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

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

On 26/06/25 10:32 am, 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

Didn't take the time to actually check that, more of a "it *may* be a pain
so let's send this together".

> leave it then. And I can actually finally review it... :)