Re: [PATCH 0/5] Add trace events for filesystem freeze/thaw events

[Dave Chinner <david@fromorbit.com>]

On Wed, Mar 02, 2016 at 11:22:54PM +0000, Al Viro wrote:
> On Thu, Mar 03, 2016 at 09:47:30AM +1100, Dave Chinner wrote:
> 
> > > So let's get it right.  Preferably - without need for boot-to-shutdown
> > > tracking just to mirror the state.  What do we really want?
> > > 
> > > * an ioctl to query the state (frozen/freezing/not frozen) for something in
> > > util-linux to use?
> > > 
> > > * /proc/fs/freezing and /proc/fs/frozen, with ->s_id of affected filesystems
> > > or, pehaps, one file with (frozen|freezing) + ->s_id?
> > > 
> > > * ability to audit on state changes?  That'd need some thought re what to
> > > do when some joker freezes the fs syslogd is logging to...
> > > 
> > > * something else entirely?
> > 
> > Such as:
> > 
> > https://lkml.org/lkml/2015/6/16/456
> 
> IIRC, there had been a weird use of vfsmounts as proxies for superblocks and
> even more strange requirements along the lines "you should just pick one and
> use only it in case if there's more than one mountpoint for this fs".  It's
> been a while, though, so I might've forgotten if that stuff got resolved in
> later thread...
> 
> Lifetime rules are really odd there; the object is killed as we are about
> to gut vfsmount for good, even though the code in there appears to assume
> that it would stay connected to mount tree all the way until that point.
> Again, that might have been resolved later; I really can't recall right now...

Agreed, there were lots of unresolved problems with that patch set
(e.g. the duplicate space accounting infrastructure, rather than
just calling vfs_statfs() to grab the free/used space from the
filesystems).

I was just making the point that we really need a generic fs event
mechanism, not just something specific to freeze/unfreeze. I've kind
of been waiting for the kdbus stuff to work itself out, because if
there's going to be a generic kernel-wide event mechanism added to
the kernel, we should just be using that...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com