Re: [v2 PATCH 6/7] mm: migrate: check mapcount for THP instead of ref count

[Yang Shi]

On Tue, Apr 13, 2021 at 8:00 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> Yang Shi <shy828301@xxxxxxxxx> writes:
>
> > The generic migration path will check refcount, so no need check refcount here.
> > But the old code actually prevents from migrating shared THP (mapped by multiple
> > processes), so bail out early if mapcount is > 1 to keep the behavior.
>
> What prevents us from migrating shared THP?  If no, why not just remove
> the old refcount checking?

We could migrate shared THP if we don't care about the bounce back and
forth between nodes as Zi Yan described. The other reason is, as I
mentioned in the cover letter,  I'd like to keep the behavior as
consistent as possible between before and after for now. The old
behavior does prevent migrating shared THP, so I did so in this
series. We definitely could optimize the behavior later on.

>
> Best Regards,
> Huang, Ying
>
> > Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
> > ---
> >  mm/migrate.c | 16 ++++------------
> >  1 file changed, 4 insertions(+), 12 deletions(-)
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index a72994c68ec6..dc7cc7f3a124 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -2067,6 +2067,10 @@ static int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page)
> >
> >       VM_BUG_ON_PAGE(compound_order(page) && !PageTransHuge(page), page);
> >
> > +     /* Do not migrate THP mapped by multiple processes */
> > +     if (PageTransHuge(page) && page_mapcount(page) > 1)
> > +             return 0;
> > +
> >       /* Avoid migrating to a node that is nearly full */
> >       if (!migrate_balanced_pgdat(pgdat, compound_nr(page)))
> >               return 0;
> > @@ -2074,18 +2078,6 @@ static int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page)
> >       if (isolate_lru_page(page))
> >               return 0;
> >
> > -     /*
> > -      * migrate_misplaced_transhuge_page() skips page migration's usual
> > -      * check on page_count(), so we must do it here, now that the page
> > -      * has been isolated: a GUP pin, or any other pin, prevents migration.
> > -      * The expected page count is 3: 1 for page's mapcount and 1 for the
> > -      * caller's pin and 1 for the reference taken by isolate_lru_page().
> > -      */
> > -     if (PageTransHuge(page) && page_count(page) != 3) {
> > -             putback_lru_page(page);
> > -             return 0;
> > -     }
> > -
> >       page_lru = page_is_file_lru(page);
> >       mod_node_page_state(page_pgdat(page), NR_ISOLATED_ANON + page_lru,
> >                               thp_nr_pages(page));