Re: [PATCH v11 00/22] fscache,erofs: fscache-based on-demand read semantics

[Chao Yu <chao@kernel.org>]

On 2022/5/9 15:40, Jeffle Xu wrote:
> changes since v10:
> - rebase to 5.18-rc5
> - append the patchset with a patch from Xin Yin, implementing the
>    asynchronous readahead (patch 22)
> 
> 
> Kernel Patchset
> ---------------
> Git tree:
> 
>      https://github.com/lostjeffle/linux.git jingbo/dev-erofs-fscache-v11
> 
> Gitweb:
> 
>      https://github.com/lostjeffle/linux/commits/jingbo/dev-erofs-fscache-v11
> 
> 
> User Guide for E2E Container Use Case
> -------------------------------------
> User guide:
> 
>      https://github.com/dragonflyoss/image-service/blob/fscache/docs/nydus-fscache.md
> 
> Video:
> 
>      https://youtu.be/F4IF2_DENXo
> 
> 
> User Daemon for Quick Test
> --------------------------
> Git tree:
> 
>      https://github.com/lostjeffle/demand-read-cachefilesd.git main
> 
> Gitweb:
> 
>      https://github.com/lostjeffle/demand-read-cachefilesd
> 
> 
> Tested-by: Zichen Tian <tianzichen@kuaishou.com>
> Tested-by: Jia Zhu <zhujia.zj@bytedance.com>
> 
> 
> 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/
> v4: https://lore.kernel.org/lkml/20220307123305.79520-1-jefflexu@linux.alibaba.com/T/#t
> v5: https://lore.kernel.org/lkml/202203170912.gk2sqkaK-lkp@intel.com/T/
> v6: https://lore.kernel.org/lkml/202203260720.uA5o7k5w-lkp@intel.com/T/
> v7: https://lore.kernel.org/lkml/557bcf75-2334-5fbb-d2e0-c65e96da566d@linux.alibaba.com/T/
> v8: https://lore.kernel.org/all/ac8571b8-0935-1f4f-e9f1-e424f059b5ed@linux.alibaba.com/T/
> v9: https://lore.kernel.org/lkml/2067a5c7-4e24-f449-4676-811d12e9ab72@linux.alibaba.com/T/
> v10:https://lore.kernel.org/all/20220425122143.56815-21-jefflexu@linux.alibaba.com/t/
> 
> 
> [Background]
> ============
> Nydus [1] is an image distribution service especially optimized for
> distribution over network. Nydus is an excellent container image
> acceleration solution, since it only pulls data from remote when needed,
> a.k.a. on-demand reading and it also supports chunk-based deduplication,
> compression, etc.
> 
> erofs (Enhanced Read-Only File System) is a filesystem designed for
> read-only scenarios. (Documentation/filesystem/erofs.rst)
> 
> Over the past months we've been focusing on supporting Nydus image service
> with in-kernel erofs format[2]. In that case, each container image will be
> organized in one bootstrap (metadata) and (optional) multiple data blobs in
> erofs format. Massive container images will be stored on one machine.
> 
> To accelerate the container startup (fetching container images from remote
> and then start the container), we do hope that the bootstrap & blob files
> could support on-demand read. That is, erofs can be mounted and accessed
> even when the bootstrap/data blob files have not been fully downloaded.
> Then it'll have native performance after data is available locally.
> 
> 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 instead.
> 
> The fscache on-demand read feature aims to be implemented in a generic way
> so that it can benefit other use cases and/or filesystems if it's
> implemented in the fscache subsystem.
> 
> [1] https://nydus.dev
> [2] https://sched.co/pcdL
> 
> 
> [Overall Design]
> ================
> Please refer to patch 7 ("cachefiles: document on-demand read mode") for
> more details.
> 
> When working in the original mode, cachefiles mainly serves as a local cache
> for remote networking fs, while in on-demand read mode, cachefiles can work
> in 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 feeds to the kernel fscache side.
> 
> The on-demand read mode relies on a simple protocol used for communication
> between kernel and user daemon.
> 
> The proposed implementation relies on the anonymous fd mechanism to avoid
> the dependence on the format of cache file. When a fscache cachefile is opened
> for the first time, an anon_fd associated with the cache file is sent to the
> 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 with the
> anon_fd, along with the requested file range. When notified, user daemon
> 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 wait 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 on-demand read]
> ========================================
> 1. Asynchronous prefetch
> In current mechanism, fscache is responsible for cache state management,
> while the data plane (fetching data from local/remote on cache miss) is
> done on the user daemon side even without any file system request driven.
> In addition, if cached data has already been available locally, fscache
> will use it instead of trapping to user space anymore.
> 
> Therefore, different from event-driven approaches, the fscache on-demand
> user daemon could also fetch data (from remote) asynchronously in the
> background just like most multi-threaded HTTP downloaders.
> 
> 2. Flexible request amplification
> Since the data plane can be independently controlled by the user daemon,
> the user daemon can also fetch more data from remote than that the file
> system actually requests for small I/O sizes. Then, fetched data in bulk
> will be available at once and fscache won't be trapped into the user
> daemon again.
> 
> 3. Support massive blobs
> This mechanism can naturally support a large amount of backing files,
> and thus can benefit the densely employed scenarios. In our use cases,
> one container image can be formed of one bootstrap (required) and
> multiple chunk-deduplicated data blobs (optional).
> 
> For example, one container image for node.js will correspond to ~20
> files in total. In densely employed environment, there could be hundreds
> of containers and thus thousands of backing files on one machine.
> 
> 
> [Following Steps]
> =================
> The following improvements are on our TODO list, and will be formed in
> shape with the development process:
> 
> - Data blobs can be shared between multiple filesystems. Whilst in the
>    current implementation, each filesystem registers a unique fscache_volume,
>    causing the backing file for the data blob can not be shared between
>    different erofs filesystems. Later we need to introduce shared domain
>    in order to share fscache_volume, so that data blobs can be shared
>    between container images to some degree.
> 
> - in-memory extent-based data sharing, e.g., different files can share
>    the same chunk of the data blob. In the current implementation, each erofs
>    file maintains its own page cache, thus the page caches for the same chunk
>    may be duplicated among multiple files sharing the same chunk.
> 
> - other useful features, including multiple cachefiles daemon support,
>    etc.
> 
> 
> Jeffle Xu (21):
>    cachefiles: extract write routine
>    cachefiles: notify the user daemon when looking up cookie
>    cachefiles: unbind cachefiles gracefully in on-demand mode
>    cachefiles: notify the user daemon when withdrawing cookie
>    cachefiles: implement on-demand read
>    cachefiles: enable on-demand read mode
>    cachefiles: add tracepoints for on-demand read mode
>    cachefiles: document on-demand read mode
>    erofs: make erofs_map_blocks() generally available
>    erofs: add fscache mode check helper
>    erofs: register fscache volume
>    erofs: add fscache context helper functions
>    erofs: add anonymous inode caching metadata for data blobs
>    erofs: add erofs_fscache_read_folios() helper
>    erofs: register fscache context for primary data blob
>    erofs: register fscache context for extra data blobs
>    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: implement fscache-based data readahead
>    erofs: add 'fsid' mount option

