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

["Yajun Deng" <yajun.deng@linux.dev>]

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.

> 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?

>> 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;