ACK: [PATCH][SRC][XENIAL] UBUNTU: SAUCE: Allow mounting datasets more than once (LP: #1759848)

[Seth Forshee]

On Mon, Jun 25, 2018 at 08:36:41PM +0100, Colin King wrote:
> From: Colin Ian King <colin.king at canonical.com>
> 
> BugLink: https://bugs.launchpad.net/bugs/1759848
>     
> Currently mounting an already mounted zfs dataset results in an
> error, whereas it is typically allowed with other filesystems.
> This causes some bad interactions with mount namespaces. Take
> this sequence for example:
>     
>  - Create a dataset
>  - Create a snapshot of the dataset
>  - Create a clone of the snapshot
>  - Create a new mount namespace
>  - Rename the original dataset
>     
> The rename results in unmounting and remounting the clone in the
> original mount namespace, however the remount fails because the
> dataset is still mounted in the new mount namespace. (Note that
> this means the mount in the new mount namespace is never being
> unmounted, so perhaps the unmount/remount of the clone isn't
> actually necessary.)
> 
> The problem here is a result of the way mounting is implemented
> in the kernel module. Since it is not mounting block devices it
> uses mount_nodev() instead of the usual mount_bdev(). However,
> mount_nodev() is written for filesystems for which each mount is
> a new instance (i.e. a new super block), and zfs should be able
> to detect when a mount request can be satisfied using an existing
> super block.
>    
> Change zpl_mount() to call sget() directly with it's own test
> callback. Passing the objset_t object as the fs data allows
> checking if a superblock already exists for the dataset, and in
> that case we just need to return a new reference for the sb's
> root dentry.
>     
> [ Sync'd from zfsutils-linux, from a patch by Seth Forshee and
>   backported to zfs 0.6.5.6. Note that this also contains
>   some zfstutils changes between 0.6.5.6-0ubuntu20 and 
>   0.6.5.6-0ubuntu24 which go also sync'd into this fix, which
>   is expected part of the zfs sync'ing ]
>     
> Signed-off-by: Colin Ian King <colin.king at canonical.com>

Backport looks good.

Acked-by: Seth Forshee <seth.forshee at canonical.com>