For erofs parts:

Acked-by: Chao Yu <chao@kernel.org>

Thanks,

> 
> Xin Yin (1):
>    erofs: change to use asynchronous io for fscache readpage/readahead
> 
>   .../filesystems/caching/cachefiles.rst        | 178 ++++++
>   fs/cachefiles/Kconfig                         |  12 +
>   fs/cachefiles/Makefile                        |   1 +
>   fs/cachefiles/daemon.c                        | 117 +++-
>   fs/cachefiles/interface.c                     |   2 +
>   fs/cachefiles/internal.h                      |  78 +++
>   fs/cachefiles/io.c                            |  76 ++-
>   fs/cachefiles/namei.c                         |  16 +-
>   fs/cachefiles/ondemand.c                      | 503 +++++++++++++++++
>   fs/erofs/Kconfig                              |  10 +
>   fs/erofs/Makefile                             |   1 +
>   fs/erofs/data.c                               |  26 +-
>   fs/erofs/fscache.c                            | 522 ++++++++++++++++++
>   fs/erofs/inode.c                              |   4 +
>   fs/erofs/internal.h                           |  49 ++
>   fs/erofs/super.c                              | 105 +++-
>   fs/erofs/sysfs.c                              |   4 +-
>   include/linux/fscache.h                       |   1 +
>   include/linux/netfs.h                         |   1 +
>   include/trace/events/cachefiles.h             | 176 ++++++
>   include/uapi/linux/cachefiles.h               |  68 +++
>   21 files changed, 1871 insertions(+), 79 deletions(-)
>   create mode 100644 fs/cachefiles/ondemand.c
>   create mode 100644 fs/erofs/fscache.c
>   create mode 100644 include/uapi/linux/cachefiles.h
>