Re: System hangs if NVMe/SSD is removed during suspend

[Mika Westerberg <mika.westerberg@linux.intel.com>]

On Fri, Oct 04, 2019 at 11:59:26AM +0200, Rafael J. Wysocki wrote:
> On Friday, October 4, 2019 10:03:40 AM CEST Mika Westerberg wrote:
> > On Thu, Oct 03, 2019 at 09:50:33AM -0700, Tejun Heo wrote:
> > > Hello, Mika.
> > > 
> > > On Wed, Oct 02, 2019 at 03:21:36PM +0300, Mika Westerberg wrote:
> > > > but from that discussion I don't see more generic solution to be
> > > > implemented.
> > > > 
> > > > Any ideas we should fix this properly?
> > > 
> > > Yeah, the only fix I can think of is not using freezable wq.  It's
> > > just not a good idea and not all that difficult to avoid using.
> > 
> > OK, thanks.
> > 
> > In that case I will just make a patch that removes WQ_FREEZABLE from
> > bdi_wq and see what people think about it :)
> 
> I guess that depends on why WQ_FREEZABLE was added to it in the first place. :-)
> 
> The reason might be to avoid writes to persistent storage after creating an
> image during hibernation, since wqs remain frozen throughout the entire
> hibernation including the image saving phase.

Good point.

> Arguably, making the wq freezable is kind of a sledgehammer approach to that
> particular issue, but in principle it may prevent data corruption from
> occurring, so be careful there.

I tried to find the commit that introduced the "freezing" and I think it
is this one:

  03ba3782e8dc writeback: switch to per-bdi threads for flushing data

Unfortunately from that commit it is not clear (at least to me) why it
calls set_freezable() for the bdi task. It does not look like it has
anything to do with blocking writes to storage while entering
hibernation but I may be mistaken.