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

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Date:   Thu, 11 Mar 2021 10:52:50 +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 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.