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

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Date:	Mon, 12 Jan 2015 17:47:05 +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 Mon, Jan 12, 2015 at 03:20:19PM +0100, Lukáš Czerner wrote:
> that's not what I meant at all. Yes, things like MMP should help.
> But what I was suggesting was not FS specific.

And what do you want to do when somebody will implement any
cluster-like or multi-root filesystem where share the same FS image
between two loop devices is expected and supported feature?

I don't think there is any such generic VFS restriction, it's about FS
driver(s).

> 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.
 
mount(8) already checks if the same backing file is not mounted on the
same mountpoint

 # mount -o loop fs.img /mnt/A
 # mount -o loop fs.img /mnt/A
 mount: fs.img is already mounted

but I don't want to make decision about anything else in userspace.

    Karel

-- 
 Karel Zak  <kzak@...hat.com>
 http://karelzak.blogspot.com
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