linux-kernel - Re: [PATCH] hugetlb: select PREEMPT_COUNT if HUGETLB_PAGE for in

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Date:   Thu, 11 Mar 2021 12:09:15 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>, tglx@...utronix.de,
        john.ogness@...utronix.de, urezki@...il.com, ast@...com,
        Eric Dumazet <edumazet@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] hugetlb: select PREEMPT_COUNT if HUGETLB_PAGE for
 in_atomic use

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