Re: [External] Re: [PATCH v3 1/6] mm: migrate: do not migrate HugeTLB page whose refcount is one

[Muchun Song]

On Tue, Jan 12, 2021 at 5:42 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Sun 10-01-21 20:40:12, Muchun Song wrote:
> > If the refcount is one when it is migrated, it means that the page
> > was freed from under us. So we are done and do not need to migrate.
>
> I would consider the following easier to understand. Feel free to reuse.
> "
> All pages isolated for the migration have an elevated reference count
> and therefore seeing a reference count equal to 1 means that the last
> user of the page has dropped the reference and the page has became
> unused and there doesn't make much sense to migrate it anymore. This has
> been done for regular pages and this patch does the same for hugetlb
> pages. Although the likelyhood of the race is rather small for hugetlb
> pages it makes sense the two code paths in sync.
> "

Thanks.

>
> >
> > This optimization is consistent with the regular pages, just like
> > unmap_and_move() does.
> >
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > Acked-by: Yang Shi <shy828301@xxxxxxxxx>
>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks.

>
> > ---
> >  mm/migrate.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 4385f2fb5d18..a6631c4eb6a6 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -1279,6 +1279,12 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
> >               return -ENOSYS;
> >       }
> >
> > +     if (page_count(hpage) == 1) {
> > +             /* page was freed from under us. So we are done. */
> > +             putback_active_hugepage(hpage);
> > +             return MIGRATEPAGE_SUCCESS;
> > +     }
> > +
> >       new_hpage = get_new_page(hpage, private);
> >       if (!new_hpage)
> >               return -ENOMEM;
> > --
> > 2.11.0
>
> --
> Michal Hocko
> SUSE Labs