[PATCH v2 00/22] netfs, afs, 9p, cifs: Rework netfs to use ->writepages() to copy to cache

David Howells
2024-04-30T14:01:09Z

Hi Christian, Willy,

Here's an updated version of my netfs writeback rewrite. There haven't
been any major fixes. So far...

The primary purpose of these patches is to rework the netfslib writeback
implementation such that pages read from the cache are written to the cache
through ->writepages(), thereby allowing the fscache page flag to be
retired.

The reworking also:

(1) builds on top of the new writeback_iter() infrastructure;

(2) makes it possible to use vectored write RPCs as discontiguous streams
of pages can be accommodated;

(3) makes it easier to do simultaneous content crypto and stream division.

(4) provides support for retrying writes and re-dividing a stream;

(5) replaces the ->launder_folio() op, so that ->writepages() is used
instead;

(6) uses mempools to allocate the netfs_io_request and netfs_io_subrequest
structs to avoid allocation failure in the writeback path.

Some code that uses the fscache page flag is retained for compatibility
purposes with nfs and ceph. The code is switched to using the synonymous
private_2 label instead and marked with deprecation comments. I have a
separate set of patches that convert cifs to use this code.

-~-

In this new implementation, writeback_iter() is used to pump folios,
progressively creating two parallel, but separate streams. Either or both
streams can contain gaps, and the subrequests in each stream can be of
variable size, don't need to align with each other and don't need to align
with the folios. (Note that more streams can be added if we have multiple
servers to duplicate data to).

Indeed, subrequests can cross folio boundaries, may cover several folios or
a folio may be spanned by multiple subrequests, e.g.:

+---+---+-----+-----+---+----------+
Folios: | | | | | | |
+---+---+-----+-----+---+----------+

+------+------+ +----+----+
Upload: | | |.....| | |
+------+------+ +----+----+

+------+------+------+------+------+
Cache: | | | | | |
+------+------+------+------+------+

Data that got read from the server that needs copying to the cache is
stored in folios that are marked dirty and have folio->private set to a
special value.

The progressive subrequest construction permits the algorithm to be
preparing both the next upload to the server and the next write to the
cache whilst the previous ones are already in progress. Throttling can be
applied to control the rate of production of subrequests - and, in any
case, we probably want to write them to the server in ascending order,
particularly if the file will be extended.

Content crypto can also be prepared at the same time as the subrequests and
run asynchronously, with the prepped requests being stalled until the
crypto catches up with them. This might also be useful for transport
crypto, but that happens at a lower layer, so probably would be harder to
pull off.

The algorithm is split into three parts:

(1) The issuer. This walks through the data, packaging it up, encrypting
it and creating subrequests. The part of this that generates
subrequests only deals with file positions and spans and so is usable
for DIO/unbuffered writes as well as buffered writes.

(2) The collector. This asynchronously collects completed subrequests,
unlocks folios, frees crypto buffers and performs any retries. This
runs in a work queue so that the issuer can return to the caller for
writeback (so that the VM can have its kswapd thread back) or async
writes.

Collection is slightly complex as the collector has to work out where
discontiguities happen in the folio list so that it doesn't try and
collect folios that weren't included in the write out.

(3) The retryer. This pauses the issuer, waits for all outstanding
subrequests to complete and then goes through the failed subrequests
to reissue them. This may involve reprepping them (with cifs, the
credits must be renegotiated and a subrequest may need splitting), and
doing RMW for content crypto if there's a conflicting change on the
server.

The patches can be found here also:

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

David

Link: https://lore.kernel.org/r/20240328163424.2781320-1-dhowells@redhat.com/ # v1

Changes
=======
ver #2)
- Rebase on v6.9-rc6 to accommodate some fixes that went upstream.
- Make netfs_begin_writethrough() actually return an error.
- Move the cifs invalidation patch to the cifs-netfs branch.
- Mark writeback_iter() EXPORT_SYMBOL_GPL.
- In p9_client_write_subreq(), use 'int len' rather than 'size_t len'
because of the varargs packet formatter.
- In netfs_perform_write(), don't wait for writeback if we don't need to.
- Don't do the AFS StoreData RPC inline, but keep doing it from a
workqueue.
- Remove a couple of redundant checks where the second used to be inside
a lock.
- Add missing linux/bio.h for BIO_MAX_VECS in cachefiles.
- Change a comma ending a statement to a semicolon.
- Make filemap_invalidate_inode() take a range.
- Make netfs_unbuffered_write_iter() use filemap_invalidate_inode().

David Howells (22):
netfs: Update i_blocks when write committed to pagecache
netfs: Replace PG_fscache by setting folio->private and marking dirty
mm: Remove the PG_fscache alias for PG_private_2
netfs: Remove deprecated use of PG_private_2 as a second writeback
flag
netfs: Make netfs_io_request::subreq_counter an atomic_t
netfs: Use subreq_counter to allocate subreq debug_index values
mm: Provide a means of invalidation without using launder_folio
9p: Use alternative invalidation to using launder_folio
afs: Use alternative invalidation to using launder_folio
netfs: Remove ->launder_folio() support
netfs: Use mempools for allocating requests and subrequests
mm: Export writeback_iter()
netfs: Switch to using unsigned long long rather than loff_t
netfs: New writeback implementation
netfs: Add some write-side stats and clean up some stat names
netfs, afs: Implement helpers for new write code
netfs, 9p: Implement helpers for new write code
netfs, cachefiles: Implement helpers for new write code
netfs: Cut over to using new writeback code
netfs: Remove the old writeback code
netfs: Miscellaneous tidy ups
netfs, afs: Use writeback retry to deal with alternate keys

fs/9p/vfs_addr.c | 60 +--
fs/afs/file.c | 8 +-
fs/afs/internal.h | 6 +-
fs/afs/validation.c | 4 +-
fs/afs/write.c | 189 ++++----
fs/cachefiles/io.c | 76 +++-
fs/ceph/addr.c | 24 +-
fs/ceph/inode.c | 2 +
fs/netfs/Makefile | 3 +-
fs/netfs/buffered_read.c | 40 +-
fs/netfs/buffered_write.c | 825 +++--------------------------------
fs/netfs/direct_write.c | 56 ++-
fs/netfs/fscache_io.c | 14 +-
fs/netfs/internal.h | 55 ++-
fs/netfs/io.c | 155 +------
fs/netfs/main.c | 55 ++-
fs/netfs/misc.c | 10 +-
fs/netfs/objects.c | 81 +++-
fs/netfs/output.c | 478 --------------------
fs/netfs/stats.c | 17 +-
fs/netfs/write_collect.c | 808 ++++++++++++++++++++++++++++++++++
fs/netfs/write_issue.c | 675 ++++++++++++++++++++++++++++
fs/nfs/file.c | 8 +-
fs/nfs/fscache.h | 6 +-
fs/nfs/write.c | 4 +-
fs/smb/client/file.c | 16 +-
include/linux/fscache.h | 22 +-
include/linux/netfs.h | 196 +++++----
include/linux/pagemap.h | 2 +
include/net/9p/client.h | 2 +
include/trace/events/netfs.h | 249 ++++++++++-
mm/filemap.c | 60 ++-
mm/page-writeback.c | 1 +
net/9p/Kconfig | 1 +
net/9p/client.c | 49 +++
35 files changed, 2502 insertions(+), 1755 deletions(-)
delete mode 100644 fs/netfs/output.c
create mode 100644 fs/netfs/write_collect.c
create mode 100644 fs/netfs/write_issue.c