[dm-devel] Re: [PATCH 2.6.19 5/5] fs: freeze_bdev with semaphore not mutex

[Rafael J. Wysocki]

On Wednesday, 8 November 2006 21:48, Nigel Cunningham wrote:
> Hi.
> 
> On Wed, 2006-11-08 at 13:10 +0100, Rafael J. Wysocki wrote:
> > On Wednesday, 8 November 2006 03:30, Alasdair G Kergon wrote:
> > > On Tue, Nov 07, 2006 at 11:49:51PM +0000, Alasdair G Kergon wrote:
> > > > I hadn't noticed that -mm patch.  I'll take a look.  
> > > 
> > > swsusp-freeze-filesystems-during-suspend-rev-2.patch
> > > 
> > > I think you need to give more thought to device-mapper
> > > interactions here.  If an underlying device is suspended
> > > by device-mapper without freezing the filesystem (the
> > > normal state) and you issue a freeze_bdev on a device
> > > above it, the freeze_bdev may never return if it attempts
> > > any synchronous I/O (as it should).
> > 
> > Well, it looks like the interactions with dm add quite a bit of
> > complexity here.
> > 
> > > Try:
> > >   while process generating I/O to filesystem on LVM
> > >   issue dmsetup suspend --nolockfs (which the lvm2 tools often do)
> > >   try your freeze_filesystems()
> > 
> > Okay, I will.
> > 
> > > Maybe: don't allow freeze_filesystems() to run when the system is in that
> > > state;
> > 
> > I'd like to avoid that (we may be running out of battery power at this point).
> > 
> > > or, use device-mapper suspend instead of freeze_bdev directly where 
> > > dm is involved;
> > 
> > How do I check if dm is involved?
> > 
> > > or skip dm devices that are already frozen - all with 
> > > appropriate dependency tracking to process devices in the right order.
> > 
> > I'd prefer this one, but probably the previous one is simpler to start with.
> 
> Shouldn't we just go for the right thing to begin with? Otherwise we'll
> just make more problems for ourselves later.
> 
> If we do this last one, I guess we want to do something like I was doing
> before (creating a list of the devices we've frozen)?

Well, the frozen superblocks have MS_FROZEN set.  What the other list is
needed for?

Rafael


-- 
You never change things by fighting the existing reality.
		R. Buckminster Fuller