Re: [PATCH] mm/rmap: do not add fully unmapped large folio to deferred split list

[Yang Shi]

On Thu, Apr 11, 2024 at 8:46 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 11.04.24 17:32, Zi Yan wrote:
> > From: Zi Yan <ziy@xxxxxxxxxx>
> >
> > In __folio_remove_rmap(), a large folio is added to deferred split list
> > if any page in a folio loses its final mapping. It is possible that
> > the folio is unmapped fully, but it is unnecessary to add the folio
> > to deferred split list at all. Fix it by checking folio mapcount before
> > adding a folio to deferred split list.
> >
> > Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> > ---
> >   mm/rmap.c | 9 ++++++---
> >   1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 2608c40dffad..d599a772e282 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1494,7 +1494,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
> >               enum rmap_level level)
> >   {
> >       atomic_t *mapped = &folio->_nr_pages_mapped;
> > -     int last, nr = 0, nr_pmdmapped = 0;
> > +     int last, nr = 0, nr_pmdmapped = 0, mapcount = 0;
> >       enum node_stat_item idx;
> >
> >       __folio_rmap_sanity_checks(folio, page, nr_pages, level);
> > @@ -1506,7 +1506,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
> >                       break;
> >               }
> >
> > -             atomic_sub(nr_pages, &folio->_large_mapcount);
> > +             mapcount = atomic_sub_return(nr_pages,
> > +                                          &folio->_large_mapcount) + 1;
>
> That becomes a new memory barrier on some archs. Rather just re-read it
> below. Re-reading should be fine here.
>
> >               do {
> >                       last = atomic_add_negative(-1, &page->_mapcount);
> >                       if (last) {
> > @@ -1554,7 +1555,9 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
> >                * is still mapped.
> >                */
> >               if (folio_test_large(folio) && folio_test_anon(folio))
> > -                     if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped)
> > +                     if ((level == RMAP_LEVEL_PTE &&
> > +                          mapcount != 0) ||
> > +                         (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped))
> >                               deferred_split_folio(folio);
> >       }
>
> But I do wonder if we really care? Usually the folio will simply get
> freed afterwards, where we simply remove it from the list.
>
> If it's pinned, we won't be able to free or reclaim, but it's rather a
> corner case ...
>
> Is it really worth the added code? Not convinced.

It is actually not only an optimization, but also fixed the broken
thp_deferred_split_page counter in /proc/vmstat.

The counter actually counted the partially unmapped huge pages (so
they are on deferred split queue), but it counts the fully unmapped
mTHP as well now. For example, when a 64K THP is fully unmapped, the
thp_deferred_split_page is not supposed to get inc'ed, but it does
now.

The counter is also useful for performance analysis, for example,
whether a workload did a lot of partial unmap or not. So fixing the
counter seems worthy. Zi Yan should have mentioned this in the commit
log.

>
> --
> Cheers,
>
> David / dhildenb
>