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

[Peter Zijlstra]

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@xxxxxxxxxxxxxx

> 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.