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

[Muchun Song]

> On Sep 29, 2026, at 15:24, David Hildenbrand (Arm) <david@xxxxxxxxxx> wrote:
> 
> On 9/27/26 04:54, Muchun Song wrote:
>> 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.

> 
> [...]
> 
>> 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.
> 
>> 
>> #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