Re: [GIT PULL] arm64 fix for 5.14

On Fri, Aug 27, 2021 at 10:10 AM Christoph Hellwig [off-list ref] 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

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