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Date:   Fri, 27 Aug 2021 09:40:41 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Will Deacon <will@kernel.org>, Christoph Hellwig <hch@lst.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Android Kernel Team <kernel-team@android.com>
Subject: Re: [GIT PULL] arm64 fix for 5.14
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On Thu, Aug 26, 2021 at 11:41:34AM -0700, Linus Torvalds wrote:
> "pfn_valid()" is more about whether you can do a "pfn_to_page()" lookup on it.
> 
> II get the feeling that the dma-mapping code should allow pages that
> are PageReserved() to be mapped - they aren't "ram" in the kernel
> sense.
> 
> Perhaps also make sure it's not the zero page (which is
> PageReserved(), but most definitely RAM).
> 
> In a PC world that would be (for example) the legacy PCI space at
> 0xa0000-0xfffff, but I could easily imagine other platforms having
> other situations.

So what would be the correct check for "this is not actually page backed
normal RAM"?