[PATCH v4 00/21] fscache,erofs: fscache-based on-demand read semantics

Jeffle Xu
2022-03-07T12:33:15Z

changes since v3:
- cachefiles: The current implementation relies on the anonymous fd mechanism to avoid
the dependence on the format of cache file. When cache file is opened
for the first time, an anon_fd associated with the cache file is sent to
user daemon. User daemon could fetch and write data to cache file with
the given anon_fd. The following write to the anon_fd will finally
call to cachefiles kernel module, which will write data to cache file in
the latest format of cache file. Thus the on-demand read mode can
keep working no matter how cache file format could change in the
future. (patch 4)
- cachefiles: the on-demand read mode reuses the existing
"/dev/cachefiles" devnode (patch 3)
- erofs: squash several commits implementing readahead into single
commit (patch 20)
- erofs: refactor the readahead routine, so that it can read multiple
pages each round (patch 20)
- patch 1 and 7 have already been cherry-picked by the maintainers, but
have not been merged to the master. Keep them here for completeness.

RFC: https://lore.kernel.org/all/YbRL2glGzjfZkVbH@B-P7TQMD6M-0146.local/t/
v1: https://lore.kernel.org/lkml/47831875-4bdd-8398-9f2d-0466b31a4382@linux.alibaba.com/T/
v2: https://lore.kernel.org/all/2946d871-b9e1-cf29-6d39-bcab30f2854f@linux.alibaba.com/t/
v3: https://lore.kernel.org/lkml/20220209060108.43051-1-jefflexu@linux.alibaba.com/T/

[Background]
============
Nydus [1] is a container image distribution service specially optimised
for distribution over network. Nydus is an excellent container image
acceleration solution, since it only pulls data from remote when it's
really needed, a.k.a. on-demand reading.

erofs (Enhanced Read-Only File System) is a filesystem specially
optimised for read-only scenarios. (Documentation/filesystem/erofs.rst)

Recently we are focusing on erofs in container images distribution
scenario [2], trying to combine it with nydus. In this case, erofs can
be mounted from one bootstrap file (metadata) with (optional) multiple
data blob files (data) stored on another local filesystem. (All these
files are actually image files in erofs disk format.)

To accelerate the container startup (fetching container image from remote
and then start the container), we do hope that the bootstrap blob file
could support demand read. That is, erofs can be mounted and accessed
even when the bootstrap/data blob files have not been fully downloaded.

That means we have to manage the cache state of the bootstrap/data blob
files (if cache hit, read directly from the local cache; if cache miss,
fetch the data somehow). It would be painful and may be dumb for erofs to
implement the cache management itself. Thus we prefer fscache/cachefiles
to do the cache management. Besides, the demand-read feature shall be
general and it can benefit other using scenarios if it can be implemented
in fscache level.

[1] https://nydus.dev
[2] https://sched.co/pcdL

[Overall Design]
================

Please refer to patch 6 ("cachefiles: document on-demand read mode") for
more details.

When working in original mode, cachefiles mainly serves as a local cache for
remote networking fs, while in on-demand read mode, cachefiles can boost the
scenario where on-demand read semantics is needed, e.g. container image
distribution.

The essential difference between these two modes is that, in original mode,
when cache miss, netfs itself will fetch data from remote, and then write the
fetched data into cache file. While in on-demand read mode, a user daemon is
responsible for fetching data and then writing to the cache file.

The on-demand read mode relies on a simple protocol used for communication
between kernel and user daemon.

The current implementation relies on the anonymous fd mechanism to avoid
the dependence on the format of cache file. When cache file is opened
for the first time, an anon_fd associated with the cache file is sent to
user daemon. With the given anon_fd, user daemon could fetch and write data
into the cache file in the background, even when kernel has not triggered
the cache miss. Besides, the write() syscall to the anon_fd will finally
call cachefiles kernel module, which will write data to cache file in
the latest format of cache file.

1. cache miss
When cache miss, cachefiles kernel module will notify user daemon the
anon_fd, along with the requested file range. When notified, user dameon
needs to fetch data of the requested file range, and then write the fetched
data into cache file with the given anonymous fd. When finished
processing the request, user daemon needs to notify the kernel.

After notifying the user daemon, the kernel read routine will hang there,
until the request is handled by user daemon. When it's awaken by the
notification from user daemon, i.e. the corresponding hole has been filled
by the user daemon, it will retry to read from the same file range.

2. cache hit
Once data is already ready in cache file, netfs will read from cache file directly.

[Advantage of fscache-based demand-read]
========================================
1. Asynchronous Prefetch
In current mechanism, fscache is responsible for cache state management,
while the data plane (fetch data from local/remote on cache miss) is
done on the user daemon side.

If data has already been ready in the backing file, the upper fs (e.g.
erofs) will read from the backing file directly and won't be trapped to
user space anymore. Thus the user daemon could fetch data (from remote)
asynchronously on the background, and thus accelerate the backing file
accessing in some degree.

2. Support massive blob files
Besides this mechanism supports a large amount of backing files, and
thus can benefit the densely employed scenario.

In our using scenario, one container image can correspond to one
bootstrap file (required) and multiple data blob files (optional). For
example, one container image for node.js will corresponds to ~20 files
in total. In densely employed environment, there could be as many as
hundreds of containers and thus thousands of backing files on one
machine.

[Test]
==========
You could start a quick test by
https://github.com/lostjeffle/demand-read-cachefilesd

Jeffle Xu (21):
fscache: export fscache_end_operation()
cachefiles: export write routine
cachefiles: introduce on-demand read mode
cachefiles: notify user daemon with anon_fd when opening cache file
cachefiles: implement on-demand read
cachefiles: document on-demand read mode
erofs: use meta buffers for erofs_read_superblock()
erofs: export erofs_map_blocks()
erofs: add mode checking helper
erofs: register global fscache volume
erofs: add cookie context helper functions
erofs: add anonymous inode managing page cache of blob file
erofs: add erofs_fscache_read_pages() helper
erofs: register cookie context for bootstrap blob
erofs: implement fscache-based metadata read
erofs: implement fscache-based data read for non-inline layout
erofs: implement fscache-based data read for inline layout
erofs: register cookie context for data blobs
erofs: implement fscache-based data read for data blobs
erofs: implement fscache-based data readahead
erofs: add 'uuid' mount option

.../filesystems/caching/cachefiles.rst | 159 +++++
fs/cachefiles/Kconfig | 11 +
fs/cachefiles/daemon.c | 576 +++++++++++++++++-
fs/cachefiles/internal.h | 48 ++
fs/cachefiles/io.c | 72 ++-
fs/cachefiles/namei.c | 16 +-
fs/erofs/Makefile | 3 +-
fs/erofs/data.c | 18 +-
fs/erofs/fscache.c | 496 +++++++++++++++
fs/erofs/inode.c | 6 +-
fs/erofs/internal.h | 30 +
fs/erofs/super.c | 106 +++-
fs/fscache/internal.h | 11 -
fs/nfs/fscache.c | 8 -
include/linux/fscache.h | 15 +
include/linux/netfs.h | 1 +
include/trace/events/cachefiles.h | 2 +
include/uapi/linux/cachefiles.h | 48 ++
18 files changed, 1526 insertions(+), 100 deletions(-)
create mode 100644 fs/erofs/fscache.c
create mode 100644 include/uapi/linux/cachefiles.h

--
2.27.0