lkml.org 
[lkml]   [2016]   [Aug]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC] usb: host: u132-hcd: Remove deprecated create_singlethread_workqueue
On Mon 01-08-16 14:00:57, Alan Stern wrote:
> 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.

Or even worse, silent hangs for GFP_NOIO requests. If the allocation
size that is issued from that context is not large (basically < order-4)
then the allocation would be retried basically for ever without invoking
the OOM killer. Now, this is rather unlikely to become a real problem
unless there is a serious flood of these GFP_NOIO allocation requests.
But the main point remains. GFP_NOIO doesn't guanratee a forward
progress. Success of such an allocation depends on on a different
context with the full reclaim capabilities (including the OOM killer).

--
Michal Hocko
SUSE Labs

\
 
 \ /
  Last update: 2016-08-01 22:01    [W:0.214 / U:0.480 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site