From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta1.migadu.com (out-86.mta1.migadu.com [95.215.58.86]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8E1B12D7398 for ; Sun, 27 Sep 2026 10:51:37 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.86 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790506299; cv=none; b=aVcRh+3KG2/Q1k/uGScszek0GbolOdOdpj4LpGjAbAOGVQy/QP4BIcSWn+uwVyipE4YEfUeAyTUa6yGFuetUV3qKrwoS6ftUrZa1OkmXxBWT9fKm01azVT/7/uChHnN/XIG9Z8IzS4llvCWqWdrHB/Ebx4RSt0vOTqmnw3IVUSU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790506299; c=relaxed/simple; bh=1vTYwK/wvoIR3O8GrBIjGhV9ooIB1stkGBEkt4hT5EY=; h=Content-Type:Mime-Version:Subject:From:In-Reply-To:Date:Cc: Message-Id:References:To; b=C5mIkl82Psa3H5StVqbHqfPlM0jDJKdmuDwPxvdK8Ltm0oxKCXwsuE0/YXKRLYJLWKDbDQChe4ik8tcSBE4uV3vnOgYKQ3qZYJRsI+nMNYcTzfui05Sxlh2usyA38IS2+eVZJOMdBgV4FGpSAmbAblSMjGeAYCCDTqVmFZ/zbEQ= 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=i5d601uz; arc=none smtp.client-ip=95.215.58.86 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="i5d601uz" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=1vTYwK/wvoIR3O8GrBIjGhV9ooIB1stkGBEkt4hT5EY=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1790506295; v=1; x=1791111095; b=i5d601uzeXfntEWeu5aEZKekL2SJaQ+BqNrPwfWm7tERCOIu/rBhChI/QbkHtvdYlkFIrfrW esa1G2NaW1Mxh0RtDEviIR0poJXlo+jyAix//Tf/xX4QVTSambMDqvTV5RBjxSljyAedn5RXPYe DkGy3aYx+H/ntzgLbcEE73yQ= X-Envelope-To: linux-kernel@vger.kernel.org Received: by mta12.migadu.com with ESMTPS id 5500dc4e2682cac7; Sun, 27 Sep 2026 10:51:35 +0000 X-Mizu-Trace-ID: 5500dc4e2682cac7 X-Migadu-Flow: FLOW_OUT Content-Type: text/plain; charset=us-ascii Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3901.100.1.1.11\)) Subject: Re: [PATCH v5 00/12] mm: Switch device DAX to section-based vmemmap optimization From: Muchun Song In-Reply-To: <20260926225105.a56f29d76b2f496c8dc2dac0@linux-foundation.org> Date: Sun, 27 Sep 2026 18:51:15 +0800 Cc: Muchun Song , David Hildenbrand , Oscar Salvador , Madhavan Srinivasan , Michael Ellerman , Jonathan Corbet , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-doc@vger.kernel.org, Lorenzo Stoakes , Mike Rapoport , Qi Zheng , Nicholas Piggin , Christophe Leroy , Randy Dunlap , Lance Yang Content-Transfer-Encoding: quoted-printable Message-Id: References: <20260927025441.741633-1-songmuchun@bytedance.com> <20260926225105.a56f29d76b2f496c8dc2dac0@linux-foundation.org> To: Andrew Morton X-Mailer: Apple Mail (2.3901.100.1.1.11) > On Sep 27, 2026, at 13:51, Andrew Morton = wrote: >=20 > On Sun, 27 Sep 2026 10:54:29 +0800 Muchun Song = wrote: >=20 >> After the HugeTLB conversion, optimized vmemmap state is described by >> the memory section and the sparse-vmemmap population path can = allocate or >> reuse shared tail vmemmap pages based on that metadata. Device DAX = still >> uses the older DAX-specific population model, including a separate = tail >> vmemmap page reservation and architecture-specific logic to locate or >> populate reusable tail pages. >>=20 >> This series makes device DAX use the same section-based model. Device = DAX >> records the compound page order from pgmap->vmemmap_shift in section >> metadata before vmemmap population, uses the common per-zone shared = tail >> vmemmap page, and drops the extra reserved tail page. The powerpc = radix >> path is updated to use the same shared tail-page helper, so the = generic >> and powerpc DAX paths follow the same reservation model. >=20 > Thanks, I've updated mm-unstable to this version. Thanks. >=20 > Sashiko asked a thing: > = https://sashiko.dev/#/patchset/20260927025441.741633-1-songmuchun@bytedanc= e.com Sashiko said page->refcount can overflow by incrementing it over 2.14 = billion times when mapping more than **524 TB** of DEV-DAX memory on a single = NUMA node, where the pages share the same node, order, and zone. I am not aware of any practical hardware configuration approaching this topology today. Handling that theoretical limit would add non-trivial lifetime or architecture-specific teardown complexity. Without a concrete hardware requirement, I prefer not to over-engineer the current series. We can = revisit it when such a system or use case becomes realistic. Thanks.