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

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Date:   Fri, 16 Dec 2016 18:10:16 +0100
From:   Robert Richter <robert.richter@...ium.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
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 15.12.16 16:07:26, Ard Biesheuvel wrote:
> 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?

When initializing the whole section it works. Maybe it uncovers
another bug. Did not yet start debugging this.

> 
> > 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)

I would prefer to check for a performance penalty *before* we put it
into stable. There is nor risk at all with the patch I am proposing.
See: https://lkml.org/lkml/2016/12/16/412

-Robert