Re: [PATCH v1 1/3] mm: Allow deferred splitting of arbitrary large anon folios

[David Hildenbrand]

On 17.07.23 18:01, Ryan Roberts wrote:
> On 17/07/2023 16:42, David Hildenbrand wrote:
> > On 17.07.23 16:31, Ryan Roberts wrote:
> > > In preparation for the introduction of large folios for anonymous
> > > memory, we would like to be able to split them when they have unmapped
> > > subpages, in order to free those unused pages under memory pressure. So
> > > remove the artificial requirement that the large folio needed to be at
> > > least PMD-sized.
> > >
> > > Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> > > Reviewed-by: Yu Zhao <yuzhao@xxxxxxxxxx>
> > > Reviewed-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
> > > ---
> > >    mm/rmap.c | 2 +-
> > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/mm/rmap.c b/mm/rmap.c
> > > index 0c0d8857dfce..2baf57d65c23 100644
> > > --- a/mm/rmap.c
> > > +++ b/mm/rmap.c
> > > @@ -1430,7 +1430,7 @@ void page_remove_rmap(struct page *page, struct
> > > vm_area_struct *vma,
> > >             * page of the folio is unmapped and at least one page
> > >             * is still mapped.
> > >             */
> > > -        if (folio_test_pmd_mappable(folio) && folio_test_anon(folio))
> > > +        if (folio_test_large(folio) && folio_test_anon(folio))
> > >                if (!compound || nr < nr_pmdmapped)
> > >                    deferred_split_folio(folio);
> >
> > !compound will always be true I guess, so nr_pmdmapped == 0 (which will always
> > be the case) will be ignored.
>
> I don't follow why !compound will always be true. This function is
> page_remove_rmap() (not folio_remove_rmap_range() which I add in a later patch).
> page_remove_rmap() can work on pmd-mapped pages where compound=true is passed in.

I was talking about the folio_test_pmd_mappable() -> folio_test_large() 
change. For folio_test_large() && !folio_test_pmd_mappable() I expect 
that we'll never pass in "compound=true".

--
Cheers,

David / dhildenb