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

[Vlastimil Babka <vbabka@suse.cz>]

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);