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

[Jan Kara]

On Fri 04-10-19 07:32:40, Jens Axboe wrote:
> On 10/4/19 5:01 AM, Mika Westerberg wrote:
> > 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.
> 
> Wow, a decade ago...
> 
> Honestly, I don't recall why these were marked freezable, and as I wrote
> in the other reply, I don't think there's a good reason for that to be
> the case.

Well, cannot it happen that the flush worker will get stuck in D state
because some subsystem is already suspended and thus hibernation fails
(because AFAIK processes in uninterruptible sleep block hibernation)?

I was also somewhat worried that the hibernation image could be
inconsistent if flush workers do something while hibernation image is being
taken but that does not seem to be a valid concern as all kernel processes
get frozen before hibernation image is taken.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR