Re: [PATCH] Use of getblk differs between locations

From: Mikulas Patocka
Date: Wed Oct 12 2005 - 14:59:43 EST

Anton Altaparmakov wrote:
On Tue, 2005-10-11 at 00:49 +0200, Mikulas Patocka wrote:
On Mon, 10 Oct 2005, Anton Altaparmakov wrote:
What should a filesystem driver do if it can't suddenly read or write any
blocks on media?

Two clear choices:

1) Switch to read-only and use the cached data to fulfil requests and
fail all others.

2) Ask the user to insert the media/plug the device back in (this is by
far the most likely cause of all requests suddenly failing) and then
continue where they left off.

It is unfortunate that Linux does not allow for 2) so you need to do 1).

I recently looked into 2) a bit and the dm multipath code is almost
enough to do exactly this.

If you configure your block device as an mpath device that queues on
path failure, and change the table to the new device location on device
re-attach, the queued up i/o will be flushed out. Almost. Right now,
when you change the table and resume the dm mapping, it does a suspend
which attempts to write out the data to a device which is no longer
there, causing it to just be dropped on the floor. If this were changed
not to do that, and perhaps set a timer so that the dirty data wouldn't
be left around forever if the device wasn't reattached, 2) would
definitely be possible.

I realize that the userspace intervention required may involve a bit of
dark magic, but my point is most of the code required on the kernel side
is already implemented.

- -Jeff

Is memory management ready for this? Can't deadlock like this happen?
- displaying dialog window needs memory, so it waits until memory will be available
- system decides to write some write-back cached data in order to free memory
- the write of these data waits until the dialog window is displayed, user inserts the device and clicks 'OK'

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