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

[David Hildenbrand <david@redhat.com>]

On 03.07.25 11:52, Dev Jain wrote:
> 
> 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.

I see, talking about orders is confusing though.

"with a present PMD mapping a folio again."

-- 
Cheers,

David / dhildenb