Re: [RFC] usb: host: u132-hcd: Remove deprecated create_singlethread_workqueue

From: Alan Stern
Date: Mon Aug 01 2016 - 15:02:45 EST


On Mon, 1 Aug 2016, Tejun Heo wrote:

> Hello,
>
> On Mon, Aug 01, 2016 at 03:50:36PM +0200, Michal Hocko wrote:
> > > All that would do is deferring the deadlock, right? I'm not sure it
> > > makes a lot of sense to protect an IO path against memory pressure
> > > half-way. It either can be depended during memory reclaim or it
> > > can't.
> >
> > Completely agreed! If the rescuer thread can block on a memory
> > allocation be it GFP_NOIO or others it is basically useless.
> ...
> > > Can MM people please chime in? The question is about USB stoage
> > > devices and memory reclaim. USB doesn't guarantee forward progress
> > > under memory pressure but tries a best-effort attempt with GFP_NOIO
> > > and ATOMIC. Is this the right thing to do?
> >
> > If any real IO depends on those devices then this is not sufficient and
> > they need some form of guarantee for progress (aka mempool).
>
> Oliver, Alan, what do you think? If USB itself can't operate without
> allocating memory during transactions, whatever USB storage drivers
> are doing isn't all that meaningful. Can we proceed with the
> workqueue patches? Also, it could be that the only thing GFP_NOIO and
> GFP_ATOMIC are doing is increasing the chance of IO failures under
> memory pressure. Maybe it'd be a good idea to reconsider the
> approach?

I agree that USB's approach to memory allocation won't prevent failures
when there is severe pressure.

However, is it possible that the _type_ of failure might be more
transparent? With GFP_NOIO, you end up with a cascading series of
allocation errors and it's obvious that something has gone very wrong.
If we were to switch to GFP_KERNEL, page-out could lead to deadlock
with no diagnostics (except perhaps for a watchdog warning).

Regardless, I would like to hear Oliver's thoughts.

Alan Stern