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

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Date:   Thu, 11 Mar 2021 12:36:51 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Michal Hocko <mhocko@...e.com>
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, 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.

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.