On 6/9/21 4:51 AM, Stefan Hajnoczi wrote: > On Tue, Jun 08, 2021 at 11:35:24AM -0400, Vivek Goyal 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 this patch proposes that we add a new prefix "fstag:" which specifies >> that identifier which follows is filesystem specific tag and its not >> a block device. Just pass this tag to filesystem and filesystem will >> figure out how to mount it. >> >> For example, "root=fstag:<tag>". >> >> In case of virtiofs, I can specify "root=fstag:myfs rootfstype=virtiofs" >> and it works. >> >> I think this should work for 9p as well. "root=fstag:myfs rootfstype=9p". >> Though I have yet to test it. >> >> This kind of syntax should be able to address wide variety of use cases >> where root device is not a block device and is simply some kind of >> tag/label understood by filesystem. > "fstag" is kind of virtio-9p/fs specific. The intended effect is really > to specify the file system source (like in mount(2)) without it being > interpreted as a block device. > > In a previous discussion David Gilbert suggested detecting file systems > that do not need a block device: > https://patchwork.kernel.org/project/linux-fsdevel/patch/20190906100324.8492-1-stefanha@xxxxxxxxxx/ > > I never got around to doing it, but can do_mounts.c just look at struct > file_system_type::fs_flags FS_REQUIRES_DEV to detect non-block device > file systems? > > That way it would know to just mount with root= as the source instead of > treating it as a block device. No root= prefix would be required and it > would handle NFS, virtiofs, virtio-9p, etc without introducing the > concept of a "tag". > > root=myfs rootfstype=virtiofs rootflags=... > > I wrote this up quickly after not thinking about the topic for 2 years, > so the idea may not work at all :). I plead for the long term goal of syntax harmony between the kernel command line and the first three fields of /etc/fstab. Let's do one thing one way, even if it is specified more than one place. HC
