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

[Matthew Wilcox <willy@infradead.org>]

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 ...