Re: [PATCH v2 2/5] mm: add PTE_MARKER_GUARD PTE marker

[David Hildenbrand]

> > > > > 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