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

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Message-ID: <561b17cd-ad82-45ca-aba2-78515873ebb8@arm.com>
Date: Thu, 26 Jun 2025 10:29:22 +0530
From: Dev Jain <dev.jain@....com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, ziy@...dia.com,
 baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, baohua@...nel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] khugepaged: Reduce race probability between
 migration and khugepaged


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.