[PATCH v2 1/4] erofs: add file-backed mount support

[Geert Uytterhoeven]

Hi Gao,

CC vfs

On Fri, Aug 30, 2024 at 5:29 AM Gao Xiang <hsiangkao at linux.alibaba.com> 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@mail.gmail.com
> [3] https://docs.kernel.org/filesystems/caching/fscache.html
> [4] https://lore.kernel.org/r/cover.1723670362.git.josef@toxicpanda.com
>
> Closes: https://github.com/containers/composefs/issues/144
> Signed-off-by: Gao Xiang <hsiangkao at linux.alibaba.com>

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.

> +       help
> +         This allows EROFS to use filesystem image files directly, without
> +         the intercession of loopback block devices or likewise. It is
> +         particularly useful for container images with numerous blobs and
> +         other sandboxes, where loop devices behave intricately.  It can also
> +         be used to simplify error-prone lifetime management of unnecessary
> +         virtual block devices.
> +
> +         Note that this feature, along with ongoing fanotify pre-content
> +         hooks, will eventually replace "EROFS over fscache."
> +
> +         If you don't want to enable this feature, say N.
> +
>  config EROFS_FS_ZIP
>         bool "EROFS Data Compression Support"
>         depends on EROFS_FS

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

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