[PATCH 1/3] lib: Add a generic copy_oldmem_page()

[Christoph Hellwig]

On Fri, Jul 10, 2020 at 08:55:42PM -0700, Palmer Dabbelt wrote:
> +ssize_t copy_oldmem_page(unsigned long pfn, char *buf,
> +			 size_t csize, unsigned long offset,
> +			 int userbuf)
> +{
> +	void *vaddr;
> +
> +	if (!csize)
> +		return 0;
> +
> +	vaddr = memremap(__pfn_to_phys(pfn), PAGE_SIZE, MEMREMAP_WB);
> +	if (!vaddr)
> +		return -ENOMEM;

Doing a memremap for every page is very inefficient.  Also I don't see
why you'd want to even do that.  All memory is in the direct mapping
for RISC-V.  For other architecture that support highmem kmap_atomic_pfn
would do the job, which is what I'd use in a generic version.