linux-kernel - Re: [RFC] usb: host: u132-hcd: Remove deprecated create_singlethread

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Date:	Mon, 1 Aug 2016 14:00:57 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Tejun Heo <tj@...nel.org>
cc:	Michal Hocko <mhocko@...nel.org>, Oliver Neukum <oneukum@...e.com>,
	Bhaktipriya Shridhar <bhaktipriya96@...il.com>,
	Geliang Tang <geliangtang@....com>,
	"GeyslanG.Bem@...yakshetra" <geyslan@...il.com>,
	Masanari Iida <standby24x7@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Saurabh Karajgaonkar <skarajga@...teon.com>,
	<linux-kernel@...r.kernel.org>, <linux-usb@...r.kernel.org>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [RFC] usb: host: u132-hcd: Remove deprecated create_singlethread_workqueue

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