[{"id":3677746,"web_url":"http://patchwork.ozlabs.org/comment/3677746/","msgid":"","date":"2026-04-15T15:58:47","subject":"Re: [PATCH v2 3/6] mm/sparse-vmemmap: Fix DAX vmemmap accounting\n with optimization","submitter":{"id":77733,"url":"http://patchwork.ozlabs.org/api/people/77733/","name":"Mike Rapoport","email":"rppt@kernel.org"},"content":"On Wed, Apr 15, 2026 at 07:14:09PM +0800, Muchun Song wrote:\n> When vmemmap optimization is enabled for DAX, the nr_memmap_pages\n> counter in /proc/vmstat is incorrect. The current code always accounts\n> for the full, non-optimized vmemmap size, but vmemmap optimization\n> reduces the actual number of vmemmap pages by reusing tail pages. This\n> causes the system to overcount vmemmap usage, leading to inaccurate\n> page statistics in /proc/vmstat.\n> \n> Fix this by introducing section_vmemmap_pages(), which returns the exact\n> vmemmap page count for a given pfn range based on whether optimization\n> is in effect.\n> \n> Fixes: 15995a352474 (\"mm: report per-page metadata information\")\n> Signed-off-by: Muchun Song \n\nAcked-by: Mike Rapoport (Microsoft) \n\n> ---\n> mm/sparse-vmemmap.c | 32 ++++++++++++++++++++++++++++----\n> 1 file changed, 28 insertions(+), 4 deletions(-)\n> \n> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c\n> index 40290fbc1db4..05e3e2b94e32 100644\n> --- a/mm/sparse-vmemmap.c\n> +++ b/mm/sparse-vmemmap.c\n> @@ -652,6 +652,29 @@ void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn)\n> \t}\n> }\n> \n> +static int __meminit section_vmemmap_pages(unsigned long pfn, unsigned long nr_pages,\n> +\t\t\t\t\t struct vmem_altmap *altmap,\n> +\t\t\t\t\t struct dev_pagemap *pgmap)\n> +{\n> +\tunsigned int order = pgmap ? pgmap->vmemmap_shift : 0;\n> +\tunsigned long pages_per_compound = 1L << order;\n> +\n> +\tVM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, min(pages_per_compound,\n> +\t\t\t\t\t\t\tPAGES_PER_SECTION)));\n> +\tVM_WARN_ON_ONCE(pfn_to_section_nr(pfn) != pfn_to_section_nr(pfn + nr_pages - 1));\n> +\n> +\tif (!vmemmap_can_optimize(altmap, pgmap))\n> +\t\treturn DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE);\n> +\n> +\tif (order < PFN_SECTION_SHIFT)\n> +\t\treturn VMEMMAP_RESERVE_NR * nr_pages / pages_per_compound;\n> +\n> +\tif (IS_ALIGNED(pfn, pages_per_compound))\n> +\t\treturn VMEMMAP_RESERVE_NR;\n> +\n> +\treturn 0;\n> +}\n> +\n> static struct page * __meminit populate_section_memmap(unsigned long pfn,\n> \t\tunsigned long nr_pages, int nid, struct vmem_altmap *altmap,\n> \t\tstruct dev_pagemap *pgmap)\n> @@ -659,7 +682,7 @@ static struct page * __meminit populate_section_memmap(unsigned long pfn,\n> \tstruct page *page = __populate_section_memmap(pfn, nr_pages, nid, altmap,\n> \t\t\t\t\t\t pgmap);\n> \n> -\tmemmap_pages_add(DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE));\n> +\tmemmap_pages_add(section_vmemmap_pages(pfn, nr_pages, altmap, pgmap));\n> \n> \treturn page;\n> }\n> @@ -670,7 +693,7 @@ static void depopulate_section_memmap(unsigned long pfn, unsigned long nr_pages,\n> \tunsigned long start = (unsigned long) pfn_to_page(pfn);\n> \tunsigned long end = start + nr_pages * sizeof(struct page);\n> \n> -\tmemmap_pages_add(-1L * (DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE)));\n> +\tmemmap_pages_add(-section_vmemmap_pages(pfn, nr_pages, altmap, pgmap));\n> \tvmemmap_free(start, end, altmap);\n> }\n> \n> @@ -679,9 +702,10 @@ static void free_map_bootmem(struct page *memmap, struct vmem_altmap *altmap,\n> {\n> \tunsigned long start = (unsigned long)memmap;\n> \tunsigned long end = (unsigned long)(memmap + PAGES_PER_SECTION);\n> +\tunsigned long pfn = page_to_pfn(memmap);\n> \n> -\tmemmap_boot_pages_add(-1L * (DIV_ROUND_UP(PAGES_PER_SECTION * sizeof(struct page),\n> -\t\t\t\t\t\t PAGE_SIZE)));\n> +\tmemmap_boot_pages_add(-section_vmemmap_pages(pfn, PAGES_PER_SECTION,\n> +\t\t\t\t\t\t altmap, pgmap));\n> \tvmemmap_free(start, end, NULL);\n> }\n> \n> -- \n> 2.20.1\n>","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","linuxppc-dev@lists.ozlabs.org"],"Delivered-To":"patchwork-incoming@legolas.ozlabs.org","Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256\n header.s=k20201202 header.b=H0i33bJZ;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=lists.ozlabs.org\n (client-ip=2404:9400:21b9:f100::1; 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Howlett\" ,\n\tVlastimil Babka ,\n\tSuren Baghdasaryan ,\n\tMichal Hocko , Nicholas Piggin ,\n\tChristophe Leroy , aneesh.kumar@linux.ibm.com,\n\tjoao.m.martins@oracle.com, linux-mm@kvack.org,\n\tlinuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org","Subject":"Re: [PATCH v2 3/6] mm/sparse-vmemmap: Fix DAX vmemmap accounting\n with optimization","Message-ID":"","References":"<20260415111412.1003526-1-songmuchun@bytedance.com>\n <20260415111412.1003526-4-songmuchun@bytedance.com>","X-Mailing-List":"linuxppc-dev@lists.ozlabs.org","List-Id":"","List-Help":"","List-Owner":"","List-Post":"","List-Archive":",\n ","List-Subscribe":",\n ,\n ","List-Unsubscribe":"","Precedence":"list","MIME-Version":"1.0","Content-Type":"text/plain; charset=us-ascii","Content-Disposition":"inline","In-Reply-To":"<20260415111412.1003526-4-songmuchun@bytedance.com>","X-Spam-Status":"No, score=-0.2 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED,\n\tDKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,SPF_HELO_NONE,SPF_PASS\n\tautolearn=disabled version=4.0.1 OzLabs 8","X-Spam-Checker-Version":"SpamAssassin 4.0.1 (2024-03-25) on lists.ozlabs.org"}}]