From: Thomas Gleixner <tglx@linutronix.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Juri Lelli <juri.lelli@redhat.com>,
linux-xtensa@linux-xtensa.org,
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Subject: Re: [patch V2 00/18] mm/highmem: Preemptible variant of kmap_atomic & friends
Date: Fri, 30 Oct 2020 00:41:39 +0100 [thread overview]
Message-ID: <87pn50ob0s.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <CAHk-=wiFxxGapdOyZHE-7LbFPk+jdfoqdeeJg0zWNQ86WvJGXg@mail.gmail.com>
On Thu, Oct 29 2020 at 16:11, Linus Torvalds wrote:
> On Thu, Oct 29, 2020 at 3:32 PM Thomas Gleixner <tglx@linutronix.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
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next prev parent reply other threads:[~2020-10-29 23:43 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-29 22:18 [patch V2 00/18] mm/highmem: Preemptible variant of kmap_atomic & friends Thomas Gleixner
2020-10-29 22:18 ` [patch V2 01/18] sched: Make migrate_disable/enable() independent of RT Thomas Gleixner
2020-10-29 22:18 ` [patch V2 02/18] mm/highmem: Un-EXPORT __kmap_atomic_idx() Thomas Gleixner
2020-10-29 22:18 ` [patch V2 03/18] highmem: Provide generic variant of kmap_atomic* Thomas Gleixner
2020-10-29 22:18 ` [patch V2 04/18] x86/mm/highmem: Use generic kmap atomic implementation Thomas Gleixner
2020-10-29 22:18 ` [patch V2 05/18] arc/mm/highmem: " Thomas Gleixner
2020-10-29 22:18 ` [patch V2 06/18] ARM: highmem: Switch to generic kmap atomic Thomas Gleixner
2020-10-29 22:18 ` [patch V2 07/18] csky/mm/highmem: " Thomas Gleixner
2020-10-29 22:18 ` [patch V2 08/18] microblaze/mm/highmem: " Thomas Gleixner
2020-10-29 22:18 ` [patch V2 09/18] mips/mm/highmem: " Thomas Gleixner
2020-10-29 22:18 ` [patch V2 10/18] nds32/mm/highmem: " Thomas Gleixner
2020-10-29 22:18 ` [patch V2 11/18] powerpc/mm/highmem: " Thomas Gleixner
2020-10-29 22:18 ` [patch V2 12/18] sparc/mm/highmem: " Thomas Gleixner
2020-10-29 22:18 ` [patch V2 13/18] xtensa/mm/highmem: " Thomas Gleixner
2020-10-29 22:18 ` [patch V2 14/18] mm/highmem: Remove the old kmap_atomic cruft Thomas Gleixner
2020-10-29 22:18 ` [patch V2 15/18] io-mapping: Cleanup atomic iomap Thomas Gleixner
2020-10-29 22:18 ` [patch V2 16/18] sched: highmem: Store local kmaps in task struct Thomas Gleixner
2020-10-29 22:18 ` [patch V2 17/18] mm/highmem: Provide kmap_local* Thomas Gleixner
2020-10-29 22:18 ` [patch V2 18/18] io-mapping: Provide iomap_local variant Thomas Gleixner
2020-10-29 23:11 ` [patch V2 00/18] mm/highmem: Preemptible variant of kmap_atomic & friends Linus Torvalds
2020-10-29 23:41 ` Thomas Gleixner [this message]
2020-10-30 9:39 ` Thomas Gleixner
2020-10-30 20:28 ` Linus Torvalds
2020-10-30 22:26 ` Thomas Gleixner
2020-10-30 22:46 ` Linus Torvalds
2020-10-30 23:26 ` Thomas Gleixner
2020-10-30 23:53 ` Thomas Gleixner
2020-10-31 13:37 ` Arnd Bergmann
2020-10-31 15:05 ` Christophe Leroy
2020-10-31 21:33 ` Arnd Bergmann
2020-10-30 7:25 ` Christoph Hellwig
2020-10-30 9:39 ` Thomas Gleixner
2020-10-30 13:06 ` Matthew Wilcox
2020-10-30 19:35 ` Thomas Gleixner
2020-11-02 1:08 ` Thomas Gleixner
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