[PATCH v7 00/31] Network fs helper library & fscache kiocb API

David Howells
2021-04-23T13:28:06Z

Here's a set of patches to do two things:

(1) Add a helper library to handle the new VM readahead interface. This
is intended to be used unconditionally by the filesystem (whether or
not caching is enabled) and provides a common framework for doing
caching, transparent huge pages and, in the future, possibly fscrypt
and read bandwidth maximisation. It also allows the netfs and the
cache to align, expand and slice up a read request from the VM in
various ways; the netfs need only provide a function to read a stretch
of data to the pagecache and the helper takes care of the rest.

(2) Add an alternative fscache/cachfiles I/O API that uses the kiocb
facility to do async DIO to transfer data to/from the netfs's pages,
rather than using readpage with wait queue snooping on one side and
vfs_write() on the other. It also uses less memory, since it doesn't
do buffered I/O on the backing file.

Note that this uses SEEK_HOLE/SEEK_DATA to locate the data available
to be read from the cache. Whilst this is an improvement from the
bmap interface, it still has a problem with regard to a modern
extent-based filesystem inserting or removing bridging blocks of
zeros. Fixing that requires a much greater overhaul.

This is a step towards overhauling the fscache API. The change is opt-in
on the part of the network filesystem. A netfs should not try to mix the
old and the new API because of conflicting ways of handling pages and the
PG_fscache page flag and because it would be mixing DIO with buffered I/O.
Further, the helper library can't be used with the old API.

This does not change any of the fscache cookie handling APIs or the way
invalidation is done.

In the near term, I intend to deprecate and remove the old I/O API
(fscache_allocate_page{,s}(), fscache_read_or_alloc_page{,s}(),
fscache_write_page() and fscache_uncache_page()) and eventually replace
most of fscache/cachefiles with something simpler and easier to follow.

The patchset contains the following parts:

(1) Some helper patches, including provision of an ITER_XARRAY iov
iterator and a function to do readahead expansion.

(2) Patches to add the netfs helper library.

(3) A patch to add the fscache/cachefiles kiocb API.

(4) Patches to add support in AFS for this.

Jeff Layton has patches to add support in Ceph for this.

With this, AFS without a cache passes all expected xfstests; with a cache,
there's an extra failure, but that's also there before these patches.
Fixing that probably requires a greater overhaul. Ceph also passes the
expected tests.

The netfs lib and fscache/cachefiles patches can be found also on:

https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=netfs-lib

The afs patches can be found on:

https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=afs-netfs-lib

I also have some supplementary patches to tidy up the handling of
PG_fscache/PG_private_2 and their contribution to page refcounting in the
core kernel here, but I haven't included them in this set and will route
them separately:

https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=netfs-lib-mm

Changes
=======

ver #7:
Put some missing compound_head() calls in the *_page_private_2()
functions[11].

Included a patch from Matthew Wilcox to make it possible to modify
the readahead_control descriptor in a filesystem without occasionally
triggering a BUG in the VM core[12].

Renamed iter_xarray_copy_pages() to iter_xarray_populate_pages() as
it doesn't copy the contents of the pages, but rather fills out a
list of pages[13].

ver #6:
Merged in some fixes and added an additional tracepoint[8], including
fixing the amalgamation of contiguous subrequests that are to be
written to the cache.

Added/merged some patches from Matthew Wilcox to make
readahead_expand() appropriately adjust the trigger for the next
readahead[9]. Also included is a patch to kerneldocify the
file_ra_state struct.

Altered netfs_write_begin() to use DEFINE_READAHEAD()[10].

Split the afs patches out into their own branch.

ver #5:
Fixed some review comments from Matthew Wilcox:

- Put a comment into netfs_readahead() to indicate why there's a loop
that puts, but doesn't unlock, "unconsumed" pages at the end when
it could just return said pages to the caller to dispose of[6].
(This is because where those pages are marked consumed).

- Use the page_file_mapping() and page_index() helper functions
rather than accessing the page struct directly[6].

- Better names for wrangling functions for PG_private_2 and
PG_fscache wrangling functions[7]. Came up with
{set,end,wait_for}_page_private_2() and aliased these for fscache.

