Re: mount does not accurately identify existing NFS mounts, and shouldn't try.

[Neil Brown <neilb@suse.com>]

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On Fri, Oct 30 2015, Karel Zak wrote:

> On Mon, Oct 19, 2015 at 12:08:41PM +1100, Neil Brown wrote:
>> 	/* Refuse the same filesystem on the same mount point */
>> 	err = -EBUSY;
>> 	if (path->mnt->mnt_sb == newmnt->mnt.mnt_sb &&
>> 	    path->mnt->mnt_root == path->dentry)
>> 		goto unlock;
>> 
>> This is the *only* time that the kernel returns EBUSY for a "mount a new
>> filesystem" request (remount can return EBUSY for other reasons I
>> think).
>> 
>> So in the case where "mount" thinks it should check if the filesystem is
>> already mounted and ignore the request if it is, the best thing it can
>> do is to just try the mount and ignore any "EBUSY" error status - treat
>> that the same as success.
>> 
>> So all that code for "is this the same mountpoint" can probably be
>> discarded.
>
> Well, we have many situations (another than mount -a) when you want to
> know if the FS is mounted, but you have not permissions for mount(2)
> or you don't want to try it (because mount(2) may trigger many another
> things in userspace).
>
>> Did I miss any important consideration there?
>
> The problem is that the way between fstab entry and mount(2) is
> sometimes pretty complex (mount.<type> helpers, loop devices,
> UUID/LABEL translation, etc).
>
> So, compare fstab with /proc/mounts is usually good compromise...
>
> Your suggestion seems usable for mount -a, but it's one small use-case
> (and for example systemd don't use "mount -a" during boot).
>
>> Unfortunately this change could be awkward to implement as /sbin/mount
>> decides whether to try the mount and (for NFS), /sbin/mount.nfs gives an
>> error message if EBUSY is returned.  Would we need to pass a "noebusy"
>> flag, but only if mount.nfs were willing to accept it....
>> 
>> Any suggestions?
>
> We have kill(PID, 0) to detect if the PID is valid, you don't have to
> parse all /proc. It would be nice to have the same for filesystems,
> something like
>
>     mount("/dev/sda1, "/mnt", "ext4", MS_NOMOUNT, NULL)

I like that!  I really do.  It returns the same error as mount would if
run by a privileged uses, or returns success without actually changing
anything.
MS_NOMOUNT is incompatible with MS_REMOUNT, MS_MOVE and several others.

I won't be providing a kernel patch straight away, but I would like to
see this happen.

Thanks,
NeilBrown

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