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

[Jeff Layton]

On Tue, 2021-09-14 at 14:54 +0100, David Howells 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@xxxxxxx [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
> 
> 

Love the diffstat.

I looked over the set and didn't see any problems with it. I also ran a
quick test with ceph+fscache and didn't run into any issues, but that's
not too surprising since this patchset doesn't really change the
codepaths that it uses.

Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>