Re: XFS and multiple mounts protection (aka preventing multiple mounts)

[Carlos Maiolino <cmaiolino@redhat.com>]

On Fri, May 05, 2017 at 01:52:53PM +0200, Gionatan Danti wrote:
> 
> 
> On 05/05/2017 13:32, Carlos Maiolino wrote:
> > 
> > This is not true, XFS can't identify mounts on different systems. It is not a
> > shared or clustered filesystem.
> > 
> 
> Ok, this confirm my finding.
> 
> To reiterate: there is *no* method to prevent multiple mounts in XFS, right?

As far as I know, don't, I can always be wrong, but, I re-checked xfs
superblock, and could not find anything that actually protects it.

> 
> > At the worst case, it would need to store something to disk saying the
> > filesystem is already mounted, and the mount process would need to read it
> > before actually mounting the filesystem. Nothing though would prevent a race
> > between two systems, or even make the filesystem unmountable after a crash,
> > needing something like xfs_repair to clean some "is_mounted" flag, which would
> > trash one of the reasons why we have a journal (fast recovery without needing an
> > fsck before mounting).
> > 
> 
> EXT4 uses a "keepalive" approach: enabling the "mmp" feature (which require
> kernel 3.10+), a specific on-disk structure is continuously (each 5 seconds,
> by default) updated with a timestamp by the mounting machine. If another
> machine tries to mount the filesystem, it sees the mmp structure changing
> and it refuse to mount.
> 

> It's not perfect, but better than nothing ;)

Eh, I really don't know much about EXT4, but this only works if the clocks are
properly adjusted (not really hard to achieve though), but still racy.

cheers.

-- 
Carlos