linux-kernel - Re: [PATCH 38/41] union-mount: Make pivot

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Message-Id: <200912010426.nB14QUt0030633@agora.fsl.cs.sunysb.edu>
Date:	Mon, 30 Nov 2009 23:26:30 -0500
From:	Erez Zadok <ezk@...sunysb.edu>
To:	Valerie Aurora <vaurora@...hat.com>
Cc:	Jan Blunck <jblunck@...e.de>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	Andy Whitcroft <apw@...onical.com>,
	Scott James Remnant <scott@...onical.com>,
	Sandu Popa Marius <sandupopamarius@...il.com>,
	Jan Rekorajski <baggins@...h.mimuw.edu.pl>,
	"J. R. Okajima" <hooanon05@...oo.co.jp>,
	Arnd Bergmann <arnd@...db.de>,
	Vladimir Dronnikov <dronnikov@...il.com>,
	Felix Fietkau <nbd@...nwrt.org>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 38/41] union-mount: Make pivot_root work with union mounts 

In message <1256152779-10054-39-git-send-email-vaurora@...hat.com>, Valerie Aurora writes:
> When moving a union mount, follow it down to the bottom layer and move
> that instead of just the top layer.
> 
> Signed-off-by: Valerie Aurora <vaurora@...hat.com>
> ---
>  fs/namespace.c |    9 +++++++++
>  1 files changed, 9 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 9b71743..6ac5fc1 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2282,6 +2282,15 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
>  	if (d_unlinked(old.dentry))
>  		goto out2;
>  	error = -EBUSY;
> +	/*
> +	 * follow_union_down() only goes one layer down.  We want the
> +	 * bottom-most layer here - if we move that around, all the
> +	 * layers on top move with it.  But if we ever allow more than
> +	 * two layers, the below two will both need to be in while()
> +	 * loops.
> +	 */

Given the nature of this comment, I'd stick an "XXX" there for easier
ability to grep-the-src-for-issues.

> +	follow_union_down(&new.mnt, &new.dentry);
> +	follow_union_down(&root.mnt, &root.dentry);
>  	if (new.mnt == root.mnt ||
>  	    old.mnt == root.mnt)
>  		goto out2; /* loop, on the same file system  */
> -- 
> 1.6.3.3
> 
> --
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Erez.
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