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

[David Hildenbrand]

On 21.10.24 19:26, Vlastimil Babka wrote:
> On 10/21/24 19:14, Lorenzo Stoakes wrote:
> > On Mon, Oct 21, 2024 at 07:00:53PM +0200, David Hildenbrand wrote:
> > > > > > 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!
>
> Well, good to know it's consistent then. As I've explained I see why zapping
> actual hwpoison makes sense for MADV_DONTNEED/MADV_FREE. That it's done also
> for uffd poison is not completely clear, but maybe it was just easier to
> implement. 

Note that in VM context "uffd poison" really just is "this was hwpoison 
on the source VM, so we mimic that on the destination VM, because the 
data *is* lost" -- so you want the exact same behavior.

For example, when a VM reboots you might just want to ZAP these hwpoison 
entries, and get fresh pages on next access.

So to me it makes sense that they are treated equally.

--
Cheers,

David / dhildenb