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DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1773091523; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=S4UPmqAJABaUK73z85xpd7p4z1y5KCxWljZJflvXjRo=; b=lvgs9JvJj2k9QFImZ28+WWvrJ3RN9lq2IPQz0SVXjNs8UMfe0T4V7KW1vPr2Nwjyf+1iMR 99dpCJuWyAmhsSM2l44dYmrgk2cU10UnG1Ve51vQ39cZ6PDobKXQ1++LZWM5504ltAlTQD aGVTDoZpWcf662p8JO5y7m1gPJDIg4E= From: Usama Arif To: Andrew Morton , npache@redhat.com, david@kernel.org, ziy@nvidia.com, linux-mm@kvack.org Cc: matthew.brost@intel.com, joshua.hahnjy@gmail.com, hannes@cmpxchg.org, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, linux-kernel@vger.kernel.org, kernel-team@meta.com, Usama Arif Subject: [PATCH v2] mm/migrate_device: document folio_get requirement before frozen PMD split Date: Mon, 9 Mar 2026 14:25:02 -0700 Message-ID: <20260309212502.3922825-1-usama.arif@linux.dev> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Stat-Signature: mwfdgj6wo5bx6kyp3tkcp8qtdj6uerah X-Rspam-User: X-Rspamd-Queue-Id: 98204140018 X-Rspamd-Server: rspam12 X-HE-Tag: 1773091525-290044 X-HE-Meta: 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 mNvAr64d rgBijm0yBya+1P1zaaqUskapc6WpNDmmcNEDpA22E2P8SdtfgFb90TidKmpbVno/3NUf45oGz9gjoGjx8Hysg/jwZuatgPkEqqJDgyWtz3EJ5YgNTFWSqYixBmJF7/skJ0jjEzAotY81m/VErWH/qMAVtgm1wEYBtXcVR9SreXhOtpNS/bydOEykUYmpKeRH4/R48rVXkAp1S3bFOoEA3q+81q/CzxuufEsncfA989kS1wbBi/9f8bEL/zhm3xvZ4iIlOEDWUycglqij5QYazbZtI3GIc4sVRLHRevh8HNmfrjlxB5NwpXweO4sMn+KJP3rfRzuMPch8BEuLkvc2fqQXt1qv2DGS7F4BVlohKOUHijNI= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: split_huge_pmd_address() with freeze=true splits a PMD migration entry into PTE migration entries, consuming one folio reference in the process. The folio_get() before it provides this reference. Add a comment explaining this relationship. The expected folio refcount at the start of migrate_vma_split_unmapped_folio() is 1. Suggested-by: Zi Yan Reviewed-by: Zi Yan Reviewed-by: Nico Pache Signed-off-by: Usama Arif --- v1 -> v2: - Remove warning if folio refcount !=1 at the start of migrate_vma_split_unmapped_folio() (David) --- mm/migrate_device.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/mm/migrate_device.c b/mm/migrate_device.c index 78c7acf02461..fbfe5715f635 100644 --- a/mm/migrate_device.c +++ b/mm/migrate_device.c @@ -908,6 +908,10 @@ static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate, unsigned long flags; int ret = 0; + /* + * take a reference, since split_huge_pmd_address() with freeze = true + * drops a reference at the end. + */ folio_get(folio); split_huge_pmd_address(migrate->vma, addr, true); ret = folio_split_unmapped(folio, 0); -- 2.47.3