linux-kernel - Re: [PATCH 0/2] virtio-fs: Add 'file' mount option

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Message-ID: <20240711143425.GA1235314@perftesting>
Date: Thu, 11 Jul 2024 10:34:25 -0400
From: Josef Bacik <josef@...icpanda.com>
To: Hanna Czenczek <hreitz@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-doc@...r.kernel.org, virtualization@...ts.linux.dev,
	Miklos Szeredi <mszeredi@...hat.com>,
	German Maglione <gmaglione@...hat.com>,
	Stefan Hajnoczi <stefanha@...hat.com>,
	Eugenio Pérez <eperezma@...hat.com>,
	Jonathan Corbet <corbet@....net>, Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH 0/2] virtio-fs: Add 'file' mount option

On Thu, Jul 11, 2024 at 10:21:35AM +0200, Hanna Czenczek wrote:
> On 10.07.24 20:42, Josef Bacik wrote:
> > On Wed, Jul 10, 2024 at 09:28:08AM +0200, Hanna Czenczek wrote:
> > > On 09.07.24 19:56, Josef Bacik wrote:
> > > > On Tue, Jul 09, 2024 at 01:19:16PM +0200, Hanna Czenczek wrote:
> > > > > Hi,
> > > > > 
> > > > > We want to be able to mount filesystems that just consist of one regular
> > > > > file via virtio-fs, i.e. no root directory, just a file as the root
> > > > > node.
> > > > > 
> > > > > While that is possible via FUSE itself (through the 'rootmode' mount
> > > > > option, which is automatically set by the fusermount help program to
> > > > > match the mount point's inode mode), there is no virtio-fs option yet
> > > > > that would allow changing the rootmode from S_IFDIR to S_IFREG.
> > > > > 
> > > > > To do that, this series introduces a new 'file' mount option that does
> > > > > precisely that.  Alternatively, we could provide the same 'rootmode'
> > > > > option that FUSE has, but as laid out in patch 1's commit description,
> > > > > that option is a bit cumbersome for virtio-fs (in a way that it is not
> > > > > for FUSE), and its usefulness as a more general option is limited.
> > > > > 
> > > > All this does is make file an alias for something a little easier for users to
> > > > read, which can easily be done in libfuse.  Add the code to lib/mount.c to alias
> > > > 'file' to turn it into rootmode=S_IFREG when it sends it to the kernel, it's not
> > > > necessary to do this in the kernel.  Thanks,
> > > This series is not about normal FUSE filesystems (file_system_type
> > > fuse_fs_type, “fuse”), but about virtio-fs (file_system_type virtio_fs_type,
> > > “virtiofs”), i.e. a case where libfuse and fusermount are not involved at
> > > all.  As far as I’m aware, mounting a virtio-fs filesystem with a
> > > non-directory root inode is currently not possible at all.
> > Ok so I think I had it backwards in my head, my apologies.
> > 
> > That being said I still don't understand why this requires a change to virtiofs
> > at all.
> > 
> > I have a virtiofs thing attached to my VM.  Inside the vm I do
> > 
> > mount -t virtiofs <name of thing I've attached to the vm> /directory
> > 
> > and then on the host machine, virtiofsd is a "normal" FUSE driver, except it's
> > talking over the socket you setup between the guest and the host.  I assume this
> > is all correct?
> > 
> > So then the question is, why does it matter what virtiofsd is exposing?  I guess
> > that's the better question.  The guest shouldn't have to care if it's a
> > directory or a file right?  The mountpoint is going to be a directory, whatever
> > is backing it shouldn't matter.  Could you describe the exact thing you're
> > trying to accomplish?  Thanks,
> 
> The mount point needs to be of the same mode as the root node of the mounted
> filesystem, or it’ll be inaccessible after mounting[1].  In this case, I
> want to export a regular file as the root node, so the root node must be a
> regular file, too:
> 
> host$ echo foo > /tmp/bar
> 
> host$ virtiofsd --shared-dir /tmp/bar --socket-path /tmp/viofsd.sock
> --sandbox none
> 
> 
> guest# mkdir /tmp/mnt-dir
> 
> guest# mount -t virtiofs virtiofs-tag /tmp/mnt-dir
> 
> guest# stat /tmp/mnt-dir
> stat: cannot statx '/tmp/mnt-dir': Input/output error
> 
> guest# cat /tmp/mnt-dir
> cat: /tmp/mnt-dir: Input/output error
> 
> guest# ls /tmp/mnt-dir
> ls: cannot access '/tmp/mnt-dir': Input/output error
> 
> guest# umount /tmp/mnt-dir
> 
> (following with this series applied)
> 
> guest# touch /tmp/mnt-file
> 
> guest# mount -t virtiofs virtiofs-tag /tmp/mnt-file -o file
> 
> guest# stat /tmp/mnt-file
>   File: /tmp/mnt-file
>   Size: 4               Blocks: 8          IO Block: 4096   regular file
> [...]
> 
> guest# cat /tmp/mnt-file
> foo
> 
> guest# ls --file-type /tmp/mnt-file
> /tmp/mnt-file
> 
> guest# ls --file-type /tmp
> mnt-dir/
> mnt-file
> [...]
> 

Got it, this makes sense, thanks for explaining it to me.  You can add

Reviewed-by: Josef Bacik <josef@...icpanda.com>

Thanks,

Josef