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Mon, 28 Sep 2026 04:26:18 +0000 X-Mizu-Trace-ID: 061b4c85c7534b56 X-Migadu-Flow: FLOW_OUT Content-Type: text/plain; charset=utf-8 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: <20260927125452.0c1ec382905482841a4faef7@linux-foundation.org> Date: Mon, 28 Sep 2026 12:25:58 +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: <0E16642B-8180-4047-B846-40E3CC52E7D8@linux.dev> References: <20260927025441.741633-1-songmuchun@bytedance.com> <20260926225105.a56f29d76b2f496c8dc2dac0@linux-foundation.org> <20260927125452.0c1ec382905482841a4faef7@linux-foundation.org> To: Andrew Morton X-Mailer: Apple Mail (2.3901.100.1.1.11) X-Stat-Signature: kg8qsu177zfjuyuxok5w33f9zb7uys39 X-Rspamd-Queue-Id: 2FF3BA0004 X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1790569579-621910 X-HE-Meta: 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 wCPHSdFd eIySinWEl7bCmYRAhNYQ9ExmhuHOkMMI9feIlLrc0t8/pWCDDgG6p8JKpiCEJDeQx0q0SFc7wWDcHuoAVImw2PwCpN7jQAhHGANedb8sw5GH/2r5vDV4ETC89p88FVHMVzXrwIQz8SgPBE5N1olMQiqZXd9vwZtuapRCBF1fLJ29R+FYzRzA+7KLbkfLhwAAhtw+ObD/Qzyjm98nzuayQac3OJMi2eVMCSo91gfZniH1Z9zncepsfmCnzgjMDNhxYDNfr6/NSt/bg0wBWDd7cRWipfIlmR0j9EPYACOvXa3rsd1hKC7ean5pqcN8fHsQ95YI5YiPV40LCmnl+H6JFTwKFKfrFvn5vV1kJUaY1AgkAGDm2KRb1hb988xpfiHCnU7MaXiStxzB9tSLNYXlnmZfngg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: > On Sep 28, 2026, at 03:54, Andrew Morton = wrote: >=20 > On Sun, 27 Sep 2026 18:51:15 +0800 Muchun Song = wrote: >=20 >>=20 >>=20 >>> 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. >>=20 >> Thanks. >>=20 >>>=20 >>> Sashiko asked a thing: >>> = https://sashiko.dev/#/patchset/20260927025441.741633-1-songmuchun@bytedanc= e.com >>=20 >> 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. >>=20 >> I am not aware of any practical hardware configuration approaching = this >> topology today. >>=20 >> 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. >=20 > OK. Presumably it would be cheap to add a check for this craziness = and > return ENOSOMETHING? Sounds right =E2=80=94 I'll send a follow-up fixup patch shortly. Muchun, Thanks.