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

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Mar 11, 2021 at 10:44:56AM +0100, Michal Hocko wrote:
> On Thu 11-03-21 10:32:24, Peter Zijlstra wrote:
> > The whole changelog reads like a trainwreck, but akpm already commented
> > on that. I picked out a small factual incorrectness, simply because if
> > you can't get that right, the whole argument looses weight.
> 
> Is there any reason why in_atomic || irq_disabled wouldn't work
> universally?

I just explained to you how you really wanted:

  in_atomic() && !irq_disabled()

> > That said, I don't think you actually need it, if as you write the lock
> > should be IRQ-safe, then you're worried about the IRQ recursion
> > deadlock:
> 
> making hugetlb_lock irqsafe is a long way as explained by Mike
> elsewhere. Not only that. The upcoming hugeltb feature to have sparse
> vmemmap for hugetlb pages will need to allocate vmemmap when hugetlb
> page is to be freed back to the allocator. That cannot happen in any
> atomic context so there will be a need to tell those contexts for
> special casing.

Then scrap the vmemmap code *NOW*. Do not merge more shit before fixing
existing problems. Especially not if that's known to make it harder to
fix the problems.