linux-kernel - Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE

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Date:   Wed, 10 Oct 2018 14:06:22 +0100
From:   Alan Jenkins <alan.christopher.jenkins@...il.com>
To:     David Howells <dhowells@...hat.com>, viro@...iv.linux.org.uk
Cc:     torvalds@...ux-foundation.org, ebiederm@...ssion.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        mszeredi@...hat.com
Subject: Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE
 [ver #12]

On 10/10/2018 14:02, David Howells wrote:
> The attached change seems to fix the lazy-umount problem.
>
> David
> ---
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 5adeeea2a4d9..d43f0fa152e9 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2472,7 +2472,7 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
>   	if (old->mnt_ns && !attached)
>   		goto out1;
>   
> -	if (old->mnt.mnt_flags & MNT_LOCKED)
> +	if (old->mnt.mnt_flags & (MNT_LOCKED | MNT_UMOUNT))
>   		goto out1;
>   
>   	if (old_path->dentry != old_path->mnt->mnt_root)

I can't test any more at the moment, as my laptop died today :). But I 
have no objection to this.

It would be more fun if there was a way to support it :), but I don't 
have a genuine reason to want it.  And you couldn't use it for fully 
general purposes anyway, because umount2( , MNT_DETACH) is defined as 
separating all the child mounts.

P.S. Regarding the issue with the namespace loop.  My strawman solution 
would be for graft_tree() to silently detach any NS file mounts that 
have a sequence number less than or equal to the namespace they are 
being mounted into.