[CRIU] [PATCH] mount: handle a circular reference in mount tree

[Pavel Emelyanov]

On 09/12/2014 03:41 PM, Andrey Vagin wrote:
> $ cat /proc/self/mountinfo
> ...
> 1 1 0:2 / / rw - rootfs rootfs rw,size=373396k,nr_inodes=93349
> ...
> 
> You can see that mnt_id and parent_mnt_id are equals here.
> This patch interpretes this case as a root mount in a tree.

While I agree with the patch, I'd like to get some info about
how this all happens from the kernel objects perspective.

The kernel's show_mountinfo() prints this like
seq_printf(m, "%i %i %u:%u ", r->mnt_id, r->mnt_parent->mnt_id,

which means, that each mnt always has a parent.

We've typically seen the 0 as root's parent. Where did the kernel
get this mount from?


> Cc: beproject criu <beprojectcriu at gmail.com>
> Cc: Christopher Covington <cov at codeaurora.org>
> Reported-by: beproject criu <beprojectcriu at gmail.com>
> Reviewed-by: Christopher Covington <cov at codeaurora.org>
> Signed-off-by: Andrey Vagin <avagin at openvz.org>
> ---
>  mount.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/mount.c b/mount.c
> index bf27052..0509ee3 100644
> --- a/mount.c
> +++ b/mount.c
> @@ -251,7 +251,12 @@ static struct mount_info *mnt_build_ids_tree(struct mount_info *list)
>  		struct mount_info *p;
>  
>  		pr_debug("\t\tWorking on %d->%d\n", m->mnt_id, m->parent_mnt_id);
> -		p = __lookup_mnt_id(list, m->parent_mnt_id);
> +
> +		if (m->mnt_id != m->parent_mnt_id)
> +			p = __lookup_mnt_id(list, m->parent_mnt_id);
> +		else /* a circular mount reference. It's rootfs or smth like it. */
> +			p = NULL;
> +
>  		if (!p) {
>  			/* This should be / */
>  			if (root == NULL && is_root_mount(m)) {
>