To me, it seems safer to split or do some other similar optimization if we find a
large folio has partial map and unmap.
I'm hoping that we can avoid any new direct users of _nr_pages_mapped if
possible.
Is _nr_pages_mapped < nr_pages a reasonable case to split as we
have known the folio has at least some subpages zapped?
I'm not sure we need this - the folio's presence on the split list will tell us
everything we need to know I think?
I agree, this is just one question to David, not my proposal. if
deferred_list is sufficient,
I prefer we use deferred_list.
I actually don't quite understand why David dislikes _nr_pages_mapped being used
though I do think _nr_pages_mapped cannot precisely reflect how a
folio is mapped
by multi-processes. but _nr_pages_mapped < nr_pages seems be safe to
tell the folio
is partially unmapped :-)