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

[Zi Yan <ziy@nvidia.com>]

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On 9 Feb 2023, at 20:57, Yajun Deng wrote:

> February 9, 2023 11:50 PM, "Zi Yan" <ziy@nvidia.com> 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.

OK. Got it. Would the code below look better?

        for (i = 0; i < MIGRATE_PCPTYPES - 1 ; i++) {
                fallback_mt = fallbacks[migratetype][i];
                if (free_area_empty(area, fallback_mt))
                        continue;
        }

        if (can_steal_fallback(order, migratetype))
                *can_steal = true;

        if (!only_stealable || *can_steal)
                return fallback_mt;

        return -1;

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


--
Best Regards,
Yan, Zi

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