linux-kernel - Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN

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Date:   Thu, 15 Dec 2016 16:07:26 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Robert Richter <robert.richter@...ium.com>
Cc:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Will Deacon <will.deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hanjun Guo <hanjun.guo@...aro.org>,
        Yisheng Xie <xieyisheng1@...wei.com>,
        James Morse <james.morse@....com>
Subject: Re: [PATCH 2/2] arm64: mm: enable CONFIG_HOLES_IN_ZONE for NUMA

On 15 December 2016 at 15:39, Robert Richter <robert.richter@...ium.com> wrote:
> I was going to do some measurements but my kernel crashes now with a
> page fault in efi_rtc_probe():
>
> [   21.663393] Unable to handle kernel paging request at virtual address 20251000
> [   21.663396] pgd = ffff000009090000
> [   21.663401] [20251000] *pgd=0000010ffff90003
> [   21.663402] , *pud=0000010ffff90003
> [   21.663404] , *pmd=0000000fdc030003
> [   21.663405] , *pte=00e8832000250707
>
> The sparsemem config requires the whole section to be initialized.
> Your patches do not address this.
>

96000047 is a third level translation fault, and the PTE address has
RES0 bits set. I don't see how this is related to sparsemem, could you
explain?

> On 14.12.16 09:11:47, Ard Biesheuvel wrote:
>> +config HOLES_IN_ZONE
>> +     def_bool y
>> +     depends on NUMA
>
> This enables pfn_valid_within() for arm64 and causes the check for
> each page of a section. The arm64 implementation of pfn_valid() is
> already expensive (traversing memblock areas). Now, this is increased
> by a factor of 2^18 for 4k page size (16384 for 64k). We need to
> initialize the whole section to avoid that.
>

I know that. But if you want something for -stable, we should have
something that is correct first, and only then care about the
performance hit (if there is one)