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

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Message-ID: <YS5fUqQYWf0AKLgD@arm.com>
Date:   Tue, 31 Aug 2021 17:56:50 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Christoph Hellwig <hch@....de>, Will Deacon <will@...nel.org>,
        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 10:16:27AM -0700, Linus Torvalds wrote:
> 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.

The suggested solution in the original thread was to change the generic
DMA code to use memblock_is_memory() instead of pfn_valid():

https://lore.kernel.org/lkml/b720e7c8-ca44-0a25-480b-05bf49d03c35@redhat.com/

Given how late we discovered this in the -rc cycle, the decision was to
revert the pfn_valid() patch. We'll re-instate it at some point but
someone needs to sanity check the other pfn_valid() call sites and the
expected semantics.

-- 
Catalin