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SAWPR12MB999165.namprd12.prod.outlook.com (2603:10b6:806:4e1::7) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.428.16; Mon, 21 Sep 2026 11:05:16 +0000 Received: from DS0PR12MB7771.namprd12.prod.outlook.com ([fe80::9a3e:791f:33b6:3d8c]) by DS0PR12MB7771.namprd12.prod.outlook.com ([fe80::9a3e:791f:33b6:3d8c%4]) with mapi id 15.21.0428.015; Mon, 21 Sep 2026 11:05:16 +0000 Message-ID: Date: Mon, 21 Sep 2026 13:05:04 +0200 User-Agent: Mozilla Thunderbird From: "Gupta, Pankaj" Subject: Re: [PATCH 3/4] mm: add shared read-only vmemmap support for FS-DAX To: Muchun Song 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 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pspG9vSUH/RvrpTq7iJmSzKNcQcKAFH18RvfCu+l78AKuhvVzxXdvk0P9nklc58FCg0wkhwuMSsGTehzqc61t9q/dOnD3JZ3M1/9YLetfYJn9c5z/uumxElhDNPUgKHKFwTT5CiHxmrYSq1g3+BcDGfSIo8Hg1fWYFOAaPIUx6WC/zVW4X86ykrIWR6/mKcGrGQbGVqI6r253wo6SccuLw+L4HGKcbT1OoRV0bK/G8VptYCD8E5jYjT4VXeIzHpLnLfK1AdFNU+z2SByuGQ/W49/g/GUWuBzPuNu+VyYIPd7iWm/rk2evRqAlBIzVzGop5brhwpXmh2sZS7UaKpt7jDkhDHGFtFGOywYtVZp912YPIvUQx2aalKACmdIb3zB3cqyp8qSAbfcWAk1asF4rVdph1D1OyP42jdKZRhSOaIKSyUwxIBl4JGiRBRLfCb1PMZA3/KVKVHas3Glp4bEFFlC7Ig== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >>> 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. >>> >>> 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. >>> >>> 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. >>> >>> 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. >>> >>> 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. >>> >>> No caller enables the mode yet. >>> >>> 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(-) >>> >>> 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) >> I liked the overall idea. >> >> Minor naming suggestion I have: >> >> 'PGMAP_VMEMMAP_OPTIMIZATION' feel too generic to me. >> >> Maybe something on the lines to reflect the actual optimization: >> >> |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. Yes, PGMAP_VMEMMAP_SHARED sounds better. Thanks, Pankaj