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

[David Hildenbrand]

On 11.04.24 21:01, Yang Shi wrote:
> 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.

Yes, all that is information that is missing from the patch description. 
If it's a fix, there should be a "Fixes:".

Likely we want to have a folio_large_mapcount() check in the code below. 
(I yet have to digest the condition where this happens -- can we have an 
example where we'd use to do the wrong thing and now would do the right 
thing as well?)
--
Cheers,

David / dhildenb