Re: [PATCH v2 3/3] khugepaged: Reduce race probability between migration and khugepaged

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

On 26/06/25 10:36 am, Lorenzo Stoakes wrote:
> On Thu, Jun 26, 2025 at 06:02:25AM +0100, Lorenzo Stoakes wrote:
>> On Thu, Jun 26, 2025 at 10:29:22AM +0530, Dev Jain wrote:
>>> On 26/06/25 10:27 am, Lorenzo Stoakes wrote:
>>>> On Thu, Jun 26, 2025 at 09:22:28AM +0530, Dev Jain wrote:
>>>>> On 25/06/25 6:58 pm, Lorenzo Stoakes wrote:
>>>>>> On Wed, Jun 25, 2025 at 11:28:06AM +0530, 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.
>>>>>> Hm is this related to the series at all? Seems somewhat unrelated?
>>>>> Not related.
>>>>>
>>>>>> Is there a Fixes, Closes, etc.? Do we need something in stable?
>>>>> We don't need anything. This is an "expected race" in the sense that
>>>>> both migration and khugepaged collapse are best effort algorithms.
>>>>> I am just seeing a test failure on my system because my system hits
>>>>> the race more often. So this patch reduces the window for the race.
>>>> Does it rely on previous patches? If not probably best to send this one
>>>> separately :)
>>> To prevent rebasing headaches for others (if any) I thought to send all together.
>>> I'll send it separately if still that is the preference.
>>>
>>>
>> Oh actually would it be a pain to rebase given the previous 2 patches? Maybe
>> leave it then. And I can actually finally review it... :)
> At the same time you'd have to update cover letter to mention and it's really
> not good practice to include unrelated patches to a series.
>
> So probably best overall to send separately but _wait_ for this series to be
> merged first :>) then you can based it on mm-new without problems.

Sure.