Move to using the alternate fallback fscache I/O API
fscache: Remove the old I/O API
fscache: Remove stats that are no longer used
fscache: Update the documentation to reflect I/O API changes
.../filesystems/caching/backend-api.rst | 138 +--
.../filesystems/caching/netfs-api.rst | 385 +-----
fs/9p/Kconfig | 1 +
fs/9p/cache.c | 137 ---
fs/9p/cache.h | 98 +-
fs/9p/v9fs.h | 9 +
fs/9p/vfs_addr.c | 195 ++-
fs/9p/vfs_file.c | 21 +-
fs/cachefiles/Makefile | 1 -
fs/cachefiles/interface.c | 15 -
fs/cachefiles/internal.h | 38 -
fs/cachefiles/io.c | 57 +-
fs/cachefiles/main.c | 1 -
fs/cachefiles/rdwr.c | 972 ---------------
fs/cifs/file.c | 64 +-
fs/cifs/fscache.c | 105 +-
fs/cifs/fscache.h | 74 +-
fs/fscache/cache.c | 6 -
fs/fscache/cookie.c | 10 -
fs/fscache/internal.h | 58 +-
fs/fscache/io.c | 137 ++-
fs/fscache/object.c | 2 -
fs/fscache/page.c | 1066 -----------------
fs/fscache/stats.c | 73 +-
fs/netfs/read_helper.c | 8 +-
fs/nfs/file.c | 14 +-
fs/nfs/fscache-index.c | 26 -
fs/nfs/fscache.c | 170 +--
fs/nfs/fscache.h | 84 +-
fs/nfs/read.c | 25 +-
fs/nfs/write.c | 7 +-
include/linux/fscache-cache.h | 131 --
include/linux/fscache.h | 445 ++-----
include/linux/netfs.h | 17 +-
34 files changed, 568 insertions(+), 4022 deletions(-)
delete mode 100644 fs/cachefiles/rdwr.c
