Re: [PATCH 00/67] fscache: Rewrite index API and management system

Jeff Layton
2021-10-19T18:08:57Z

On Mon, 2021-10-18 at 15:50 +0100, David Howells wrote:
> Here's a set of patches that rewrites and simplifies the fscache index API
> to remove the complex operation scheduling and object state machine in
> favour of something much smaller and simpler. It is built on top of the
> set of patches that removes the old API[1].
>
> The operation scheduling API was intended to handle sequencing of cache
> operations, which were all required (where possible) to run asynchronously
> in parallel with the operations being done by the network filesystem, while
> allowing the cache to be brought online and offline and interrupt service
> with invalidation.
>
> However, with the advent of the tmpfile capacity in the VFS, an opportunity
> arises to do invalidation much more easily, without having to wait for I/O
> that's actually in progress: Cachefiles can simply cut over its file
> pointer for the backing object attached to a cookie and abandon the
> in-progress I/O, dismissing it upon completion.
>
> Future work there would involve using Omar Sandoval's vfs_link() with
> AT_LINK_REPLACE[2] to allow an extant file to be displaced by a new hard
> link from a tmpfile as currently I have to unlink the old file first.
>
> These patches can also simplify the object state handling as I/O operations
> to the cache don't all have to be brought to a stop in order to invalidate
> a file. To that end, and with an eye on to writing a new backing cache
> model in the future, I've taken the opportunity to simplify the indexing
> structure.
>
> I've separated the index cookie concept from the file cookie concept by
> type now. The former is now called a "volume cookie" (struct
> fscache_volume) and there is a container of file cookies. There are then
> just the two levels. All the index cookieage is collapsed into a single
> volume cookie, and this has a single printable string as a key. For
> instance, an AFS volume would have a key of something like
> "afs,example.com,1000555", combining the filesystem name, cell name and
> volume ID. This is freeform, but must not have '/' chars in it.
>

Given the indexing changes, what sort of behavior should we expect when
upgrading from old-style to new-style indexes? Do they just not match,
and we end up downloading new copies of all the data and the old stale
stuff eventually gets culled?

Ditto for downgrades -- can we expect sane behavior if someone runs an
old kernel on top of an existing fscache that was populated by a new
kernel?

