linux-kernel - Re: [PATCH V2 1/2] arm64/mm: Fix pfn_valid() for ZONE

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Message-ID: <20210211115354.GB29894@willie-the-truck>
Date:   Thu, 11 Feb 2021 11:53:54 +0000
From:   Will Deacon <will@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Mark Rutland <mark.rutland@....com>,
        David Hildenbrand <david@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-kernel@...r.kernel.org, Mike Rapoport <rppt@...ux.ibm.com>,
        linux-mm@...ck.org,
        Jérôme Glisse <jglisse@...hat.com>,
        James Morse <james.morse@....com>,
        Dan Williams <dan.j.williams@...el.com>,
        Robin Murphy <robin.murphy@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH V2 1/2] arm64/mm: Fix pfn_valid() for ZONE_DEVICE based
 memory

On Fri, Feb 05, 2021 at 06:55:53PM +0000, Will Deacon wrote:
> On Wed, Feb 03, 2021 at 09:20:39AM +0530, Anshuman Khandual wrote:
> > On 2/2/21 6:26 PM, David Hildenbrand wrote:
> > > On 02.02.21 13:51, Will Deacon wrote:
> > >> On Tue, Feb 02, 2021 at 01:39:29PM +0100, David Hildenbrand wrote:
> > >>> As I expressed already, long term we should really get rid of the arm64
> > >>> variant and rather special-case the generic one. Then we won't go out of
> > >>> sync - just as it happened with ZONE_DEVICE handling here.
> > >>
> > >> Why does this have to be long term? This ZONE_DEVICE stuff could be the
> > >> carrot on the stick :)
> > > 
> > > Yes, I suggested to do it now, but Anshuman convinced me that doing a
> > > simple fix upfront might be cleaner --- for example when it comes to
> > > backporting :)
> > 
> > Right. The current pfn_valid() breaks for ZONE_DEVICE memory and this fixes
> > the problem in the present context which can be easily backported if required.
> > 
> > Changing or rather overhauling the generic code with new configs as proposed
> > earlier (which I am planning to work on subsequently) would definitely be an
> > improvement for the current pfn_valid() situation in terms of maintainability
> > but then it should not stop us from fixing the problem now.
> 
> Alright, I've mulled this over a bit. I don't agree that this patch helps
> with maintainability (quite the opposite, in fact), but perfection is the
> enemy of the good so I'll queue the series for 5.12. However, I'll revert
> the changes at the first sign of a problem, so please do work towards a
> generic solution which can replace this in the medium term.

... and dropped. These patches appear to be responsible for a boot
regression reported by CKI:

https://lore.kernel.org/r/cki.8D1CB60FEC.K6NJMEFQPV@redhat.com

Will