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

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Date:   Thu, 11 Mar 2021 13:02:17 +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 12:36:51, Peter Zijlstra wrote:
> On Thu, Mar 11, 2021 at 12:09:15PM +0100, Michal Hocko wrote:
> 
> > 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
> 
> What I'm saying is that if irq_disabled(), the that interrupt cannot
> happen, so the second spin_lock cannot happen, so the deadlock cannot
> happen.
> 
> So: '!irqs_disabled() && in_atomic()' is sufficient to avoid the IRQ
> recursion deadlock.

OK, then I understand your point now. I thought you were arguing
an actual deadlock scenario. As I've said irq_disabled check would be
needed for sleeping operations that we already do.

> Also, Linus hates constructs like this:
> 
>   https://lkml.kernel.org/r/CAHk-=wht7kAeyR5xEW2ORj7m0hibVxZ3t+2ie8vNHLQfdbN2_g@mail.gmail.com
> 
> > 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.
> 
> Just to be clear:
> 
> NAK on this patch and any and all ductape crap. Fix it properly, make
> hugetlb_lock, spool->lock IRQ-safe, move the workqueue into the CMA
> thing.
> 
> The code really doesn't look _that_ complicated.

Fair enough. As I've said I am not a great fan of this patch either
but it is a quick fix for a likely long term problem. If reworking the
hugetlb locking is preferable then be it.
-- 
Michal Hocko
SUSE Labs