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

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Message-ID: <aGZQUIfiLEMXZ_Cu@localhost.localdomain>
Date: Thu, 3 Jul 2025 11:41:36 +0200
From: Oscar Salvador <osalvador@...e.de>
To: Dev Jain <dev.jain@....com>
Cc: akpm@...ux-foundation.org, david@...hat.com, ziy@...dia.com,
	baolin.wang@...ux.alibaba.com, lorenzo.stoakes@...cle.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] khugepaged: Reduce race probability between migration
 and khugepaged

On Thu, Jul 03, 2025 at 11:18:23AM +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.
> 
> 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@....com>

David's comment refering to a 'present PMD entry' seems more clear to
me, but

Acked-by: Oscar Salvador <osalvador@...e.de>

 

-- 
Oscar Salvador
SUSE Labs