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

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Date:	Fri, 9 Jan 2015 12:01:59 +0000
From:	Stefan Hajnoczi <stefanha@...hat.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	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 03:51:23AM +0000, Al Viro wrote:
> On Fri, Jan 09, 2015 at 03:44:16AM +0000, Dexuan Cui wrote:
> > The 'mount' utility allows me to mount 1 file systems multiple times at different
> > mount points, like "mount /dev/sdb1 /a; mount /dev/sdb1 /b".
> > 
> > I tried to write from /a and /b at the same time and it seems everything is OK
> > and no data corruption happens.
> > I tried only  ext4 only.
> > 
> > Can somebody please tell me if this usage is safe?
> 
> Yes.

Can you explain under what conditions mounting the same file system
twice will work?

I guess the kernel is looking up the block device and then sharing the
superblock if the file system is already mounted on this block device?

If I use two loop devices for the same underlying storage it does not
work:

  # mount -o loop /var/tmp/ext4.img /tmp/a
  # mount -o loop /var/tmp/ext4.img /tmp/b
  # touch /tmp/a/a
  # ls /tmp/b
  lost+found
  # umount /tmp/a
  # umount /tmp/b
  # mount -o loop /var/tmp/ext4.img /tmp/a
  [1078357.297245] EXT4-fs error (device loop0): ext4_lookup:1441: inode #2: comm ls: deleted inode referenced: 12

Stefan

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