Re: [PATCH] Use of getblk differs between locations

[Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>]

> 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'

Mikulas
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