On 29.08.24 10:07, Miklos Szeredi wrote:
On Tue, 9 Jul 2024 at 13:19, Hanna Czenczek <hreitz@xxxxxxxxxx> 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.
I wonder if this is needed at all for virtiofs, which could easily do
the FUSE_INIT request synchronously with mount(2) and the server could
just tell the client the root mode explicitly in the FUSE_INIT reply,
or could just fetch it with a separate FUSE_GETATTR.
That would be great. I thought it would be necessary to install the
superblock before sending FUSE_INIT, so I thought this wasn’t possible.
I honestly have no idea how to go about it on a technical level,
though. Naïvely, I think we’d need to split off the tail of
fuse_fill_super_common() (everything starting from the
fuse_get_root_inode() call) into a separate function, which in case of
virtio-fs we’d call once we get the FUSE_INIT reply. (For
non-virtio-fs, we could just call it immediately after
fuse_fill_super_common().)
But we can’t return from fuse_fill_super() until that root node is set
up, can we? If so, we‘d need to await that FUSE_INIT reply in that
function. Can we do that?
Why regular fuse doesn't do this? That's because a single threaded
server can only be supported if the mount(2) syscall returns before
any request need processing. Virtiofs doesn't suffer from this at
all, AFAICS.
Does this make sense?
It does!
Hanna
