On Wed, Nov 28, 2018 at 11:27:17AM -0800, David Miller wrote:

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 28 Nov 2018 10:00:06 -0800

quoted

Not all memory is accessible even to the kernel. If you have memory
that shows up in the last page of phys_addr_t, you just mark it
reserved at boot-time.

It's not the physical memory at the end that needs to be reserved.

It's the IOMMU mapping arena.

True, if and only if you have an IOMMU.

Where there isn't an IOMMU, then we'd have to reserve every page that
that translates to a bus address in the top 4K of dma_addr_t on any
bus in the system - that means knowing early in the kernel
initialisation about all buses in the system so we can detect and
reserve these pages.

I don't think that's trivial to do.

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