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

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Message-ID: <20170105112407.GU4930@rric.localdomain>
Date:   Thu, 5 Jan 2017 12:24:07 +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 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. 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.

-Robert