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Tue, 08 Sep 2026 08:34:31 +0000 X-Mizu-Trace-ID: ef271c2c22e4d45d X-Migadu-Flow: FLOW_OUT Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3864.700.51.1.1\)) Subject: Re: [PATCH v2 06/11] mm/sparse-vmemmap: switch device DAX to shared tail vmemmap pages From: Muchun Song In-Reply-To: <1fc41803-a0b3-48ca-84a6-caf965fb11f9@linux.dev> Date: Tue, 8 Sep 2026 16:34:09 +0800 Cc: Muchun Song , Andrew Morton , 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 , Nicholas Piggin , Christophe Leroy , Randy Dunlap Content-Transfer-Encoding: quoted-printable Message-Id: References: <20260908030335.96549-1-songmuchun@bytedance.com> <20260908030335.96549-7-songmuchun@bytedance.com> <1fc41803-a0b3-48ca-84a6-caf965fb11f9@linux.dev> To: Qi Zheng X-Mailer: Apple Mail (2.3864.700.51.1.1) X-Rspamd-Server: rspam11 X-Rspam-User: X-Stat-Signature: aojnioh3bpzzkjb6yfgggzasikyqm74i X-Rspamd-Queue-Id: 2369E20009 X-HE-Tag: 1788856482-133202 X-HE-Meta: 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 Q6fMm6ot W3hwLNxiw+5BiO8bVkKc4q14+PCVKtd9B8Y+cqqiKwA+fMbXNnDjtP/YCJcu4w5V7s38OncvNf4ONSP/PEgNkA90GdUYayr7FbIWkn/F0sMJmMq9mQY5oylFDjYGvDSzkI+W4DkBYYh8cuyYmx7Y1ZqOxwm3UF2dgBONZcqMWNJAs9A0pjE+yyIlGCcQ/QikP/uzGwjihjnvvAwBkYiN3dPJzTkvqQg4EiG/in29iQczNb7YEx5RsPqz16idxutkr42D8Ozp6Wx/5fcZp5FSUVTpkA6DvaiGmV8O1 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: > On Sep 8, 2026, at 15:46, Qi Zheng wrote: >=20 >=20 >=20 > On 9/8/26 11:03 AM, Muchun Song wrote: >> HugeTLB vmemmap optimization now uses per-zone shared tail vmemmap = pages. >> Device DAX has not been switched to that mechanism yet. >> Switch device DAX to vmemmap_shared_tail_page() as well. This aligns = DAX >> with HugeTLB by using the common per-zone shared tail vmemmap page. >> The optimization is enabled only for DEV-DAX through = pgmap->vmemmap_shift, >> which is assigned when a DEV-DAX device is initialized. Unlike = FS-DAX, >> DEV-DAX does not modify tail struct pages, so sharing them is safe. >=20 > Got it =E2=80=94 adding this description makes things significantly = clearer. Aha, also makes Sashiko happy. This time, it is no longer confused with FS-DAX. >=20 >> Since the shared tail page can now back ZONE_DEVICE vmemmap mappings, >> initialize its entries with PG_reserved for device zones. Also skip >> poisoning vmemmap-optimizable sections while their struct pages may = be >> shared. >> Signed-off-by: Muchun Song >> --- >> v2: >> - Explain why sharing tail vmemmap pages is safe for DEV-DAX >> (suggested by Qi Zheng) >> --- >> include/linux/mmzone.h | 10 +++++++++ >> mm/memory_hotplug.c | 5 +++-- >> mm/sparse-vmemmap.c | 47 = ++++++++++++++---------------------------- >> 3 files changed, 28 insertions(+), 34 deletions(-) >=20 > Acked-by: Qi Zheng Thanks. >=20 > Thanks, > Qi