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

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