Re: [xfs-masters] Re: freeze vs freezer

[Dave Chinner <david@fromorbit.com>]

On Sun, Jun 29, 2008 at 11:37:31PM -0700, Jeremy Fitzhardinge wrote:
> Dave Chinner wrote:
>> On Mon, Jun 30, 2008 at 01:22:47AM +0200, Rafael J. Wysocki wrote:
>>> On Monday, 30 of June 2008, Dave Chinner wrote:
>>>> On Thu, Jun 26, 2008 at 05:09:10PM +0200, Pavel Machek wrote:
>>>>>>> Is this the same thing the per-device IO-queue-freeze patches for
>>>>>>> HDAPS also
>>>>>>> need to do?  If so, you may want to talk to Elias Oltmanns
>>>>>>> <eo@nebensachen.de> about it.  Added to CC.
>>>>>>>             
>>>>>> Thanks for the heads up Henrique. Even though these issues seem to be
>>>>>> related up to a certain degree, there probably are some important
>>>>>> differences. When suspending a system, the emphasis is on leaving the
>>>>>> system in a consistent state (think of journalled file systems), whereas
>>>>>> disk shock protection is mainly concerned with stopping I/O as soon as
>>>>>> possible. As yet, I cannot possibly say to what extend these two
>>>>>> concepts can be reconciled in the sense of sharing some common code.
>>>>>>           
>>>>> Actually, I believe requirements are same.
>>>>>
>>>>> 'don't do i/o in dangerous period'.
>>>>>
>>>>> swsusp will just do sync() before entering dangerous period. That
>>>>> provides consistent-enough state...
>>>>>         
>>>> As I've said many times before - if the requirement is "don't do
>>>> I/O" then you have to freeze the filesystem. In no way does 'sync'
>>>> prevent filesystems from doing I/O.....
>>>>       
>>> Well, it seems we can handle this on the block layer level, by temporarily
>>> replacing the elevator with something that will selectively prevent fs I/O
>>> from reaching the layers below it.
>>
>> Why? What part of freeze_bdev() doesn't work for you?
>
> Well, my original problem - which is still an issue - is that a process  
> writing to a frozen XFS filesystem is stuck in D state, and therefore  
> cannot be frozen as part of suspend.

Silly me - how could I forget the three headed monkey getting in
the way of our happy trip to beer island?

Seriously, though, how is stopping I/O in the elevator is going to
change that?  What do you do with a sync I/O (read or write)? The
process is going to have to go to sleep somewhere in D state waiting
for that I/O to complete.  If you're going to intercept such
processes somewhere else to do something magic, then why not put
that magic in vfs_check_frozen()?

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com