From nobody Thu Apr 9 09:25:49 2026 Received: from out-179.mta0.migadu.com (out-179.mta0.migadu.com [91.218.175.179]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 62362346AE5 for ; Mon, 9 Mar 2026 21:25:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.179 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773091527; cv=none; b=STShTQqSDC9vz/Wz1trn7BRCy7AP5KwWjTJUj3Fk1lLkwJgIjyX1KInlSaojXDld8Oa8zq7UV7AOCNSwWGfRaGrVPu8o4M0WF348NvlCYYZ2gAzRicvYj3rMDQ/anbGTuZCyTXzTe8NaTFSNJx9nOcohFchSV2EmB+cUNMvRnE4= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773091527; c=relaxed/simple; bh=RUZQxH7uOZqIDKIJ2xtZyjfq56VMoEIKYIVsS+CGqNo=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=EN48gnM3xj/+1FMYjo9nsu0HFhoIHm+P2D20Terb86DKd937BEbq+i9YLgaWsOgKwdgVZ813vT3UvvCBXcNLCz/7u27fTCWNZXgufVTuJ0+/DesLrtU5u/uk30VBFUlKB/2X/UxmQycV2YQx61ENEs+CaR4gFuzNkVbOHByPjo4= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=lvgs9JvJ; arc=none smtp.client-ip=91.218.175.179 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="lvgs9JvJ" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. 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> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Migadu-Flow: FLOW_OUT Content-Type: text/plain; charset="utf-8" split_huge_pmd_address() with freeze=3Dtrue 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 Acked-by: David Hildenbrand (Arm) Reviewed-by: Wei Yang --- v1 -> v2: - Remove warning if folio refcount !=3D1 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 mig= rate_vma *migrate, unsigned long flags; int ret =3D 0; =20 + /* + * take a reference, since split_huge_pmd_address() with freeze =3D true + * drops a reference at the end. + */ folio_get(folio); split_huge_pmd_address(migrate->vma, addr, true); ret =3D folio_split_unmapped(folio, 0); --=20 2.47.3