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Date: Tue, 14 Sep 2021 11:30:29 -0400
From: David Wysochanski <dwysocha@...hat.com>
To: David Howells <dhowells@...hat.com>
Cc: Trond Myklebust <trondmy@...merspace.com>,
Anna Schumaker <anna.schumaker@...app.com>,
Steve French <sfrench@...ba.org>,
Dominique Martinet <asmadeus@...ewreck.org>,
linux-nfs <linux-nfs@...r.kernel.org>,
v9fs-developer@...ts.sourceforge.net,
Trond Myklebust <trond.myklebust@...merspace.com>,
linux-cachefs <linux-cachefs@...hat.com>,
linux-cifs <linux-cifs@...r.kernel.org>,
Shyam Prasad N <nspmangalore@...il.com>,
Jeff Layton <jlayton@...hat.com>,
Matthew Wilcox <willy@...radead.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
Linus Torvalds <torvalds@...ux-foundation.org>,
linux-afs@...ts.infradead.org, ceph-devel@...r.kernel.org,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 0/8] fscache: Replace and remove old I/O API
On Tue, Sep 14, 2021 at 9:55 AM David Howells <dhowells@...hat.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.
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