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

Huang, Ying
2021-04-14T03:01:00Z

Yang Shi <shy828301@gmail.com> 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?

Best Regards,
Huang, Ying

> Signed-off-by: Yang Shi <shy828301@gmail.com>
> ---
> 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));