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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