Re: [PATCH v6 0/6] ksm: support tracking KSM-placed zero-pages

[David Hildenbrand]

> > If we'd really want to identify whether a zeropage was deduplciated by
> > KSM, we could try storing that information inside the PTE instead of
>
> this is interesting, but needs caution, for the reason you mention below
>
> > inside the RMAP. Then, we could directly adjust the counter when zapping
> > the shared zeropage or during MADV_DONTNEED/when unmerging.
> >
> > Eventually, we could simply say that
> > * !pte_dirty(): zeropage placed during fault
> > * pte_dirty(): zeropage placed by KSM
> >
> > Then it would also be easy to adjust counters and unmerge. We'd limit
> > this handling to known-working architectures initially (spec64 still has

^ I meant sparc64 here. We can (and should) have a testcase that 
deduplicates the shared zeropage using KSM and makes sure that writes 
properly lead to a write fault.

> > the issue that pte_mkdirty() will set a pte writable ... and my patch to
> > fix that was not merged yet). We'd have to double-check all
> > pte_mkdirty/pte_mkclean() callsites.
>
> this will be... interesting

IIRC, most code that touches the dirty bit makes sure that it operates 
on a proper struct page (via vm_normal_folio()). 
madvise_free_pte_range() is one such user. But we have to double-check.

--
Thanks,

David / dhildenb