Re: [PATCH v5 05/12] mm/sparse-vmemmap: prepare DAX vmemmap population for compound page orders

On Sep 29, 2026, at 15:24, David Hildenbrand (Arm) [off-list ref] wrote:

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

quoted

Device DAX still uses vmemmap_populate_compound_pages() to populate its
compound-page vmemmap mappings. That helper allocates the head and first
tail vmemmap pages explicitly, then reuses the first tail page for the
remaining tail page mappings.

Device DAX is being moved to the section-based vmemmap optimization
infrastructure, but it cannot switch to the generic section-based
population path yet. Once a later patch records the DAX compound page
order in section metadata, DAX head and first-tail PFNs can look
optimizable to the generic helpers as well.

Add a DAX-specific population flag for this transition. It keeps DAX

Well, you're not adding flag, your reusing an existing one and renaming it?

And then you're specifying it on more paths.

You're right. The commit message need to be more precise.

[...]

quoted

mm/sparse-vmemmap.c | 27 +++++++++++++++------------
1 file changed, 15 insertions(+), 12 deletions(-)

diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index e4dae98ba7f8..2457ea2c6dca 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c

@@ -35,8 +35,8 @@

/*
 * Flags for vmemmap_populate_range and friends.
 */
-/* Get a ref on the head page struct page, for ZONE_DEVICE compound pages */
-#define VMEMMAP_POPULATE_PAGEREF 0x0001
+/* Vmemmap population for ZONE_DEVICE compound pages */
+#define VMEMMAP_POPULATE_DAX 0x0001

Cleaner.

quoted

#include "internal.h"
#include "mm_init.h"

@@ -243,13 +243,17 @@ static inline struct page *vmemmap_shared_tail_page(unsigned int order,

#endif

static __meminit void *vmemmap_alloc_pte(unsigned long pfn, int node,
-  			struct vmem_altmap *altmap)
+ 			struct vmem_altmap *altmap, unsigned long flags)
{
	struct zone *zone;
	struct page *page;
	const unsigned int order = pfn_to_section_compound_order(pfn);

- 	if (!vmemmap_optimizable_pfn(pfn))
+ 	/*
+ 	 * Device DAX still relies on vmemmap_populate_compound_pages() for
+ 	 * head/first-tail allocation and tail-page reuse.
+	 */
+ 	if (!vmemmap_optimizable_pfn(pfn) || flags & VMEMMAP_POPULATE_DAX)
		return vmemmap_alloc_block_buf(PAGE_SIZE, node, altmap);

zone = pfn_to_zone(pfn, node);

@@ -271,7 +275,7 @@ static pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, in

		pte_t entry;

		if (ptpfn == (unsigned long)-1) {
- 			void *p = vmemmap_alloc_pte(pfn, node, altmap);
+ 			void *p = vmemmap_alloc_pte(pfn, node, altmap, flags);

		if (!p)
			return NULL;

@@ -286,7 +290,7 @@ static pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, in

			 * and through vmemmap_populate_compound_pages() when
			 * slab is available.
 			 */
- 			if (flags & VMEMMAP_POPULATE_PAGEREF)
+ 			if (flags & VMEMMAP_POPULATE_DAX)
				get_page(pfn_to_page(ptpfn));
		}
		entry = pfn_pte(ptpfn, PAGE_KERNEL);

@@ -546,6 +550,7 @@ static int __meminit vmemmap_populate_compound_pages(unsigned long start_pfn,

	unsigned long size, addr;
	pte_t *pte;
	int rc;
+ 	unsigned long flags = VMEMMAP_POPULATE_DAX;

const and at the top?

No problem.

Thanks,
Muchun

-- 
Cheers,

David