linux-kernel - Re: [PATCH v2] khugepaged: Reduce race probability between migration and khugepaged

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Message-ID: <474399c9-21cf-409c-883b-85b437979503@arm.com>
Date: Thu, 3 Jul 2025 15:10:38 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: David Hildenbrand <david@...hat.com>, Dev Jain <dev.jain@....com>,
 akpm@...ux-foundation.org
Cc: ziy@...dia.com, baolin.wang@...ux.alibaba.com,
 lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, baohua@...nel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] khugepaged: Reduce race probability between migration
 and khugepaged



On 03/07/25 2:55 PM, David Hildenbrand wrote:
> On 03.07.25 07:48, Dev Jain wrote:
>> Suppose a folio is under migration, and khugepaged is also trying to
>> collapse it. collapse_pte_mapped_thp() will retrieve the folio from the
>> page cache via filemap_lock_folio(), thus taking a reference on the folio
>> and sleeping on the folio lock, since the lock is held by the migration
>> path. Migration will then fail in
>> __folio_migrate_mapping -> folio_ref_freeze. Reduce the probability of
>> such a race happening (leading to migration failure) by bailing out
>> if we detect a PMD is marked with a migration entry.
>>
>> This fixes the migration-shared-anon-thp testcase failure on Apple M3.
>>
>> Note that, this is not a "fix" since it only reduces the chance of
>> interference of khugepaged with migration, wherein both the kernel
>> functionalities are deemed "best-effort".
>>
>> Signed-off-by: Dev Jain <dev.jain@....com>
>> ---
>>
>> v1->v2:
>>   - Remove SCAN_PMD_MIGRATION, merge into SCAN_PMD_MAPPED (David, Anshuman)
>>   - Add a comment (Lorenzo)
>>
>> v1:
>>   - https://lore.kernel.org/all/20250630044837.4675-1-dev.jain@arm.com/
>>
>>   mm/khugepaged.c | 9 +++++++++
>>   1 file changed, 9 insertions(+)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 1aa7ca67c756..3fdefc4f4984 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -941,6 +941,15 @@ static inline int check_pmd_state(pmd_t *pmd)
>>         if (pmd_none(pmde))
>>           return SCAN_PMD_NONE;
>> +
>> +    /*
>> +     * The folio may be under migration when khugepaged is trying to
>> +     * collapse it. Migration success or failure will eventually end
>> +     * up with the PMD still pointing to a PMD-order folio, so return
>> +     * SCAN_PMD_MAPPED.
> 
> Nit: the last part (, so return ..) is obvious from the code.
> 
> I would have written
> 
> /*
>  * The folio may be under migration when khugepaged is trying to
>  * collapse it. Migration success or failure will eventually end
>  * up with a present PMD entry again.
>  */

+1

> 
> 
> Acked-by: David Hildenbrand <david@...hat.com>
>

Reviewed-by: Anshuman Khandual <anshuman.khandual@....com>