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

[Alasdair G Kergon]

On Fri, Nov 10, 2006 at 12:11:46AM +0100, Pavel Machek wrote:
> ? Not sure if I quite understand, but if dm breaks sync... something
> is teribly wrong with dm. And we do simple sys_sync()... so I do not
> think we have a problem.
 
If you want to handle arbitrary kernel state, you might have a device-mapper
device somewhere lower down the stack of devices that is queueing any I/O
that reaches it.  So anything waiting for I/O completion will wait until 
the dm process that suspended that device has finished whatever it is doing
- and that might be a quick thing carried out by a userspace lvm tool, or
a long thing carried out by an administrator using dmsetup.

I'm guessing you need a way of detecting such state lower down the stack
then optionally either aborting the operation telling the user it can't be
done at present; waiting for however long it takes (perhaps for ever if
the admin disappeared); or more probably skipping those devices on a 
'best endeavours' basis.

I'm suggesting sysfs or something built on bd_claim might offer a mechanism 
for traversing the stack.

Alasdair
-- 
agk at redhat.com