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

[Vlastimil Babka <vbabka@suse.cz>]

On 2/9/23 09:44, Yajun Deng wrote:
> February 9, 2023 4:12 PM, "Vlastimil Babka" <vbabka@suse.cz> wrote:
> 
>> 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?
>> 
> 
> No, I'm not sure if it is called with a non-mergeable migratetype.
> It is just for extra safety.

OK, I agree with that.

>> 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...
>> 
> 
> Yes, this comment should be changed.
> So do I need to submit a v2 patch?

Please do, with my acked-by.

>>> 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;