Re: [PATCH v5 06/12] mm/sparse-vmemmap: set compound page order for device DAX

On 9/29/26 10:22, Muchun Song wrote:

quoted

On Sep 29, 2026, at 15:30, David Hildenbrand (Arm) [off-list ref] wrote:

On 9/27/26 04:54, Muchun Song wrote:

quoted

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
---

[...]>

quoted

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?

Yes, this is already handled.

For a partial addition to a normal early section, after updating the
subsection map we return the existing boot-time memmap here:

	if (nr_pages < PAGES_PER_SECTION && early_section(ms))
		return pfn_to_page(pfn);

Therefore, neither section_set_compound_order_range() nor
populate_section_memmap() is called. The fully populated boot memmap is
simply reused, and no vmemmap optimization is attempted.

Perfect, thanks

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David