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

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Date:   Fri, 20 Oct 2023 11:27:24 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     "Yin, Fengwei" <fengwei.yin@...el.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>,
        Yosry Ahmed <yosryahmed@...gle.com>
Cc:     akpm@...ux-foundation.org, mgorman@...hsingularity.net,
        hughd@...gle.com, vbabka@...e.cz, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: migrate: record the mlocked page status to remove
 unnecessary lru drain



On 10/20/2023 10:54 AM, Yin, Fengwei wrote:
> 
> 
> On 10/20/2023 10:45 AM, Baolin Wang wrote:
>>
>>
>> On 10/20/2023 10:30 AM, Yin, Fengwei wrote:
>>>
>>>
>>> On 10/20/2023 10:09 AM, Baolin Wang wrote:
>>>>
>>>>
>>>> On 10/19/2023 8:07 PM, Yin, Fengwei wrote:
>>>>>
>>>>>
>>>>> On 10/19/2023 4:51 PM, Baolin Wang wrote:
>>>>>>
>>>>>>
>>>>>> 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@...dia.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().
>>>>>
>>>>> Yes. This will clear mlock bit.
>>>>>
>>>>> What about set dst folio mlocked if source is before try_to_migrate_one()? And
>>>>> then check whether dst folio is mlocked after? And need clear mlocked if migration
>>>>> fails. I suppose the change is minor. Just a thought. Thanks.
>>>>
>>>> IMO, this will break the mlock related statistics in mlock_folio() when the remove_migration_pte() rebuilds the mlock status and mlock count.
>>>>
>>>> Another concern I can see is that, during the page migration, a concurrent munlock() can be called to clean the VM_LOCKED flags for the VMAs, so the remove_migration_pte() should not rebuild the mlock status and mlock count. But the dst folio's mlcoked status is still remained, which is wrong.
>>>>
>>>> So your suggested apporach seems not easy, and I think my patch is simple with re-using existing __migrate_folio_record() and __migrate_folio_extract() :)
>>>
>>> Can these concerns be addressed by clear dst mlocked after lru_add_drain() but before
>>> remove_migration_pte()?
>>
>> IMHO, that seems too hacky to me. I still prefer to rely on the migration process of the mlcock pages.
> 
> BTW, Yosry tried to address the overlap of field lru and mlock_count:
> https://lore.kernel.org/lkml/20230618065719.1363271-1-yosryahmed@google.com/
> But the lore doesn't group all the patches.

Thanks for the information. I'd like to review and test if this work can 
continue.