> I've also eliminated all pointers back from fscache into the network
> filesystem. This required the duplication of a little bit of data in the
> cookie (cookie key, coherency data and file size), but it's not actually
> that much. This gets rid of problems with making sure we keep netfs data
> structures around so that the cache can access them.
>
> I have changed afs throughout the patch series, but I also have patches for
> 9p, nfs and cifs. Jeff Layton is handling ceph support.
>
>
> BITS THAT MAY BE CONTROVERSIAL
> ==============================
>
> There are some bits I've added that may be controversial:
>
> (1) I've provided a flag, S_KERNEL_FILE, that cachefiles uses to check if
> a files is already being used by some other kernel service (e.g. a
> duplicate cachefiles cache in the same directory) and reject it if it
> is. This isn't entirely necessary, but it helps prevent accidental
> data corruption.
>
> I don't want to use S_SWAPFILE as that has other effects, but quite
> possibly swapon() should set S_KERNEL_FILE too.
>
> Note that it doesn't prevent userspace from interfering, though
> perhaps it should.
>
> (2) Cachefiles wants to keep the backing file for a cookie open whilst we
> might need to write to it from network filesystem writeback. The
> problem is that the network filesystem unuses its cookie when its file
> is closed, and so we have nothing pinning the cachefiles file open and
> it will get closed automatically after a short time to avoid
> EMFILE/ENFILE problems.
>
> Reopening the cache file, however, is a problem if this is being done
> due to writeback triggered by exit(). Some filesystems will oops if
> we try to open a file in that context because they want to access
> current->fs or suchlike.
>
> To get around this, I added the following:
>
> (A) An inode flag, I_PINNING_FSCACHE_WB, to be set on a network
> filesystem inode to indicate that we have a usage count on the
> cookie caching that inode.
>
> (B) A flag in struct writeback_control, unpinned_fscache_wb, that is
> set when __writeback_single_inode() clears the last dirty page
> from i_pages - at which point it clears I_PINNING_FSCACHE_WB and
> sets this flag.
>
> This has to be done here so that clearing I_PINNING_FSCACHE_WB can
> be done atomically with the check of PAGECACHE_TAG_DIRTY that
> clears I_DIRTY_PAGES.
>
> (C) A function, fscache_set_page_dirty(), which if it is not set, sets
> I_PINNING_FSCACHE_WB and calls fscache_use_cookie() to pin the
> cache resources.
>
> (D) A function, fscache_unpin_writeback(), to be called by
> ->write_inode() to unuse the cookie.
>
> (E) A function, fscache_clear_inode_writeback(), to be called when the
> inode is evicted, before clear_inode() is called. This cleans up
> any lingering I_PINNING_FSCACHE_WB.
>
> The network filesystem can then use these tools to make sure that
> fscache_write_to_cache() can write locally modified data to the cache
> as well as to the server.
>
> For the future, I'm working on write helpers for netfs lib that should
> allow this facility to be removed by keeping track of the dirty
> regions separately - but that's incomplete at the moment and is also
> going to be affected by folios, one way or another, since it deals
> with pages.
>
>
> These patches can be found also on:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=fscache-rewrite-indexing
>
> David
>
> Link: https://lore.kernel.org/r/163363935000.1980952.15279841414072653108.stgit@warthog.procyon.org.uk [1]
> Link: https://lore.kernel.org/r/cover.1580251857.git.osandov@fb.com/ [2]
>
> ---
> Dave Wysochanski (3):
> NFS: Convert fscache_acquire_cookie and fscache_relinquish_cookie
> NFS: Convert fscache_enable_cookie and fscache_disable_cookie
> NFS: Convert fscache invalidation and update aux_data and i_size
>
> David Howells (63):
> mm: Stop filemap_read() from grabbing a superfluous page
> vfs: Provide S_KERNEL_FILE inode flag
> vfs, fscache: Force ->write_inode() to occur if cookie pinned for writeback
> afs: Handle len being extending over page end in write_begin/write_end
> afs: Fix afs_write_end() to handle len > page size
> nfs, cifs, ceph, 9p: Disable use of fscache prior to its rewrite
> fscache: Remove the netfs data from the cookie
> fscache: Remove struct fscache_cookie_def
> fscache: Remove store_limit* from struct fscache_object
> fscache: Remove fscache_check_consistency()
> fscache: Remove fscache_attr_changed()
> fscache: Remove obsolete stats
> fscache: Remove old I/O tracepoints
> fscache: Temporarily disable fscache_invalidate()
> fscache: Disable fscache_begin_operation()
> fscache: Remove the I/O operation manager
> fscache: Rename fscache_cookie_{get,put,see}()
> cachefiles: Remove tree of active files and use S_CACHE_FILE inode flag
> cachefiles: Don't set an xattr on the root of the cache
> cachefiles: Remove some redundant checks on unsigned values
> cachefiles: Prevent inode from going away when burying a dentry
> cachefiles: Simplify the pathwalk and save the filename for an object
> cachefiles: trace: Improve the lookup tracepoint
> cachefiles: Remove separate backer dentry from cachefiles_object
> cachefiles: Fold fscache_object into cachefiles_object
