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Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN_ZONE for NUMA

From	Will Deacon <will.deacon@arm.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN_ZONE for NUMA
Date	2017-01-05 13:10 +0100
Message-ID	<sWf1L-2MX-9@gated-at.bofh.it> (permalink)
References	(1 earlier) <sOdTb-4EN-3@gated-at.bofh.it> <sVTNE-5e8-5@gated-at.bofh.it> <sVUgF-5oR-13@gated-at.bofh.it> <sVUql-5J1-3@gated-at.bofh.it> <sWf1L-2MX-11@gated-at.bofh.it>
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On Thu, Jan 05, 2017 at 12:24:07PM +0100, Robert Richter wrote:
> On 04.01.17 14:02:23, Will Deacon wrote:
> > Using early_pfn_valid feels like a bodge to me, since having pfn_valid
> > return false for something that early_pfn_valid says is valid (and is
> > therefore initialised in the memmap) makes the NOMAP semantics even more
> > confusing.
> 
> The concern I have had with HOLES_IN_ZONE is that it enables
> pfn_valid_within() for arm64. This means that each pfn of a section is
> checked which is done only once for the section otherwise. With up to
> 2^18 pages per section we traverse the memblock list by that factor
> more often. There could be a performance regression.

There could be, but we're trying to fix a bug here. I wouldn't have
thought that walking over pfns like that is done very often.

> I haven't numbers yet, since the fix causes another kernel crash. And,
> this is the next problem I have. The crash doesn't happen otherwise. So,
> either it uncovers another bug or the fix is incomplete. Though the
> changes look like it should work. This needs more investigation.

I really can't see how the fix causes a crash, and I couldn't reproduce
it on any of my boards, nor could any of the Linaro folk afaik. Are you
definitely running mainline with just these two patches from Ard?

Will

Thread

Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN_ZONE for NUMA Will Deacon <will.deacon@arm.com> - 2017-01-04 14:30 +0100
  Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN_ZONE for NUMA Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2017-01-04 15:00 +0100
    Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN_ZONE for NUMA Will Deacon <will.deacon@arm.com> - 2017-01-04 15:10 +0100
      Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN_ZONE for NUMA Will Deacon <will.deacon@arm.com> - 2017-01-05 13:10 +0100
        Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN_ZONE for NUMA Will Deacon <will.deacon@arm.com> - 2017-01-06 13:20 +0100
          Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN_ZONE for NUMA Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2017-01-06 13:30 +0100

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