Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

Paul E. McKenney
2020-08-15T14:23:28Z

On Sat, Aug 15, 2020 at 07:18:39AM -0700, Paul E. McKenney wrote:
> On Sat, Aug 15, 2020 at 10:42:50AM +0200, Peter Zijlstra wrote:
> > On Sat, Aug 15, 2020 at 01:14:53AM +0200, Thomas Gleixner wrote:
> >
> > > #1 trivial fix is to force switching to an high prio thread or a soft
> > > interrupt which does the allocation
> >
> > Yeah, push the alocation out to another context. I did consider it, but
> > why bother?
> >
> > Also, raising a softirq can't be done from every context, that's a whole
> > new problem. You can do irq_work I suppose, but not all architectures
> > support the self-IPI yet.
> >
> > All in all, it's just more complexity than the fairly trivial
> > __alloc_page_lockless().
> >
> > Whichever way around, we can't rely on the allocation.
>
> One way to enforce that would be to put something like this at the
> beginning of the __alloc_page_lockless() function:
>
> if (IS_ENABLED(CONFIG_PROVE_LOCKING) && (prandom_u32() & 0xffff))
> return NULL;

Right, too early in the morning. :-/

This "slight" variation might include a bit of usefulness along with
the convincing:

if (IS_ENABLED(CONFIG_PROVE_LOCKING) && !(prandom_u32() & 0xff))
return NULL;

Plus failing one out of 256 times is likely a better choice than once
out of 65536 times, especially for the occasional caller of this
function.

Thanx, Paul

> I am sure that there is a better choice than CONFIG_PROVE_LOCKING.
> But whatever the choice, there is nothing quite like the occasional
> allocation failure during testing to convince people that such failure
> really can happen.
>
> Thanx, Paul