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

[Dev Jain <dev.jain@arm.com>]

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@arm.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.
>  */
>
Thanks for the suggestion, but

PMD pointing to PMD-order folio necessarily implies present PMD entry,

but the converse is not true? For example it may point to a PTE table.

So I wanted to make it explicitly clear that the transient entry will be

converted to a PMD-THP in the end.


>
> Acked-by: David Hildenbrand <david@redhat.com>
Thanks!