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Also the existing logic is that existing markers (HW poison, uffd-simulated HW
poison, uffd wp marker) are retained and no error raised on MADV_DONTNEED, and
no error on MADV_FREE either, so it'd be consistent with existing behaviour.

HW poison / uffd-simulated HW poison are expected to be zapped: it's just
like a mapped page with HWPOISON. So that is correct.

Well, poison is _not_ zapped on MADV_DONTNEED but _is_ on MADV_FREE :) anyway, I

Huh?

madvise_dontneed_single_vma()->zap_page_range_single(details=NULL)->unmap_single_vma(details=NULL)
... zap_pte_range()

} else if (is_hwpoison_entry(entry) ||
	   is_poisoned_swp_entry(entry)) {
	if (!should_zap_cows(details))
		continue;
	...

Should just zap them.

What am I missing?

Yeah ok it's me who's missing something here, I hadn't noticed details == NULL
so should_zap_cows() is true, my mistake!

It's confusing code ... I once had a patch to call it 
"should_zap_anon_folios" etc, but it would only slightly make it clearer 
what is actually happening.

In any case we explicitly add code here to prevent guard pages from going. I
will correct everything where I wrongly say otherwise, doh!

Right, that's also one of the reasons I don't think "poison" terminology 
is the best fit, because there are som subtle differences. At least the 
marker does not mention "poison" but PTE_MARKER_GUARD, which I think is 
a very good naming :)

-- 
Cheers,

David / dhildenb