Re: [PATCH] mm: migrate: record the mlocked page status to remove unnecessary lru drain

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 10/19/2023 4:22 PM, Yin Fengwei wrote:
> Hi Baolin,
> 
> On 10/19/23 15:25, Baolin Wang wrote:
>>
>>
>> On 10/19/2023 2:09 PM, Huang, Ying wrote:
>>> Zi Yan <ziy@nvidia.com> writes:
>>>
>>>> On 18 Oct 2023, at 9:04, Baolin Wang wrote:
>>>>
>>>>> When doing compaction, I found the lru_add_drain() is an obvious hotspot
>>>>> when migrating pages. The distribution of this hotspot is as follows:
>>>>>      - 18.75% compact_zone
>>>>>         - 17.39% migrate_pages
>>>>>            - 13.79% migrate_pages_batch
>>>>>               - 11.66% migrate_folio_move
>>>>>                  - 7.02% lru_add_drain
>>>>>                     + 7.02% lru_add_drain_cpu
>>>>>                  + 3.00% move_to_new_folio
>>>>>                    1.23% rmap_walk
>>>>>               + 1.92% migrate_folio_unmap
>>>>>            + 3.20% migrate_pages_sync
>>>>>         + 0.90% isolate_migratepages
>>>>>
>>>>> The lru_add_drain() was added by commit c3096e6782b7 ("mm/migrate:
>>>>> __unmap_and_move() push good newpage to LRU") to drain the newpage to LRU
>>>>> immediately, to help to build up the correct newpage->mlock_count in
>>>>> remove_migration_ptes() for mlocked pages. However, if there are no mlocked
>>>>> pages are migrating, then we can avoid this lru drain operation, especailly
>>>>> for the heavy concurrent scenarios.
>>>>
>>>> lru_add_drain() is also used to drain pages out of folio_batch. Pages in folio_batch
>>>> have an additional pin to prevent migration. See folio_get(folio); in folio_add_lru().
>>>
>>> lru_add_drain() is called after the page reference count checking in
>>> move_to_new_folio().  So, I don't this is an issue.
>>
>> Agree. The purpose of adding lru_add_drain() is to address the 'mlock_count' issue for mlocked pages. Please see commit c3096e6782b7 and related comments. Moreover I haven't seen an increase in the number of page migration failures due to page reference count checking after this patch.
> 
> I agree with your. My understanding also is that the lru_add_drain() is only needed
> for mlocked folio to correct mlock_count. Like to hear the confirmation from Huge.
> 
> 
> But I have question: why do we need use page_was_mlocked instead of check
> folio_test_mlocked(src)? Does page migration clear the mlock flag? Thanks.

Yes, please see the call trace: try_to_migrate_one() ---> 
page_remove_rmap() ---> munlock_vma_folio().