Re: [PATCH mm-unstable v2 1/6] mm: Add folio_estimated_mapcount()

[David Hildenbrand]

On 28.01.23 01:48, Jane Chu wrote:
> On 1/26/2023 12:37 AM, David Hildenbrand wrote:
> > On 25.01.23 23:09, Vishal Moola wrote:
> [..]
> > The issue is that we're not estimating the mapcount of the folio, so the
> > name is very misleading ... I think you really want to avoid the term
> > mapcount completely in that context. We're just using the #mappings of
> > the first subpage to determine something differently.
> >
> > Thinking about it, I guess "folio_estimated_sharers()" might be what we
> > actually want to name it. Then you can comment how we estimate sharers
> > by looking at into how many page tables the first subpage is currently
> > mapped, and assume the same holds true for the other subpages.
> >
> > It's unreliable because other subpages might behave differently, we
> > might not be holding the pagelock to stabilize, and we're not looking at
> > indirect mappings via the swapcache. But it's a comapratively good
> > estimate for most scenarios I guess.
>
> Hmm, how about simply call it folio_hpage_mapcount(),
> folio_firstpage_mapcount(), or, folio_cover_mapcount() ?

All not better IMHO.

folio_estimated_subpage_mapcount() is a bit too verbose for my taste and ...

> It is used to replace page_mapcount() in that sense -
> https://lore.kernel.org/linux-mm/Y9MDJuPWsk9820xD@x1n/T/#me0531cfb9e82ad5ca88804c727d69cc6b9b33ffa
>
> 	if (flags & (MPOL_MF_MOVE_ALL) ||
> 	    (flags & MPOL_MF_MOVE && folio_estimated_mapcount(folio) == 1 &&
> 	     !hugetlb_pmd_shared(pte))) {
> 		if (isolate_hugetlb(folio, qp->pagelist) &&

... what we want to have here is an estimation on the number of sharers. 
[actually, we would want it precise, but that's hard to achieve ... ]

--
Thanks,

David / dhildenb