[LSF/MM/BPF TOPIC] MM: Mapcount Madness

[David Hildenbrand <david@redhat.com>]

As PTE-mapped large folios become more relevant (mTHP [1]) and there is 
the desire to shrink the metadata allocated for such large folios as 
well (memdesc [2]), how we track folio mappings gets more relevant. Over 
the years, we used folio mapping information to answer various 
questions: is this folio mapped by somebody else? do we have to COW on 
write fault? how do we adjust memory statistics? ...

Let's talk about ongoing work in the mapcount area, get a common 
understanding of what the users of the different mapcounts are and what 
the implications of removing some would be: which questions could we 
answer differently, which questions would we not be able to answer 
precisely anymore, and what would be the implications of such changes?

For example, can we tolerate some imprecise memory statistics? How 
expressive is the PSS when large folios are only partially mapped? Would 
we need a transition period and glue changes to a new CONFIG_ option? Do 
we really have to support THP and friends on 32bit?

[1] 
https://patchwork.kernel.org/project/linux-mm/cover/20231207161211.2374093-1-ryan.roberts@arm.com/#25628022
[2] https://kernelnewbies.org/MatthewWilcox/Memdescs

-- 
Cheers,

David / dhildenb