Hi Jan, On Mon, Sep 30, 2024 at 4:18 PM Jan Kara <jack@xxxxxxx> wrote: > On Tue 24-09-24 11:21:59, Geert Uytterhoeven wrote: > > On Fri, Aug 30, 2024 at 5:29 AM Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx> wrote: > > > It actually has been around for years: For containers and other sandbox > > > use cases, there will be thousands (and even more) of authenticated > > > (sub)images running on the same host, unlike OS images. > > > > > > Of course, all scenarios can use the same EROFS on-disk format, but > > > bdev-backed mounts just work well for OS images since golden data is > > > dumped into real block devices. However, it's somewhat hard for > > > container runtimes to manage and isolate so many unnecessary virtual > > > block devices safely and efficiently [1]: they just look like a burden > > > to orchestrators and file-backed mounts are preferred indeed. There > > > were already enough attempts such as Incremental FS, the original > > > ComposeFS and PuzzleFS acting in the same way for immutable fses. As > > > for current EROFS users, ComposeFS, containerd and Android APEXs will > > > be directly benefited from it. > > > > > > On the other hand, previous experimental feature "erofs over fscache" > > > was once also intended to provide a similar solution (inspired by > > > Incremental FS discussion [2]), but the following facts show file-backed > > > mounts will be a better approach: > > > - Fscache infrastructure has recently been moved into new Netfslib > > > which is an unexpected dependency to EROFS really, although it > > > originally claims "it could be used for caching other things such as > > > ISO9660 filesystems too." [3] > > > > > > - It takes an unexpectedly long time to upstream Fscache/Cachefiles > > > enhancements. For example, the failover feature took more than > > > one year, and the deamonless feature is still far behind now; > > > > > > - Ongoing HSM "fanotify pre-content hooks" [4] together with this will > > > perfectly supersede "erofs over fscache" in a simpler way since > > > developers (mainly containerd folks) could leverage their existing > > > caching mechanism entirely in userspace instead of strictly following > > > the predefined in-kernel caching tree hierarchy. > > > > > > After "fanotify pre-content hooks" lands upstream to provide the same > > > functionality, "erofs over fscache" will be removed then (as an EROFS > > > internal improvement and EROFS will not have to bother with on-demand > > > fetching and/or caching improvements anymore.) > > > > > > [1] https://github.com/containers/storage/pull/2039 > > > [2] https://lore.kernel.org/r/CAOQ4uxjbVxnubaPjVaGYiSwoGDTdpWbB=w_AeM6YM=zVixsUfQ@xxxxxxxxxxxxxx > > > [3] https://docs.kernel.org/filesystems/caching/fscache.html > > > [4] https://lore.kernel.org/r/cover.1723670362.git.josef@xxxxxxxxxxxxxx > > > > > > Closes: https://github.com/containers/composefs/issues/144 > > > Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx> > > > > Thanks for your patch, which is now commit fb176750266a3d7f > > ("erofs: add file-backed mount support"). > > > > > --- > > > v2: > > > - should use kill_anon_super(); > > > - add O_LARGEFILE to support large files. > > > > > > fs/erofs/Kconfig | 17 ++++++++++ > > > fs/erofs/data.c | 35 ++++++++++++--------- > > > fs/erofs/inode.c | 5 ++- > > > fs/erofs/internal.h | 11 +++++-- > > > fs/erofs/super.c | 76 +++++++++++++++++++++++++++++---------------- > > > 5 files changed, 100 insertions(+), 44 deletions(-) > > > > > > diff --git a/fs/erofs/Kconfig b/fs/erofs/Kconfig > > > index 7dcdce660cac..1428d0530e1c 100644 > > > --- a/fs/erofs/Kconfig > > > +++ b/fs/erofs/Kconfig > > > @@ -74,6 +74,23 @@ config EROFS_FS_SECURITY > > > > > > If you are not using a security module, say N. > > > > > > +config EROFS_FS_BACKED_BY_FILE > > > + bool "File-backed EROFS filesystem support" > > > + depends on EROFS_FS > > > + default y > > > > I am a bit reluctant to have this default to y, without an ack from > > the VFS maintainers. > > Well, we generally let filesystems do whatever they decide to do unless it > is a affecting stability / security / maintainability of the whole system. > In this case I don't see anything that would be substantially different > than if we go through a loop device. So although the feature looks somewhat > unusual I don't see a reason to nack it or otherwise interfere with > whatever the fs maintainer wants to do. Are you concerned about a > particular problem? I was just wondering if there are any issues with accessing files directly. If you're fine with it, I am, too. Thanks! Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds
