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

Paul E. McKenney
2020-08-15T22:10:56Z

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;

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