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

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Date:   Fri, 4 Oct 2019 14:01:51 +0300
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Tejun Heo <tj@...nel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        AceLan Kao <acelan.kao@...onical.com>,
        Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: System hangs if NVMe/SSD is removed during suspend

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.