RE: [PATCH] mm/hwpoison: Clear PRESENT bit for kernel 1:1 mappings of poison pages

[Luck, Tony]

> Persistent memory does have unpoisoning and would require this inverse
> operation - see drivers/nvdimm/pmem.c pmem_clear_poison() and core.c
> nvdimm_clear_poison().

Nice.  Well this code will need to cooperate with that ... in particular if the page
is in an area that can be unpoisoned ... then we should do that *instead* of marking
the page not present (which breaks up huge/large pages and so affects performance).

Instead of calling it "arch_unmap_pfn" it could be called something like arch_handle_poison()
and do something like:

void arch_handle_poison(unsigned long pfn)
{
	if this is a pmem page && pmem_clear_poison(pfn)
		return
	if this is a nvdimm page && nvdimm_clear_poison(pfn)
		return
	/* can't clear, map out from 1:1 region */
	... code from my patch ...
}

I'm just not sure how those first two "if" bits work ... particularly in terms of CONFIG dependencies and system
capabilities.  Perhaps each of pmem and nvdimm could register their unpoison functions and this code could
just call each in turn?

-Tony