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 10:30:08 +0800
From:   "Yin, Fengwei" <fengwei.yin@...el.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.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 10:09 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()?


Regards
Yin, Fengwei