Moved the taking of/dropping a page ref for the PG_private_2 flag
into the set and end functions.

ver #4:
Fixed some review comments from Christoph Hellwig, including dropping
the export of rw_verify_area()[3] and some minor stuff[4].

Moved the declaration of readahead_expand() to a better location[5].

Rebased to v5.12-rc2 and added a bunch of references into individual
commits.

Dropped Ceph support - that will go through the maintainer's tree.

Added interface documentation for the netfs helper library.

ver #3:
Rolled in the bug fixes.

Adjusted the functions that unlock and wait for PG_fscache according
to Linus's suggestion[1].

Hold a ref on a page when PG_fscache is set as per Linus's
suggestion[2].

Dropped NFS support and added Ceph support.

ver #2:
Fixed some bugs and added NFS support.

Link: https://lore.kernel.org/r/CAHk-=wh+2gbF7XEjYc=HV9w_2uVzVf7vs60BPz0gFA=+pUm3ww@mail.gmail.com/ [1]
Link: https://lore.kernel.org/r/CAHk-=wjgA-74ddehziVk=XAEMTKswPu1Yw4uaro1R3ibs27ztw@mail.gmail.com/ [2]
Link: https://lore.kernel.org/r/20210216102614.GA27555@lst.de/ [3]
Link: https://lore.kernel.org/r/20210216084230.GA23669@lst.de/ [4]
Link: https://lore.kernel.org/r/20210217161358.GM2858050@casper.infradead.org/ [5]
Link: https://lore.kernel.org/r/20210321014202.GF3420@casper.infradead.org/ [6]
Link: https://lore.kernel.org/r/20210321105309.GG3420@casper.infradead.org/ [7]
Link: https://lore.kernel.org/r/161781041339.463527.18139104281901492882.stgit@warthog.procyon.org.uk/ [8]
Link: https://lore.kernel.org/r/20210407201857.3582797-1-willy@infradead.org/ [9]
Link: https://lore.kernel.org/r/1234933.1617886271@warthog.procyon.org.uk/ [10]
Link: https://lore.kernel.org/r/20210408145057.GN2531743@casper.infradead.org/ [11]
Link: https://lore.kernel.org/r/20210421170923.4005574-1-willy@infradead.org/ [12]
Link: https://lore.kernel.org/r/27c369a8f42bb8a617672b2dc0126a5c6df5a050.camel@kernel.org [13]

References
==========

These patches have been published for review before, firstly as part of a
larger set:

Link: https://lore.kernel.org/r/158861203563.340223.7585359869938129395.stgit@warthog.procyon.org.uk/

Link: https://lore.kernel.org/r/159465766378.1376105.11619976251039287525.stgit@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/159465784033.1376674.18106463693989811037.stgit@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/159465821598.1377938.2046362270225008168.stgit@warthog.procyon.org.uk/

Link: https://lore.kernel.org/r/160588455242.3465195.3214733858273019178.stgit@warthog.procyon.org.uk/

Then as a cut-down set:

Link: https://lore.kernel.org/r/161118128472.1232039.11746799833066425131.stgit@warthog.procyon.org.uk/ # v1

Link: https://lore.kernel.org/r/161161025063.2537118.2009249444682241405.stgit@warthog.procyon.org.uk/ # v2

Link: https://lore.kernel.org/r/161340385320.1303470.2392622971006879777.stgit@warthog.procyon.org.uk/ # v3

Link: https://lore.kernel.org/r/161539526152.286939.8589700175877370401.stgit@warthog.procyon.org.uk/ # v4

Link: https://lore.kernel.org/r/161653784755.2770958.11820491619308713741.stgit@warthog.procyon.org.uk/ # v5

Link: https://lore.kernel.org/r/161789062190.6155.12711584466338493050.stgit@warthog.procyon.org.uk/ # v6

Proposals/information about the design has been published here:

Link: https://lore.kernel.org/r/24942.1573667720@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/2758811.1610621106@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/1441311.1598547738@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/160655.1611012999@warthog.procyon.org.uk/

And requests for information:

Link: https://lore.kernel.org/r/3326.1579019665@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/4467.1579020509@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/3577430.1579705075@warthog.procyon.org.uk/

I've posted partial patches to try and help 9p and cifs along:

Link: https://lore.kernel.org/r/1514086.1605697347@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/1794123.1605713481@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/241017.1612263863@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/270998.1612265397@warthog.procyon.org.uk/

