Re: [PATCH 21/46] hugetlb: use struct hugetlb_pte for walk_hugetlb_range

[David Hildenbrand]

> > > 1. The RFC v2 way, which is head-only, and we increment the compound
> > > mapcount for each PT mapping we have. So a PTE-mapped 2M page,
> > > compound_mapcount=512, subpage->_mapcount=0 (ignoring the -1 bias).
> > > 2. The THP-like way. If we are fully mapping the hugetlb page with the
> > > hstate-level PTE, we increment the compound mapcount, otherwise we
> > > increment subpage->_mapcount.
> > > 3. The RFC v1 way (the way you have suggested above), which is
> > > head-only, and we increment the compound mapcount if the hstate-level
> > > PTE is made present.
> > >
> > > With #1 and #2, there is no concern with folio_mapcount(). But with
> > > #3, folio_mapcount() for a PTE-mapped 2M page mapped in a single VMA
> > > would yield 1 instead of 512 (right?). That's what I mean.
> >
> > My 2 cents:
> >
> > The mapcount is primarily used (in hugetlb context) to
> >
> > (a) Detect if a page might be shared. mapcount > 1 implies that two
> > independent page table hierarchies are mapping the page. We care about
> > mapcount == 1 vs. mapcount != 1.
> >
> > (b) Detect if unmapping was sucessfull. We care about mapcount == 0 vs.
> > mapcount != 0.
> >
> > For hugetlb, I don't see why we should care about the subpage mapcount
> > at all.
>
> Agreed -- it shouldn't really matter all that much.
>
> > For (a) it's even good to count "somehow mapped into a single page table
> > structure" as "mapcount == 1" For (b), we don't care as long as "still
> > mapped" implies "mapcount != 0".
>
> Thanks for your thoughts, David. So it sounds like you're still
> squarely in the #3 camp. :)

Well, yes. As long as we can get it implemented in a clean way ... :)

--
Thanks,

David / dhildenb