Re: [PATCH] hugetlb: select PREEMPT_COUNT if HUGETLB_PAGE for in_atomic use

[Michal Hocko <mhocko@suse.com>]

On Thu 11-03-21 10:52:50, Peter Zijlstra wrote:
> On Thu, Mar 11, 2021 at 10:44:56AM +0100, Michal Hocko wrote:
> > On Thu 11-03-21 10:32:24, Peter Zijlstra wrote:
> > > The whole changelog reads like a trainwreck, but akpm already commented
> > > on that. I picked out a small factual incorrectness, simply because if
> > > you can't get that right, the whole argument looses weight.
> > 
> > Is there any reason why in_atomic || irq_disabled wouldn't work
> > universally?
> 
> I just explained to you how you really wanted:
> 
>   in_atomic() && !irq_disabled()

Sorry for being dense but I do not follow. You have provided the
following example
  spin_lock(&A);
  <IRQ>
        spin_lock(&A);

if A == hugetlb_lock then we should never reenter with
free_huge_page
	if (in_atomic() || irq_disabled())
		schedule_in_wq();
	else
		free_directly()

because hugetlb_lock is never held in irq context other than from
put_page (aka the above) path which will explicitly defer the handling and
thus the lock to a different context. We need to check for irq_disabled
because of the sleeping paths in the freeing path. Or do I miss
something?

From the code simplicity POV (and hugetlb has grown a lot of complexity)
it would be really easiest to make sure __free_huge_page to be called
from a non-atomic process context. There are few ways to do that
- defer each call to a WQ - user visible which sucks
- defer from atomic or otherwise non-sleeping contextx - requires
  reliable in_atomic AFAICS
- defer sleeping operations - makes the code flow more complex and it
  would be again user visible in some cases.

So I would say we are in "pick your own poison" kind of situation.
-- 
Michal Hocko
SUSE Labs