Re: [RFC PATCH 0/8] fscache: Replace and remove old I/O API

[David Wysochanski <dwysocha@redhat.com>]

On Tue, Sep 14, 2021 at 11:30 AM David Wysochanski <dwysocha@redhat.com> wrote:
>
> On Tue, Sep 14, 2021 at 9:55 AM David Howells <dhowells@redhat.com> wrote:
> >
> >
> > 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 "deprecated" 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-iter-3
> >
> > David
> >
> > Link: https://lore.kernel.org/r/YO17ZNOcq+9PajfQ@mit.edu [1]
> > ---
> > David Howells (8):
> >       fscache: Generalise the ->begin_read_operation method
> >       fscache: Implement an alternate I/O interface to replace the old API
> >       nfs: Move to using the alternate (deprecated) fscache I/O API
> >       9p: (untested) Convert to using the netfs helper lib to do reads and caching
> >       cifs: (untested) Move to using the alternate (deprecated) 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         |  386 +-----
> >  fs/9p/Kconfig                                 |    1 +
> >  fs/9p/cache.c                                 |  137 ---
> >  fs/9p/cache.h                                 |   98 +-
> >  fs/9p/v9fs.h                                  |    9 +
> >  fs/9p/vfs_addr.c                              |  174 ++-
> >  fs/9p/vfs_file.c                              |   21 +-
> >  fs/cachefiles/Makefile                        |    1 -
> >  fs/cachefiles/interface.c                     |   15 -
> >  fs/cachefiles/internal.h                      |   38 -
> >  fs/cachefiles/io.c                            |   28 +-
> >  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                               |  140 ++-
> >  fs/fscache/object.c                           |    2 -
> >  fs/fscache/page.c                             | 1066 -----------------
> >  fs/fscache/stats.c                            |   73 +-
> >  fs/nfs/file.c                                 |   14 +-
> >  fs/nfs/fscache-index.c                        |   26 -
> >  fs/nfs/fscache.c                              |  161 +--
> >  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                       |  418 ++-----
> >  include/linux/netfs.h                         |   17 +-
> >  33 files changed, 508 insertions(+), 3992 deletions(-)
> >  delete mode 100644 fs/cachefiles/rdwr.c
> >
> >
>
> I tested an earlier version of these with NFS, which identified a
> couple issues which you fixed.  Last I checked my unit tests and
> xfstests were looking good. I'll do some testing on this latest branch
> / patches and report back.

For the series, you can add
Tested-by: Dave Wysochanski <dwysocha@redhat.com>

Testing was limited to NFS enabled code paths.  I ran custom unit
tests, as well as a series of xfstest generic runs with various NFS
versions, both fscache enabled and not enabled, as well as various NFS
servers, comparing 5.15.0-rc1 runs vs runs with these patches.  I did
not see any failures with these new patches that were not already
present with 5.15.0-rc1.

Here are the list of xfstest generic runs:
1. Hammerspace (pNFS flexfiles) version 4.6.3-166: vers=4.1,fsc
2. Hammerspace (pNFS flexfiles) version 4.6.3-166: vers=4.2
3. Netapp (pNFS filelayout) version 9.5RC1: vers=4.1
4. Netapp (pNFS filelayout) version 9.5RC1: vers=4.1,fsc
5. Red Hat version 8.2 (kernel-4.18.0-193.el8): vers=4.2,fsc
6. Red Hat version 8.2 (kernel-4.18.0-193.el8): vers=4.0,fsc
7. Red Hat version 8.2 (kernel-4.18.0-193.el8): vers=3,fsc