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

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Date:   Thu, 5 Jan 2017 13:22:00 +0100
From:   Robert Richter <robert.richter@...ium.com>
To:     Will Deacon <will.deacon@....com>
CC:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "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 05.01.17 12:08:20, Will Deacon wrote:
> 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.

The bug happens on a small number of machines depending on the memory
layout. The fix affects all systems. And right know the impact is
unclear.

> > 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?

Yes, just both patches applied. Various other solutions were working.

-Robert