Re: [PATCH] mm/page_alloc: optimize the loop in find_suitable_fallback()

Vlastimil Babka
2023-02-09T08:12:36Z

On 2/9/23 03:44, Yajun Deng wrote:
> There is no need to execute the next loop if it not return in the first
> loop. So add a break at the end of the loop.
>
> There are only three rows in fallbacks, so reduce the first index size
> from MIGRATE_TYPES to MIGRATE_PCPTYPES.
>
> Signed-off-by: Yajun Deng <yajun.deng@linux.dev>

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

> ---
> mm/page_alloc.c | 11 +++++------
> 1 file changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1113483fa6c5..536e8d838fb5 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2603,7 +2603,7 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
> *
> * The other migratetypes do not have fallbacks.
> */
> -static int fallbacks[MIGRATE_TYPES][MIGRATE_PCPTYPES - 1] = {
> +static int fallbacks[MIGRATE_PCPTYPES][MIGRATE_PCPTYPES - 1] = {
> [MIGRATE_UNMOVABLE] = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE },
> [MIGRATE_MOVABLE] = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE },
> [MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE, MIGRATE_MOVABLE },
> @@ -2861,7 +2861,7 @@ int find_suitable_fallback(struct free_area *area, unsigned int order,
> int i;
> int fallback_mt;
>
> - if (area->nr_free == 0)
> + if (area->nr_free == 0 || !migratetype_is_mergeable(migratetype))

Just curious, did you the check for extra safety or did you find (by running
or code inspection) that this can be indeed called with a non-mergeable
migratetype, and cause out of bounds access of the shrinked fallbacks array?

BTW, I noticed the commment on migratetype_is_mergeable() contains:

"See fallbacks[MIGRATE_TYPES][3] in page_alloc.c. "

Should probably change it to e.g. "See fallbacks[][] array ..." so we don't
have to keep it in exact sync...

> return -1;
>
> *can_steal = false;
> @@ -2873,11 +2873,10 @@ int find_suitable_fallback(struct free_area *area, unsigned int order,
> if (can_steal_fallback(order, migratetype))
> *can_steal = true;
>
> - if (!only_stealable)
> - return fallback_mt;
> -
> - if (*can_steal)
> + if (!only_stealable || *can_steal)
> return fallback_mt;
> + else
> + break;
> }
>
> return -1;