Re: [PATCH v5 06/12] mm/sparse-vmemmap: set compound page order for device DAX
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From: "David Hildenbrand (Arm)" <david@kernel.org>
Date: 2026-09-29 07:30:12
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On 9/27/26 04:54, Muchun Song wrote:
Device DAX can use vmemmap optimization only when a full section is populated with a compound-page geometry. Record that geometry as the compound page order in section metadata before populating the section, so later vmemmap accounting and population decisions can use the section state directly. Clear the compound page order when the section becomes empty again. Also reject partial additions to a section that already has optimized vmemmap mappings. compound_nr_pages() determines how many struct pages to initialize with a section as the smallest granularity. A section therefore cannot safely mix optimized and ordinary vmemmap layouts. Partial additions continue to use ordinary vmemmap population, so they do not save vmemmap memory. Such additions are uncommon, and the lost saving is negligible. Signed-off-by: Muchun Song <redacted> Acked-by: Qi Zheng <qi.zheng@linux.dev> --- v3: - Update the subject and commit message to use compound page order terminology - Use EOPNOTSUPP instead of ENOTSUPP v2: - Explain why optimized and ordinary layouts cannot share a section (suggested by Qi Zheng) - Collect Acked-by from Qi Zheng ---
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quoted hunk ↗ jump to hunk
static struct page * __meminit section_activate(int nid, unsigned long pfn,@@ -838,8 +840,13 @@ static struct page * __meminit section_activate(int nid, unsigned long pfn, struct mem_section *ms = __pfn_to_section(pfn); struct mem_section_usage *usage = NULL; struct page *memmap; + unsigned int order; int rc; + order = vmemmap_can_optimize(altmap, pgmap) ? pgmap->vmemmap_shift : 0; + if (nr_pages < PAGES_PER_SECTION && section_compound_order(ms)) + return ERR_PTR(-EOPNOTSUPP);
Hm. Why should we support optimizing the vmemmap in case we fall into the same memory section as boot memory? In that case, there already is a memmap allocated during boot for the entire section. IOW, we really shouldn't mess with the vmemmap in case we have an early section. But maybe I am missing something and this is already disallowed? -- Cheers, David