Re: [PATCH v5] erofs: accept source file descriptor via fsconfig

[Gao Xiang <xiang@kernel.org>]

On Tue, Jul 28, 2026 at 06:05:32PM +0200, Giuseppe Scrivano wrote:
> Allow userspace to pass an already-opened file descriptor as the mount
> source instead of a path string.  This is useful for tools that already
> hold an fd to the image, such as composefs reusing an existing erofs
> backing file.
> 
> Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
> ---
> v4: https://lore.kernel.org/linux-fsdevel/20260717134147.1602735-1-gscrivan@redhat.com/
> v3: https://lore.kernel.org/linux-fsdevel/20260714154917.489993-1-gscrivan@redhat.com/
> v2: https://lore.kernel.org/linux-fsdevel/20260711071137.4130824-1-gscrivan@redhat.com/
> v1: https://lore.kernel.org/linux-fsdevel/ak5GfvVfWLJU1EwK@debian/
> 
>  Documentation/filesystems/erofs.rst | 15 ++++++
>  fs/erofs/super.c                    | 73 ++++++++++++++++++++++++-----
>  2 files changed, 77 insertions(+), 11 deletions(-)
> 
> diff --git a/Documentation/filesystems/erofs.rst b/Documentation/filesystems/erofs.rst
> index 4230884fb359..774e8b236d09 100644
> --- a/Documentation/filesystems/erofs.rst
> +++ b/Documentation/filesystems/erofs.rst
> @@ -139,6 +139,21 @@ inode_share            Enable inode page sharing for this filesystem.  Inodes wi
>                         page cache.
>  ===================    =========================================================
>  
> +File-backed mounts
> +==================
> +
> +When CONFIG_EROFS_FS_BACKED_BY_FILE is enabled, EROFS file-backed images
> +can be mounted directly without a loopback block device.  The backing file
> +can be given either as a path, or as an already-opened file descriptor.
> +
> +When a file descriptor is used, the kernel resolves its path and records it
> +so that /proc/mounts and similar interfaces can still report the mount
> +source.
> +
> +Only regular files are accepted as backing files; to mount an image that
> +resides on a block device, use the traditional block device mount path
> +instead.

Reviewed-by: Gao Xiang <xiang@kernel.org>

Hi Christian, Jan, could you ack this (`fc->source` filling when
`source_fd` is used) if possible? I wish this behavior can be
consistent even vfs source_fd is implemented later.

Thanks,
Gao Xiang