> cachefiles: Change to storing file* rather than dentry*
> cachefiles: trace: Log coherency checks
> cachefiles: Trace truncations
> cachefiles: Trace read and write operations
> cachefiles: Round the cachefile size up to DIO block size
> cachefiles: Don't use XATTR_ flags with vfs_setxattr()
> fscache: Replace the object management state machine
> cachefiles: Trace decisions in cachefiles_prepare_read()
> cachefiles: Make cachefiles_write_prepare() check for space
> fscache: Automatically close a file that's been unused for a while
> fscache: Add stats for the cookie commit LRU
> fscache: Move fscache_update_cookie() complete inline
> fscache: Remove more obsolete stats
> fscache: Note the object size during invalidation
> vfs, fscache: Force ->write_inode() to occur if cookie pinned for writeback
> afs: Render cache cookie key as big endian
> cachefiles: Use tmpfile/link
> fscache: Rewrite invalidation
> cachefiles: Simplify the file lookup/creation/check code
> fscache: Provide resize operation
> cachefiles: Put more information in the xattr attached to the cache file
> fscache: Implement "will_modify" parameter on fscache_use_cookie()
> fscache: Add support for writing to the cache
> fscache: Make fscache_clear_page_bits() conditional on cookie
> fscache: Make fscache_write_to_cache() conditional on cookie
> afs: Copy local writes to the cache when writing to the server
> afs: Invoke fscache_resize_cookie() when handling ATTR_SIZE for setattr
> afs: Add O_DIRECT read support
> afs: Skip truncation on the server of data we haven't written yet
> afs: Make afs_write_begin() return the THP subpage
> cachefiles, afs: Drive FSCACHE_COOKIE_NO_DATA_TO_READ
> nfs: Convert to new fscache volume/cookie API
> 9p: Use fscache indexing rewrite and reenable caching
> 9p: Copy local writes to the cache when writing to the server
> netfs: Display the netfs inode number in the netfs_read tracepoint
> cachefiles: Add tracepoints to log errors from ops on the backing fs
> cachefiles: Add error injection support
> cifs: Support fscache indexing rewrite (untested)
>
> Jeff Layton (1):
> fscache: disable cookie when doing an invalidation for DIO write
>
>
> fs/9p/cache.c | 184 +----
> fs/9p/cache.h | 23 +-
> fs/9p/v9fs.c | 14 +-
> fs/9p/v9fs.h | 13 +-
> fs/9p/vfs_addr.c | 55 +-
> fs/9p/vfs_dir.c | 13 +-
> fs/9p/vfs_file.c | 7 +-
> fs/9p/vfs_inode.c | 24 +-
> fs/9p/vfs_inode_dotl.c | 3 +-
> fs/9p/vfs_super.c | 3 +
> fs/afs/Makefile | 3 -
> fs/afs/cache.c | 68 --
> fs/afs/cell.c | 12 -
> fs/afs/file.c | 83 +-
> fs/afs/fsclient.c | 18 +-
> fs/afs/inode.c | 101 ++-
> fs/afs/internal.h | 36 +-
> fs/afs/main.c | 14 -
> fs/afs/super.c | 1 +
> fs/afs/volume.c | 15 +-
> fs/afs/write.c | 170 +++-
> fs/afs/yfsclient.c | 12 +-
> fs/cachefiles/Kconfig | 8 +
> fs/cachefiles/Makefile | 3 +
> fs/cachefiles/bind.c | 186 +++--
> fs/cachefiles/daemon.c | 20 +-
> fs/cachefiles/error_inject.c | 46 ++
> fs/cachefiles/interface.c | 660 +++++++--------
> fs/cachefiles/internal.h | 191 +++--
> fs/cachefiles/io.c | 310 +++++--
> fs/cachefiles/key.c | 203 +++--
> fs/cachefiles/main.c | 20 +-
> fs/cachefiles/namei.c | 978 ++++++++++------------
> fs/cachefiles/volume.c | 128 +++
> fs/cachefiles/xattr.c | 367 +++------
> fs/ceph/Kconfig | 2 +-
> fs/cifs/Makefile | 2 +-
> fs/cifs/cache.c | 105 ---
> fs/cifs/cifsfs.c | 11 +-
> fs/cifs/cifsglob.h | 5 +-
> fs/cifs/connect.c | 3 -
> fs/cifs/file.c | 37 +-
> fs/cifs/fscache.c | 201 ++---
> fs/cifs/fscache.h | 51 +-
> fs/cifs/inode.c | 18 +-
> fs/fs-writeback.c | 8 +
> fs/fscache/Kconfig | 4 +
> fs/fscache/Makefile | 6 +-
> fs/fscache/cache.c | 541 ++++++-------
> fs/fscache/cookie.c | 1262 ++++++++++++++---------------
> fs/fscache/fsdef.c | 98 ---
> fs/fscache/internal.h | 213 +----
> fs/fscache/io.c | 405 ++++++---
> fs/fscache/main.c | 134 +--
> fs/fscache/netfs.c | 74 --
> fs/fscache/object.c | 1123 -------------------------
> fs/fscache/operation.c | 633 ---------------
> fs/fscache/page.c | 84 --
> fs/fscache/proc.c | 43 +-
> fs/fscache/stats.c | 202 ++---
> fs/fscache/volume.c | 449 ++++++++++
> fs/netfs/read_helper.c | 2 +-
> fs/nfs/Makefile | 2 +-
> fs/nfs/client.c | 4 -
> fs/nfs/direct.c | 2 +
> fs/nfs/file.c | 7 +-
> fs/nfs/fscache-index.c | 114 ---
> fs/nfs/fscache.c | 264 ++----
> fs/nfs/fscache.h | 89 +-
> fs/nfs/inode.c | 11 +-
> fs/nfs/super.c | 7 +-
> fs/nfs/write.c | 1 +
> include/linux/fs.h | 4 +
> include/linux/fscache-cache.h | 463 +++--------
> include/linux/fscache.h | 626 +++++++-------
> include/linux/netfs.h | 4 +-
> include/linux/nfs_fs_sb.h | 9 +-
> include/linux/writeback.h | 1 +
> include/trace/events/cachefiles.h | 483 ++++++++---
> include/trace/events/fscache.h | 631 +++++++--------
> include/trace/events/netfs.h | 5 +-
> 81 files changed, 5140 insertions(+), 7295 deletions(-)
> delete mode 100644 fs/afs/cache.c
> create mode 100644 fs/cachefiles/error_inject.c
> create mode 100644 fs/cachefiles/volume.c
> delete mode 100644 fs/cifs/cache.c
> delete mode 100644 fs/fscache/fsdef.c
> delete mode 100644 fs/fscache/netfs.c
> delete mode 100644 fs/fscache/object.c
> delete mode 100644 fs/fscache/operation.c
> create mode 100644 fs/fscache/volume.c
> delete mode 100644 fs/nfs/fscache-index.c
>
>

--
Jeff Layton <jlayton@kernel.org>