linux-kernel - Re: [PATCH v2 1/2] init/do_mounts.c: Add a path to boot from tag based filesystems

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Date:   Fri, 18 Jun 2021 09:07:14 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtio-fs-list <virtio-fs@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        David Howells <dhowells@...hat.com>,
        Richard Weinberger <richard.weinberger@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        v9fs-developer@...ts.sourceforge.net
Subject: Re: [PATCH v2 1/2] init/do_mounts.c: Add a path to boot from tag
 based filesystems

On Mon, 14 Jun 2021 at 19:45, Vivek Goyal <vgoyal@...hat.com> wrote:
>
> We want to be able to mount virtiofs as rootfs and pass appropriate
> kernel command line. Right now there does not seem to be a good way
> to do that. If I specify "root=myfs rootfstype=virtiofs", system
> panics.
>
> virtio-fs: tag </dev/root> not found
> ..
> ..
> [ end Kernel panic - not syncing: VFS: Unable to mount root fs on
> +unknown-block(0,0) ]
>
> Basic problem here is that kernel assumes that device identifier
> passed in "root=" is a block device. But there are few execptions
> to this rule to take care of the needs of mtd, ubi, NFS and CIFS.
>
> For example, mtd and ubi prefix "mtd:" or "ubi:" respectively.
>
> "root=mtd:<identifier>" or "root=ubi:<identifier>"
>
> NFS and CIFS use "root=/dev/nfs" and CIFS passes "root=/dev/cifs" and
> actual root device details come from filesystem specific kernel
> command line options.
>
> virtiofs does not seem to fit in any of the above categories. In fact
> we have 9pfs which can be used to boot from but it also does not
> have a proper syntax to specify rootfs and does not fit into any of
> the existing syntax. They both expect a device "tag" to be passed
> in a device to be mounted. And filesystem knows how to parse and
> use "tag".
>
> So there seem to be a class of filesystems which specify root device
> using a "tag" which is understood by the filesystem. And filesystem
> simply expects that "tag" to be passed as "source" of mount and
> how to mount filesystem using that "tag" is responsibility of filesystem.
>
> This patch proposes that we internally create a list of filesystems
> which pass a "tag" in "root=<tag>" and expect that tag to be passed
> as "source" of mount. With this patch I can boot into virtiofs rootfs
> with following syntax.
>
> "root=myfs rootfstype=virtiofs rw"

The syntax and the implementation looks good.

Acked-by: Miklos Szeredi <mszeredi@...hat.com>