[dm-devel] Re: [PATCH 3/3] Add timeout feature

[Dave Chinner]

On Wed, Jul 09, 2008 at 01:20:31AM +0200, Pavel Machek wrote:
> On Wed 2008-07-09 09:10:27, Dave Chinner wrote:
> > On Mon, Jul 07, 2008 at 01:07:31PM +0200, Pavel Machek wrote:
> > > Hi!
> > > 
> > > > I still disagree with this whole patch.  There is not reason to let
> > > > the freeze request timeout - an auto-unfreezing will only confuse the
> > > > hell out of the caller.  The only reason where the current XFS freeze
> > > > call can hang and this would be theoretically useful is when the
> > > 
> > > What happens when someone dirties so much data that vm swaps out
> > > whatever process that frozen the filesystem?
> > 
> > a) you can't dirty a frozen filesystem - by definition a frozen
> >    filesystem is a *clean filesystem* and *cannot be dirtied*.
> 
> Can you stop me?
> 
> mmap("/some/huge_file", MAP_SHARED);
> 
> then write to memory mapping?

Sure - we can put a hook in ->page_mkwrite() to prevent it.  We
don't right now because nobody in the real world really cares if one
half of a concurrent user data change is in the old snapshot or the
new one......

> > b) Swap doesn't write through the filesystem
> > c) you can still read from a frozen filesystem to page your
> >    executable?? in.
> 
> atime modification should mean dirty data, right?

Metadata, not data. If that's really a problem (and it never has
been for XFS because we always allow in memory changes to atime)
then touch_atime could be easily changed to avoid this...

> And dirty data mean
> memory pressure, right? 

If you walk enough inodes while the filesystem is frozen, it
theoretically could happen.  Typically a filesystem is only for a
few seconds at a time so in the real world this has never, ever been
a problem.

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com