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

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

On 03/07/25 3:39 pm, David Hildenbrand wrote:
> 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."

Sure. Shall respin.