[BUG 5.14] arm64/mm: dma memory mapping fails (in some cases)

[Christoph Hellwig]

On Sat, Sep 18, 2021 at 12:22:47AM +0300, Mike Rapoport wrote:
> I did some digging and it seems that the most "generic" way to check if a
> page is in RAM is page_is_ram(). It's not 100% bullet proof as it'll give
> false negatives for architectures that do not register "System RAM", but
> those are not using dma_map_resource() anyway and, apparently, never would.

The downside of page_is_ram is that it looks really expensiv for
something done at dma mapping time.