Re: [dm-devel] [PATCH] Fix over-zealous flush_disk when changing device size.

[Christoph Hellwig <hch infradead org>]

On Thu, Feb 17, 2011 at 04:50:57PM +1100, NeilBrown wrote:
> 
> Hi Andrew (and others)
>  I wonder if you would review the following for me and comment.

Please send think in this area through -fsdevel next time, thanks!

> There are two cases when we call flush_disk.
> In one, the device has disappeared (check_disk_change) so any
> data will hold becomes irrelevant.
> In the oter, the device has changed size (check_disk_size_change)
> so data we hold may be irrelevant.
> 
> In both cases it makes sense to discard any 'clean' buffers,
> so they will be read back from the device if needed.

Does it?  If the device has disappeared we can't read them back anyway.
If the device has resized to a smaller size the same is true about
those buffers that have gone away, and if it has resized to a larger
size invalidating anything doesn't make sense at all.  I think this
area needs more love than a quick kill_dirty hackjob.

> In the former case it makes sense to discard 'dirty' buffers
> as there will never be anywhere safe to write the data.  In the
> second case it *does*not* make sense to discard dirty buffers
> as that will lead to file system corruption when you simply enlarge
> the containing devices.

Doing anything like this at the buffer cache layer or inode cache layer
doesn't make any sense.  If a device goes away or shrinks below the
filesystem size the filesystem simply needs to be shut down and in te
former size the admin needs to start a manual repair.  Trying to do
any botch jobs in lower layer never works in practice.

For now I think the best short term fix is to simply revert commit
608aeef17a91747d6303de4df5e2c2e6899a95e8

	"Call flush_disk() after detecting an online resize."