Re: [PATCH 09/13] mm/munlock: delete smp_mb() from __pagevec_lru_add_fn()

Vlastimil Babka
2022-02-11T17:43:44Z

On 2/6/22 22:45, Hugh Dickins wrote:
> My reading of comment on smp_mb__after_atomic() in __pagevec_lru_add_fn()
> says that it can now be deleted; and that remains so when the next patch
> is added.

Agree with moderate certainty.

> Signed-off-by: Hugh Dickins <hughd@google.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
> mm/swap.c | 37 +++++++++----------------------------
> 1 file changed, 9 insertions(+), 28 deletions(-)
>
> diff --git a/mm/swap.c b/mm/swap.c
> index 682a03301a2c..3f770b1ea2c1 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -1025,37 +1025,18 @@ static void __pagevec_lru_add_fn(struct folio *folio, struct lruvec *lruvec)
>
> VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
>
> + folio_set_lru(folio);
> /*
> - * A folio becomes evictable in two ways:
> - * 1) Within LRU lock [munlock_vma_page() and __munlock_pagevec()].
> - * 2) Before acquiring LRU lock to put the folio on the correct LRU
> - * and then
> - * a) do PageLRU check with lock [check_move_unevictable_pages]
> - * b) do PageLRU check before lock [clear_page_mlock]
> - *
> - * (1) & (2a) are ok as LRU lock will serialize them. For (2b), we need
> - * following strict ordering:
> - *
> - * #0: __pagevec_lru_add_fn #1: clear_page_mlock
> - *
> - * folio_set_lru() folio_test_clear_mlocked()
> - * smp_mb() // explicit ordering // above provides strict
> - * // ordering
> - * folio_test_mlocked() folio_test_lru()
> + * Is an smp_mb__after_atomic() still required here, before
> + * folio_evictable() tests PageMlocked, to rule out the possibility
> + * of stranding an evictable folio on an unevictable LRU? I think
> + * not, because munlock_page() only clears PageMlocked while the LRU
> + * lock is held.
> *
> - *
> - * if '#1' does not observe setting of PG_lru by '#0' and
> - * fails isolation, the explicit barrier will make sure that
> - * folio_evictable check will put the folio on the correct
> - * LRU. Without smp_mb(), folio_set_lru() can be reordered
> - * after folio_test_mlocked() check and can make '#1' fail the
> - * isolation of the folio whose mlocked bit is cleared (#0 is
> - * also looking at the same folio) and the evictable folio will
> - * be stranded on an unevictable LRU.
> + * (That is not true of __page_cache_release(), and not necessarily
> + * true of release_pages(): but those only clear PageMlocked after
> + * put_page_testzero() has excluded any other users of the page.)
> */
> - folio_set_lru(folio);
> - smp_mb__after_atomic();
> -
> if (folio_evictable(folio)) {
> if (was_unevictable)
> __count_vm_events(UNEVICTABLE_PGRESCUED, nr_pages);