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

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Date:   Thu, 11 Mar 2021 09:25:49 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Michal Hocko <mhocko@...e.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     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 3/11/21 4:02 AM, Michal Hocko wrote:
> 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.

Thanks you Michal and Peter.  This patch was mostly about starting a
discussion, as this topic came up in a couple different places.  I
included the 'train wreck' of how we got here just for a bit of history.

I'll start working on a proper fix.
-- 
Mike Kravetz