linux-kernel - Re: [PATCH 2.6.19 5/5] fs: freeze

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Date:	Wed, 8 Nov 2006 15:25:33 +0000
From:	Alasdair G Kergon <agk@...hat.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Alasdair G Kergon <agk@...hat.com>, David Chinner <dgc@....com>,
	Eric Sandeen <sandeen@...hat.com>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, Ingo Molnar <mingo@...e.hu>,
	Srinivasa DS <srinivasa@...ibm.com>
Subject: Re: [PATCH 2.6.19 5/5] fs: freeze_bdev with semaphore not mutex

On Wed, Nov 08, 2006 at 03:43:26PM +0100, Rafael J. Wysocki wrote:
> Will it be enough to cover the interactions with dm?

There are cases where you *cannot* freeze the filesystem (unless
you wait for userspace processes to finish what they are doing) -
and only dm can tell you that.

The freeze_filesystems() code ought to do it's best in any given 
circumstances within the constraints.

So:
  Get the filesystem's block device.
  Check the full tree of devices that that block device depends upon
and for any device that belongs to device-mapper check if it is suspended.
If it is, skip the original device.

  struct mapped_device *dm_get_md(dev_t dev);
  int dm_suspended(struct mapped_device *md);
  void dm_put(struct mapped_device *md);

Handling the tree is the difficult bit, but sysfs could help.
[You can get the device-mapper dependencies with:
  struct mapped_device *dm_get_md(dev_t dev);
  struct dm_table *dm_get_table(struct mapped_device *md);
  struct list_head *dm_table_get_devices(struct dm_table *t);
  void dm_table_put(struct dm_table *t);
  void dm_put(struct mapped_device *md);
]

Consider that you could have an already-frozen filesystem containing a loop
device containing a device-mapper device containing a not-frozen filesystem.
You won't be able to freeze that top filesystem because the I/O would
queue lower down the stack.  (Similar problem if the device-mapper device
in the stack was suspended.)

Alasdair
-- 
agk@...hat.com
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