Re: [PATCH] mm: migrate: Fix THP's mapcount on isolation

[David Hildenbrand]

On 23.11.22 17:07, Matthew Wilcox wrote:
> On Wed, Nov 23, 2022 at 09:56:38AM +0100, David Hildenbrand wrote:
> > But we do have an even better helper in place already:
> > mm/huge_memory.c:can_split_folio()
> >
> > Which cares about
> >
> > a) Swapcache for THP: each subpage could be in the swapcache
> > b) Requires the caller to hold one reference to be safe
> >
> > But I am a bit confused about the "extra_pins" for !anon. Where do the
> > folio_nr_pages() references come from?
>
> When we add a folio to the page cache, we increment its refcount by
> folio_nr_pages() instead of by 1.  I suspect this is no longer needed
> (if it was ever needed) and it could be changed.  See
> __filemap_add_folio():
>
>          long nr = 1;
>          if (!huge) {
>                  nr = folio_nr_pages(folio);
>          folio_ref_add(folio, nr);
>
> > So *maybe* it makes sense to factor out can_split_folio() and call it
> > something like: "folio_maybe_additionally_referenced"  [to clearly
> > distinguish it from "folio_maybe_dma_pinned" that cares about actual page
> > pinning (read/write page content)].
> >
> > Such a function could return false positives/negatives due to races and the
> > caller would have to hold one reference and be able to deal with the
> > semantics.
>
> I don't like the 'pextra_pins' parameter to a generic function ...

Right, that part should remain khugepaged specific. The assumption would 
be, that the caller of the generic function holds exactly one additional 
reference.

--
Thanks,

David / dhildenb