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Message-ID: <CAMuHMdVqa2Mjqtqv0q=uuhBY1EfTaa+X6WkG7E2tEnKXJbTkNg@mail.gmail.com>
Date: Tue, 24 Sep 2024 11:21:59 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Gao Xiang <hsiangkao@...ux.alibaba.com>
Cc: linux-erofs@...ts.ozlabs.org, LKML <linux-kernel@...r.kernel.org>,
Al Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v2 1/4] erofs: add file-backed mount support
Hi Gao,
CC vfs
On Fri, Aug 30, 2024 at 5:29 AM Gao Xiang <hsiangkao@...ux.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@...ux.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@...ux-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
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