Re: [PATCH v2] mm/page_alloc: optimize find_suitable_fallback() and fallbacks array

[Yajun Deng]

February 9, 2023 11:50 PM, "Zi Yan" <ziy@xxxxxxxxxx> wrote:

> On 9 Feb 2023, at 5:11, 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.
> 
> Can you explain why? If it is the case, MIGRATE_UNMOVABLE cannot fall back
> to MIGRATE_MOVABLE? And MIGRATE_MOVABLE cannot fall back to MIGRATE_UNMOVABLE?
> And MIGRATE_RECLAIMABLE cannot fall back to MIGRATE_MOVABLE?
> 

The return in the loop is only related to 'order', 'migratetype' and 'only_stealable'
variables. Even if it execute the next loop, it can't change the result. So the loop
can be broken if the first loop can't return.

>> At the same time, add !migratetype_is_mergeable() before the loop and
>> reduce the first index size from MIGRATE_TYPES to MIGRATE_PCPTYPES in
>> fallbacks array.
> 
> You sent a patch: https://lore.kernel.org/all/20230203100132.1627787-1-yajun.deng@xxxxxxxxx/T/#u,
> why not squash this one into that? Why do
> we need two separate small patches working on the same code?
> 

Yes, this is better, but I overlooked this one when I sent the first patch. It is already merged.

As Vlastimil Babka said, reduce the first index from MIGRATE_TYPES to MIGRATE_PCPTYPES may be 
cause out of bounds access of the shrinked fallbacks array If we don't judge the range of 
migratetype. But this doesn't happen with the second index.

> Thanks.
> 
>> Signed-off-by: Yajun Deng <yajun.deng@xxxxxxxxx>
>> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
>> ---
>> include/linux/mmzone.h | 2 +-
>> mm/page_alloc.c | 11 +++++------
>> 2 files changed, 6 insertions(+), 7 deletions(-)
>> 
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index ab94985ee7d9..0a817b8c7fb2 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -85,7 +85,7 @@ static inline bool is_migrate_movable(int mt)
>> * Check whether a migratetype can be merged with another migratetype.
>> *
>> * It is only mergeable when it can fall back to other migratetypes for
>> - * allocation. See fallbacks[MIGRATE_TYPES][3] in page_alloc.c.
>> + * allocation. See fallbacks[][] array in page_alloc.c.
>> */
>> static inline bool migratetype_is_mergeable(int mt)
>> {
>> 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))
>> 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;
>> --
>> 2.25.1
> 
> --
> Best Regards,
> Yan, Zi