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

[Hanna Czenczek <hreitz@redhat.com>]

On 11.07.24 16:34, Josef Bacik wrote:
> 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@toxicpanda.com>

Thanks!

Hanna