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

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Message-ID: <CA+aCy1Esm-2NGvJFG4_22rNaXiOqx1f2beCPpSCcovrXFf07+A@mail.gmail.com>
Date:	Sat, 10 Jan 2015 01:01:19 +0530
From:	Pranay Srivastava <pranjas@...il.com>
To:	Lukáš Czerner <lczerner@...hat.com>
Cc:	Stefan Hajnoczi <stefanha@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>, kzak@...hat.com,
	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?

Hi

On Fri, Jan 9, 2015 at 9:22 PM, Lukáš Czerner <lczerner@...hat.com> wrote:
> On Fri, 9 Jan 2015, Stefan Hajnoczi wrote:
>
>> 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
>
> Because now you have two block devices backed by the same file.
> Which as Christoph pointed out is equivalent of accessing it from
> two different systems.
>
> What you probably want to do is this:
>
> # losetup --show -f /var/tmp/ext4.img
> /dev/loop0
>
> mount /dev/loop0 /tmp/a
> mount /dev/loop0 /tmp/b
>
> 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 ?

It's not the mount. If you see mount_bdev it checks for this and uses
the existing super block from the first mount
for all subsequent mounts of that block device.

When block devices differ, as in with 2 loop devices they have
different bdev and that's why it doesn't work with 2
loop devices.

>
> -Lukas
>
>
>>   # 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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