David
---
David Howells (28):
iov_iter: Add ITER_XARRAY
mm: Add set/end/wait functions for PG_private_2
mm: Implement readahead_control pageset expansion
netfs: Make a netfs helper module
netfs: Documentation for helper library
netfs, mm: Move PG_fscache helper funcs to linux/netfs.h
netfs, mm: Add set/end/wait_on_page_fscache() aliases
netfs: Provide readahead and readpage netfs helpers
netfs: Add tracepoints
netfs: Gather stats
netfs: Add write_begin helper
netfs: Define an interface to talk to a cache
netfs: Add a tracepoint to log failures that would be otherwise unseen
fscache, cachefiles: Add alternate API to use kiocb for read/write to cache
afs: Disable use of the fscache I/O routines
afs: Pass page into dirty region helpers to provide THP size
afs: Print the operation debug_id when logging an unexpected data version
afs: Move key to afs_read struct
afs: Don't truncate iter during data fetch
afs: Log remote unmarshalling errors
afs: Set up the iov_iter before calling afs_extract_data()
afs: Use ITER_XARRAY for writing
afs: Wait on PG_fscache before modifying/releasing a page
afs: Extract writeback extension into its own function
afs: Prepare for use of THPs
afs: Use the fs operation ops to handle FetchData completion
afs: Use new netfs lib read helper API
afs: Use the netfs_write_begin() helper

Matthew Wilcox (Oracle) (3):
mm/filemap: Pass the file_ra_state in the ractl
fs: Document file_ra_state
mm/readahead: Handle ractl nr_pages being modified

Documentation/filesystems/index.rst | 1 +
Documentation/filesystems/netfs_library.rst | 526 ++++++++
fs/Kconfig | 1 +
fs/Makefile | 1 +
fs/afs/Kconfig | 1 +
fs/afs/dir.c | 225 ++--
fs/afs/file.c | 483 ++------
fs/afs/fs_operation.c | 4 +-
fs/afs/fsclient.c | 108 +-
fs/afs/inode.c | 7 +-
fs/afs/internal.h | 59 +-
fs/afs/rxrpc.c | 150 +--
fs/afs/write.c | 657 +++++-----
fs/afs/yfsclient.c | 82 +-
fs/cachefiles/Makefile | 1 +
fs/cachefiles/interface.c | 5 +-
fs/cachefiles/internal.h | 9 +
fs/cachefiles/io.c | 420 +++++++
fs/ext4/verity.c | 2 +-
fs/f2fs/file.c | 2 +-
fs/f2fs/verity.c | 2 +-
fs/fscache/Kconfig | 1 +
fs/fscache/Makefile | 1 +
fs/fscache/internal.h | 4 +
fs/fscache/io.c | 116 ++
fs/fscache/page.c | 2 +-
fs/fscache/stats.c | 1 +
fs/netfs/Kconfig | 23 +
fs/netfs/Makefile | 5 +
fs/netfs/internal.h | 97 ++
fs/netfs/read_helper.c | 1185 +++++++++++++++++++
fs/netfs/stats.c | 59 +
include/linux/fs.h | 24 +-
include/linux/fscache-cache.h | 4 +
include/linux/fscache.h | 50 +-
include/linux/netfs.h | 234 ++++
include/linux/pagemap.h | 42 +-
include/net/af_rxrpc.h | 2 +-
include/trace/events/afs.h | 74 +-
include/trace/events/netfs.h | 261 ++++
mm/filemap.c | 65 +-
mm/internal.h | 7 +-
mm/readahead.c | 101 +-
net/rxrpc/recvmsg.c | 9 +-
44 files changed, 4003 insertions(+), 1110 deletions(-)
create mode 100644 Documentation/filesystems/netfs_library.rst
create mode 100644 fs/cachefiles/io.c
create mode 100644 fs/fscache/io.c
create mode 100644 fs/netfs/Kconfig
create mode 100644 fs/netfs/Makefile
create mode 100644 fs/netfs/internal.h
create mode 100644 fs/netfs/read_helper.c
create mode 100644 fs/netfs/stats.c
create mode 100644 include/linux/netfs.h
create mode 100644 include/trace/events/netfs.h