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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Thomas Gleixner <tglx@linutronix.de>
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Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends
Date: Sat, 19 Sep 2020 10:18:54 -0700
Message-ID: <CAHk-=wiYGyrFRbA1cc71D2-nc5U9LM9jUJesXGqpPnB7E4X1YQ@mail.gmail.com> (raw)
In-Reply-To: <20200919091751.011116649@linutronix.de>

On Sat, Sep 19, 2020 at 2:50 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> this provides a preemptible variant of kmap_atomic & related
> interfaces. This is achieved by:

Ack. This looks really nice, even apart from the new capability.

The only thing I really reacted to is that the name doesn't make sense
to me: "kmap_temporary()" seems a bit odd.

Particularly for an interface that really is basically meant as a
better replacement of "kmap_atomic()" (but is perhaps also a better
replacement for "kmap()").

I think I understand how the name came about: I think the "temporary"
is there as a distinction from the "longterm" regular kmap(). So I
think it makes some sense from an internal implementation angle, but I
don't think it makes a lot of sense from an interface name.

I don't know what might be a better name, but if we want to emphasize
that it's thread-private and a one-off, maybe "local" would be a
better naming, and make it distinct from the "global" nature of the
old kmap() interface?

However, another solution might be to just use this new preemptible
"local" kmap(), and remove the old global one entirely. Yes, the old
global one caches the page table mapping and that sounds really
efficient and nice. But it's actually horribly horribly bad, because
it means that we need to use locking for them. Your new "temporary"
implementation seems to be fundamentally better locking-wise, and only
need preemption disabling as locking (and is equally fast for the
non-highmem case).

So I wonder if the single-page TLB flush isn't a better model, and
whether it wouldn't be a lot simpler to just get rid of the old
complex kmap() entirely, and replace it with this?

I agree we can't replace the kmap_atomic() version, because maybe
people depend on the preemption disabling it also implied. But what
about replacing the non-atomic kmap()?

Maybe I've missed something.  Is it because the new interface still
does "pagefault_disable()" perhaps?

But does it even need the pagefault_disable() at all? Yes, the
*atomic* one obviously needed it. But why does this new one need to
disable page faults?

[ I'm just reading the patches, I didn't try to apply them and look at
the end result, so I might have missed something ]

The only other worry I would have is just test coverage of this
change. I suspect very few developers use HIGHMEM. And I know the
various test robots don't tend to test 32-bit either.

But apart from that question about naming (and perhaps replacing
kmap() entirely), I very much like it.

                        Linus

  parent reply index

Thread overview: 54+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-19  9:17 Thomas Gleixner
2020-09-19  9:17 ` [patch RFC 01/15] mm/highmem: Un-EXPORT __kmap_atomic_idx() Thomas Gleixner
2020-09-21  6:23   ` Christoph Hellwig
2020-09-19  9:17 ` [patch RFC 02/15] highmem: Provide generic variant of kmap_atomic* Thomas Gleixner
2020-09-21  6:28   ` Christoph Hellwig
2020-09-19  9:17 ` [patch RFC 03/15] x86/mm/highmem: Use generic kmap atomic implementation Thomas Gleixner
2020-09-19  9:17 ` [patch RFC 04/15] arc/mm/highmem: " Thomas Gleixner
2020-09-19  9:17 ` [patch RFC 05/15] ARM: highmem: Switch to generic kmap atomic Thomas Gleixner
2020-09-19  9:17 ` [patch RFC 06/15] csky/mm/highmem: " Thomas Gleixner
2020-09-23  0:05   ` Guo Ren
2020-09-19  9:17 ` [patch RFC 07/15] microblaze/mm/highmem: " Thomas Gleixner
2020-09-19  9:17 ` [patch RFC 08/15] mips/mm/highmem: " Thomas Gleixner
2020-09-19  9:18 ` [patch RFC 09/15] nds32/mm/highmem: " Thomas Gleixner
2020-09-19  9:18 ` [patch RFC 10/15] powerpc/mm/highmem: " Thomas Gleixner
2020-09-19  9:18 ` [patch RFC 11/15] sparc/mm/highmem: " Thomas Gleixner
2020-09-19  9:18 ` [patch RFC 12/15] xtensa/mm/highmem: " Thomas Gleixner
2020-09-19  9:18 ` [patch RFC 13/15] mm/highmem: Remove the old kmap_atomic cruft Thomas Gleixner
2020-09-19  9:18 ` [patch RFC 14/15] sched: highmem: Store temporary kmaps in task struct Thomas Gleixner
2020-09-19  9:18 ` [patch RFC 15/15] mm/highmem: Provide kmap_temporary* Thomas Gleixner
2020-09-19 10:35 ` [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends Daniel Vetter
2020-09-19 10:37   ` Daniel Vetter
2020-09-20  6:23     ` Thomas Gleixner
2020-09-20  8:23       ` Daniel Vetter
2020-09-20 17:24         ` Thomas Gleixner
2020-09-19 17:18 ` Linus Torvalds [this message]
2020-09-19 17:39   ` Matthew Wilcox
2020-09-19 19:13     ` Linus Torvalds
2020-09-21 19:58     ` Ira Weiny
2020-09-20  6:41   ` Thomas Gleixner
2020-09-20  8:49     ` Thomas Gleixner
2020-09-20 16:57       ` Linus Torvalds
2020-09-20 17:40         ` Thomas Gleixner
2020-09-20 17:42           ` Linus Torvalds
2020-09-20 17:58             ` Linus Torvalds
2020-09-21  7:39             ` Thomas Gleixner
2020-09-21 16:24               ` Linus Torvalds
2020-09-21 19:27                 ` Thomas Gleixner
2020-09-23  8:40                   ` peterz
2020-09-23 13:35                     ` Thomas Gleixner
2020-09-23 15:52                     ` Steven Rostedt
2020-09-23 20:55                       ` Thomas Gleixner
2020-09-23 21:12                         ` Steven Rostedt
2020-09-24  6:57                           ` Thomas Gleixner
2020-09-24 12:32                             ` Steven Rostedt
2020-09-24 12:42                               ` Peter Zijlstra
2020-09-24 13:51                                 ` Steven Rostedt
2020-09-24 13:58                                   ` Peter Zijlstra
2020-09-24 17:55                               ` Thomas Gleixner
2020-09-24 18:58                                 ` Steven Rostedt
2020-09-24  8:27                       ` peterz
2020-09-24 19:36                         ` Daniel Bristot de Oliveira
2020-09-23 10:19                   ` peterz
2020-09-23 12:33                     ` Thomas Gleixner
2020-09-23 14:33                   ` Thomas Gleixner

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