linux-kernel - Re: [patch V2 00/18] mm/highmem: Preemptible variant of kmap

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Date:   Fri, 30 Oct 2020 00:41:39 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul McKenney <paulmck@...nel.org>,
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        Daniel Vetter <daniel@...ll.ch>,
        Ard Biesheuvel <ardb@...nel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Christoph Hellwig <hch@....de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
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        Max Filippov <jcmvbkbc@...il.com>,
        linux-xtensa@...ux-xtensa.org
Subject: Re: [patch V2 00/18] mm/highmem: Preemptible variant of kmap_atomic & friends

On Thu, Oct 29 2020 at 16:11, Linus Torvalds wrote:
> On Thu, Oct 29, 2020 at 3:32 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>>
>> Though I wanted to share the current state of affairs before investigating
>> that further. If there is consensus in going forward with this, I'll have a
>> deeper look into this issue.
>
> Me likee. I think this looks like the right thing to do.
>
> I didn't actually apply the patches, but just from reading them it
> _looks_ to me like you do the migrate_disable() unconditionally, even
> if it's not a highmem page..
>
> That sounds like it might be a good thing for debugging, but not
> necessarily great in general.
>
> Or am I misreading things?

No, you're not misreading it, but doing it conditionally would be a
complete semantical disaster. kmap_atomic*() also disables preemption
and pagefaults unconditionaly.  If that wouldn't be the case then every
caller would have to have conditionals like 'if (CONFIG_HIGHMEM)' or
worse 'if (PageHighMem(page)'.

Let's not go there.

Migrate disable is a less horrible plague than preempt and pagefault
disable even if the scheduler people disagree due to the lack of theory
backing that up :)

The charm of the new interface is that users still can rely on per
cpuness independent of being on a highmem plagued system. For non
highmem systems the extra migrate disable/enable is really a minor
nuissance.

Thanks,

        tglx