Re: Duplicate network filesystems

[NeilBrown <neilb@suse.com>]

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On Mon, Feb 11 2019, Aurélien Aptel wrote:

> Hi,
>
> In fs/namespace.c do_add_mount() we do this check:
>
> 	/* 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;
>
> So that mount fails with EBUSY. But for networked filesystems (at least
> cifs and nfs) you can do this:
>
> mount //foo /mnt -o A
> mount //foo /mnt -o B # different options
>
> Since the SB are different it works, fine.
>
> But mounting a 3rd time with options A succeeds, where from a user POV I
> would have expected to fail.

Why? 
>
> So to recap:
>
> mount //foo /mnt -o A
> mount //foo /mnt -o A
> # EBUSY => expected behaviour
>
> mount //foo /mnt -o A
> mount //foo /mnt -o B
> # ok => expected behaviour
>
> mount //foo /mnt -o A
> mount //foo /mnt -o B
> mount //foo /mnt -o A
> # ok => what?
>
> Shouldn't we check the stack of filesystems mounted at the path instead of
> just the last one?

Why?

I think that the main reason that -EBUSY is important is that people
often run "mount -a" and don't expect filesystems that are already
mounted to be mounted again.  The current behaviour achieves that.

What is your use-case for wanted -EBUSY in some other circumstance?

NeilBrown

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