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imf18.hostedemail.com; dkim=none; spf=pass (imf18.hostedemail.com: domain of dev.jain@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=dev.jain@arm.com; dmarc=pass (policy=none) header.from=arm.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1751601883; a=rsa-sha256; cv=none; b=at3E31kPuBHpZHVbcMCQAUtLXzGKiU/9Wcub3lU5eqViLdNOIXqrqYV6OEz21L9IayQmFh N4uSLM0iZSMPraTBq69V1iWvyqffjOBkwvyRLOQQLbNeHSW99WuwrPR22pg0Xb0sxNSVpq Lyw9qvJIwn0N6d512o30MA7/uCs4/Dw= Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id DA334152B; Thu, 3 Jul 2025 21:04:27 -0700 (PDT) Received: from MacBook-Pro.blr.arm.com (MacBook-Pro.blr.arm.com [10.164.18.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id A013C3F63F; Thu, 3 Jul 2025 21:04:37 -0700 (PDT) From: Dev Jain To: akpm@linux-foundation.org, david@redhat.com Cc: ziy@nvidia.com, baolin.wang@linux.alibaba.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, baohua@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Dev Jain , Oscar Salvador , Anshuman Khandual Subject: [PATCH v3] khugepaged: Reduce race probability between migration and khugepaged Date: Fri, 4 Jul 2025 09:34:17 +0530 Message-Id: <20250704040417.63826-1-dev.jain@arm.com> X-Mailer: git-send-email 2.39.5 (Apple Git-154) MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Stat-Signature: so48t6ugw7wanjqtqps3km9og97gzdun X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 0B0FD1C0005 X-HE-Tag: 1751601882-977201 X-HE-Meta: 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 PaZDuXBh qWxvcr5bVmf8USvLvxcgyr+gh+xPqP8aEVIaVoDGcfB33l8QrxC53X2qI7iVwsZHpjEeJnlmqTNH1PkHPpcHRj/rTLUiUX+e0mOOmqUb7WQPdXQ6gJZZx0UBl0VnNKCdiwVbNo+aIlZwq7bxUEiI+Z8sWitUAayRR34O48AKcP/nJJ9Wuvl91rz17a6x0D+4rBWCGTWoQtivSuyRoeDtwHiR/0NYY8oCW5aiDH9+y5Ltq1XE/v6JYbLTKbZOmxV1ZQxuxXr0bFZkAhBxvZ7OWVll1ERK5+uYXTysv X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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". Acked-by: David Hildenbrand Acked-by: Oscar Salvador Reviewed-by: Anshuman Khandual Reviewed-by: Zi Yan Reviewed-by: Baolin Wang Signed-off-by: Dev Jain --- v2->v3: - Improve comment (David) 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 | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 1aa7ca67c756..a55fb1dcd224 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -941,6 +941,14 @@ 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 a present PMD mapping a folio again. + */ + if (is_pmd_migration_entry(pmde)) + return SCAN_PMD_MAPPED; if (!pmd_present(pmde)) return SCAN_PMD_NULL; if (pmd_trans_huge(pmde)) -- 2.30.2