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

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Date:	Tue, 2 Aug 2016 10:18:01 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Oliver Neukum <oneukum@...e.com>
Cc:	Tejun Heo <tj@...nel.org>, Geliang Tang <geliangtang@....com>,
	Johannes Weiner <hannes@...xchg.org>,
	Bhaktipriya Shridhar <bhaktipriya96@...il.com>,
	"GeyslanG.Bem@...yakshetra" <geyslan@...il.com>,
	Masanari Iida <standby24x7@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Vlastimil Babka <vbabka@...e.cz>,
	Mel Gorman <mgorman@...hsingularity.net>,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
	Saurabh Karajgaonkar <skarajga@...teon.com>
Subject: Re: [RFC] usb: host: u132-hcd: Remove deprecated
 create_singlethread_workqueue

On Tue 02-08-16 10:06:12, Oliver Neukum wrote:
> On Mon, 2016-08-01 at 10:20 -0400, Tejun Heo wrote:
> > Hello,
> 
> > > 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
> 
> It cannot. The IO must be described to the hardware with a data
> structure in memory.
> 
> > 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?
> 
> We had actual deadlocks with GFP_KERNEL. It seems to me that the SCSI
> layer can deal with IO that cannot be completed due to a lack of memory
> at least somewhat, but a deadlock within a driver would obviously be
> deadly. So I don't think that mempools would remove the need for
> GFP_NOIO as there are places in usbcore we cannot enter the page
> laundering path from. They are an additional need.

OK, I guess there is some misunderstanding here. I believe that Tejun
wasn't arguing to drop GFP_NOIO. It might be really needed for the dead
lock avoidance. No question about that. The whole point is that
WQ_RECLAIM might be completely pointless because a rescuer wouldn't help
much if the work item would do GFP_NOIO and get stuck in the page
allocator.
-- 
Michal Hocko
SUSE Labs