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@xxxxxxxxxxxxxxxxx/T/ v2: https://lore.kernel.org/all/2946d871-b9e1-cf29-6d39-bcab30f2854f@xxxxxxxxxxxxxxxxx/t/ v3: https://lore.kernel.org/lkml/20220209060108.43051-1-jefflexu@xxxxxxxxxxxxxxxxx/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
