linux-kernel - Re: [GIT PULL] arm64 fix for 5.14

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Date:   Fri, 27 Aug 2021 10:16:27 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [GIT PULL] arm64 fix for 5.14

On Fri, Aug 27, 2021 at 10:10 AM Christoph Hellwig <hch@....de> wrote:
>
> They CCed me on their earlier discussion, but I did not catch up on it
> until you responded to the pull request  If I understood it correct it
> was about a platform device mapping a MMIO region (like a PCI bar),
> but something about section alignment cause pfn_valid to mistrigger.

Yeah, so I can easily see the maxpfn numbers can easily end up being
rounded up to a whole memory section etc.

I think my suggested solution should JustWork(tm) - exactly because if
the area is then in that "this pfn is valid" area, it will
double-check the actual underlying page.

That said, I think x86 avoids the problem another way - by just making
sure max_pfn is exact. That works too, as long as there are no holes
in the RAM map that might be used for PCI BAR's.

So I think arm could fix it that way too, depending on their memory layout.

(x86 has that traditional hole at A0000-FFFFF, but it's special enough
that it presumably is never an issue).

                  Linus