> On Sep 29, 2026, at 15:24, David Hildenbrand (Arm) <[email protected]> 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
