linux-ext4 - Re: Can one file system be R/W mounted multiple times in Linux?

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Message-ID: <alpine.LFD.2.00.1501121514550.32312@localhost.localdomain>
Date:	Mon, 12 Jan 2015 15:20:19 +0100 (CET)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	Karel Zak <kzak@...hat.com>
cc:	Stefan Hajnoczi <stefanha@...hat.com>,
	Al Viro <viro@...IV.linux.org.uk>,
	Dexuan Cui <decui@...rosoft.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"'linux-ext4'" <linux-ext4@...r.kernel.org>
Subject: Re: Can one file system be R/W mounted multiple times in Linux?

On Mon, 12 Jan 2015, Karel Zak wrote:

> Date: Mon, 12 Jan 2015 14:00:58 +0100
> From: Karel Zak <kzak@...hat.com>
> To: Lukáš Czerner <lczerner@...hat.com>
> Cc: Stefan Hajnoczi <stefanha@...hat.com>, Al Viro <viro@...IV.linux.org.uk>,
>     Dexuan Cui <decui@...rosoft.com>,
>     "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
>     'linux-ext4' <linux-ext4@...r.kernel.org>
> Subject: Re: Can one file system be R/W mounted multiple times in Linux?
> 
> On Fri, Jan 09, 2015 at 04:52:11PM +0100, Lukáš Czerner wrote:
> > Now the question is, whether 'mount' can be a bit smarter than that
> > and just mount the already existing block device instead of creating
> > new one ? Karel ?
> 
> Well, the question is how smart is too smart :-) And Al is right, problem
> is in races.
> 
> Anyway, I'd like to avoid complex FS specific code in mount(8). IMHO
> it's FS driver responsibility to avoid (or support) FS sharing between
> more block devices.
> 
> BTW, for ext4 you can enable ETX4_FEATURE_INCOMPAT_MMP to protect
> against multi-mount.
> 
>     Karel

Hi Karel,

that's not what I meant at all. Yes, things like MMP should help.
But what I was suggesting was not FS specific.

mount(8) could in case of '-o loop' look whether there is already a
block device associated with the file and if so use it instead of
creating new one.

However this might race with device removal among other things and I
am not sure whether there is any easy solution.

Thanks!
-Lukas