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Mon, 21 Sep 2026 09:34:18 +0000 X-Mizu-Trace-ID: a40a32e2570dfc55 X-Migadu-Flow: FLOW_OUT Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3864.700.51.1.1\)) Subject: Re: [PATCH 3/4] mm: add shared read-only vmemmap support for FS-DAX From: Muchun Song In-Reply-To: <4bc1f4de-40ad-46f0-8d37-4c4e0cad0ad6@amd.com> Date: Mon, 21 Sep 2026 17:33:56 +0800 Cc: Muchun Song , Andrew Morton , Dan Williams , David Hildenbrand , linux-mm@kvack.org, nvdimm@lists.linux.dev, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-cxl@vger.kernel.org, Vishal Verma , Dave Jiang , Alison Schofield , Mike Rapoport , Oscar Salvador , Ira Weiny , Jan Kara , Matthew Wilcox , Lorenzo Stoakes , Vlastimil Babka , Michal Hocko , Qi Zheng Content-Transfer-Encoding: quoted-printable Message-Id: <8EA6782C-7A3D-411F-91CD-031AB97CD024@linux.dev> References: <20260903122128.12264-1-songmuchun@bytedance.com> <20260903122128.12264-4-songmuchun@bytedance.com> <4bc1f4de-40ad-46f0-8d37-4c4e0cad0ad6@amd.com> To: "Gupta, Pankaj" X-Mailer: Apple Mail (2.3864.700.51.1.1) X-Rspamd-Server: rspam04 X-Rspam-User: X-Stat-Signature: hnm95tz68qmcd1yhso5adg4qjqrbb461 X-Rspamd-Queue-Id: 6C1291C0002 X-HE-Tag: 1789983259-723179 X-HE-Meta: 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 GW6ZHI4e U7e5SxQN9JOrNvxJxeaNlqfENLCMaNGhebst1vlH6gZZsA48sdv5m3sE9ovCDU+nBh0tXer043/6SLYR6t5vRjvG9xfanGL+eDwmrgznqVUNrJmYZHlTFkRK9CD0qqEHdSBUwq2d/FiE+3xQHRb/dxNFmgs6iwMpyhssYWsLULCYuivzjVOAhTV+qL9DtW9OZ+eGRSRpPpisq33o4CLK6xfyBCjdHpoB1Piim6lxX2n3MS9nLpRD9x8L9NSLk6IExHbYYWuw+lln4BJCrmqk/hWzAmS79H6/eNmeZ7pXmBS0da1MrhNhEnpiGKA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: > On Sep 21, 2026, at 14:42, Gupta, Pankaj wrote: >=20 >>=20 >> FS-DAX registers persistent-memory ranges as ZONE_DEVICE memory, and = the >> kernel normally allocates and initializes vmemmap storage for every >> advertised PFN up front. Sparse pmem images and workloads that only = use the >> DAX direct-access path may never need writable per-PFN state for most = of >> that range, but still pay the memory and initialization cost. >>=20 >> Add an opt-in dev_pagemap mode that populates FS-DAX vmemmap PTEs = from a >> shared read-only metadata page. The shared page is initialized with = the >> common ZONE_DEVICE and dev_pagemap state, so every PFN still has a = valid >> struct page representation while private metadata allocation is = deferred. >>=20 >> This relies on sizeof(struct page) being a power of two, so each = vmemmap >> page contains a naturally aligned and repeatable set of struct page = slots. >> It also requires architecture support for runtime vmemmap remapping, >> because shared mappings must be replaced with private writable pages = before >> a PFN can enter userspace mappings. >>=20 >> The initial implementation is deliberately limited to a single >> memory-block-aligned range. That is not a fundamental requirement, = but keeps >> the registration and teardown paths simple; support for multiple = ranges or >> less strict alignment can be added later. >>=20 >> Provide vmemmap_materialize_page() to replace shared mappings in the >> requested metadata range with private writable copies. A later patch = will >> call it from the FS-DAX fault path. >>=20 >> No caller enables the mode yet. >>=20 >> Signed-off-by: Muchun Song >> --- >> include/linux/memremap.h | 11 ++++++- >> mm/memremap.c | 38 ++++++++++++++++++++++-- >> mm/mm_init.c | 11 +++++++ >> mm/sparse-vmemmap.c | 64 = +++++++++++++++++++++++++++++++++++++--- >> 4 files changed, 116 insertions(+), 8 deletions(-) >>=20 >> diff --git a/include/linux/memremap.h b/include/linux/memremap.h >> index e3c2ccf872a8..21c9b6aeef67 100644 >> --- a/include/linux/memremap.h >> +++ b/include/linux/memremap.h >> @@ -9,6 +9,7 @@ >> struct resource; >> struct device; >> +struct page; >> /** >> * struct vmem_altmap - pre-allocated storage for vmemmap_populate >> @@ -108,7 +109,8 @@ struct dev_pagemap_ops { >> void (*folio_split)(struct folio *head, struct folio *tail); >> }; >> -#define PGMAP_ALTMAP_VALID (1 << 0) >> +#define PGMAP_ALTMAP_VALID BIT(0) >> +#define PGMAP_VMEMMAP_OPTIMIZATION BIT(1) >=20 > I liked the overall idea. >=20 > Minor naming suggestion I have: >=20 > 'PGMAP_VMEMMAP_OPTIMIZATION' feel too generic to me. >=20 > Maybe something on the lines to reflect the actual optimization: >=20 > |PGMAP_SHARED_VMEMMAP or PGMAP_VMEMMAP_ON_DEMAND or some_other_name?| Maybe PGMAP_VMEMMAP_SHARED? It describes the shared vmemmap backing more precisely. I prefer SHARED over ON_DEMAND. If you are OK with this, I will keep it for the next version. Thanks, Muchun