Re: [PATCH v4 4/5] virtiofs: Skip submounts in sget_fc()

On Fri, 21 May 2021 10:50:34 +0200
Miklos Szeredi [off-list ref] wrote:

On Fri, 21 May 2021 at 10:39, Greg Kurz [off-list ref] wrote:

quoted

On Fri, 21 May 2021 10:26:27 +0200
Miklos Szeredi [off-list ref] wrote:

quoted

On Thu, 20 May 2021 at 17:47, Greg Kurz [off-list ref] wrote:

quoted

All submounts share the same virtio-fs device instance as the root
mount. If the same virtiofs filesystem is mounted again, sget_fc()
is likely to pick up any of these submounts and reuse it instead of
the root mount.

On the server side:

# mkdir ${some_dir}
# mkdir ${some_dir}/mnt1
# mount -t tmpfs none ${some_dir}/mnt1
# touch ${some_dir}/mnt1/THIS_IS_MNT1
# mkdir ${some_dir}/mnt2
# mount -t tmpfs none ${some_dir}/mnt2
# touch ${some_dir}/mnt2/THIS_IS_MNT2

On the client side:

# mkdir /mnt/virtiofs1
# mount -t virtiofs myfs /mnt/virtiofs1
# ls /mnt/virtiofs1
mnt1 mnt2
# grep virtiofs /proc/mounts
myfs /mnt/virtiofs1 virtiofs rw,seclabel,relatime 0 0
none on /mnt/mnt1 type virtiofs (rw,relatime,seclabel)
none on /mnt/mnt2 type virtiofs (rw,relatime,seclabel)

And now remount it again:

# mount -t virtiofs myfs /mnt/virtiofs2
# grep virtiofs /proc/mounts
myfs /mnt/virtiofs1 virtiofs rw,seclabel,relatime 0 0
none on /mnt/mnt1 type virtiofs (rw,relatime,seclabel)
none on /mnt/mnt2 type virtiofs (rw,relatime,seclabel)
myfs /mnt/virtiofs2 virtiofs rw,seclabel,relatime 0 0
# ls /mnt/virtiofs2
THIS_IS_MNT2

Submount mnt2 was picked-up instead of the root mount.

quoted

Why is this a problem?

It seems very weird to mount the same filesystem again
and to end up in one of its submounts. We should have:

# ls /mnt/virtiofs2
mnt1 mnt2

Okay, sorry, I understand the problem.  The solution is wrong,
however: the position of the submount on that list is no indication
that it's the right one (it's possible that the root sb will go away
and only a sub-sb will remain).

Ah... I had myself convinced this could not happen, i.e. you can't
unmount a parent sb with a sub-sb still mounted.

How can this happen ?

Even just setting a flag in the root, indicating that it's the root
isn't fully going to solve the problem.

Here's issue in full:

case 1:  no connection for "myfs" exists
    - need to create fuse_conn, sb

case 2: connection for "myfs" exists but only sb for submount

How would we know this sb isn't a root sb ?

    - only create sb for root, reuse fuse_conn

case 3: connection for "myfs" as well as root sb exists
   - reuse sb

I'll think about how to fix this properly, it's probably going to be
rather more involved...

Sure. BTW I'm wondering why we never reuse sbs for submounts ?

Thanks,
Miklos

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