Re: [PATCH 2/6] mm/sparse-vmemmap: support device DAX in common vmemmap path

[Qi Zheng <qi.zheng@linux.dev>]

On 9/13/26 4:37 PM, Muchun Song wrote:
> The common vmemmap population path cannot yet handle optimized Device DAX
> mappings on its own. It uses pfn_to_zone() to find the shared tail page,
> but Device DAX populates its vmemmap at runtime before the ZONE_DEVICE span
> is initialized.
> 
> Teach the common path to use device_zone() for runtime optimized vmemmap
> population while retaining pfn_to_zone() for early boot. This allows the
> same path to support both early boot mappings and Device DAX.
> 
> The backing PFN supplied by the Device DAX-specific population path is no
> longer used, allowing the redundant lookup and population code to be
> removed later.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>   mm/sparse-vmemmap.c | 44 +++++++++++++++++++-------------------------
>   1 file changed, 19 insertions(+), 25 deletions(-)
> 
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index 878d29a4e862..e83821768c12 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -208,18 +208,27 @@ static __meminit void *vmemmap_alloc_pte(unsigned long pfn, int node,
>   	struct page *page;
>   	const unsigned int order = pfn_to_section_compound_order(pfn);
>   
> -	/*
> -	 * Device DAX still relies on vmemmap_populate_compound_pages() for
> -	 * head/first-tail allocation and tail-page reuse.
> -	 */
>   	if (!vmemmap_optimizable_pfn(pfn))
>   		return vmemmap_alloc_block_buf(PAGE_SIZE, node, altmap);
>   
> -	zone = pfn_to_zone(pfn, node);
> +	/*
> +	 * At runtime (slab available), only ZONE_DEVICE pages trigger vmemmap
> +	 * optimization, so device_zone() suffices. Note that pfn_to_zone()
> +	 * cannot be used at runtime because the zone span is not set up now.
> +	 */
> +	zone = slab_is_available() ? device_zone(node) : pfn_to_zone(pfn, node);
>   	page = vmemmap_shared_tail_page(order, zone);
>   	if (!page)
>   		return NULL;
>   
> +	/*
> +	 * When a PTE entry is freed, a free_pages() call occurs. This get_page()
> +	 * pairs with put_page_testzero() on the freeing path. This can only occur
> +	 * when slab is available.
> +	 */
> +	if (slab_is_available())

Would it make sense to introduce a helper function that wraps
slab_is_available() for better readability? Also, it might be worth
adding a comment to the helper function as well.

At least for me, encountering this always causes a moment of confusion.

> +		get_page(page);
> +
>   	return page_address(page);
>   }
>   
> @@ -231,27 +240,12 @@ static pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, in
>   
>   	if (pte_none(ptep_get(pte))) {
>   		pte_t entry;
> +		void *p = vmemmap_alloc_pte(pfn, node, altmap);
>   
> -		if (ptpfn == (unsigned long)-1) {
> -			void *p = vmemmap_alloc_pte(pfn, node, altmap);
> -
> -			if (!p)
> -				return NULL;
> -			ptpfn = PHYS_PFN(__pa(p));
> -		} else {
> -			/*
> -			 * When a PTE/PMD entry is freed from the init_mm
> -			 * there's a free_pages() call to this page allocated
> -			 * above. Thus this get_page() is paired with the
> -			 * put_page_testzero() on the freeing path.
> -			 * This can only called by certain ZONE_DEVICE path,
> -			 * and through vmemmap_populate_compound_pages() when
> -			 * slab is available.
> -			 */
> -			if (slab_is_available())
> -				get_page(pfn_to_page(ptpfn));
> -		}
> -		entry = pfn_pte(ptpfn, PAGE_KERNEL);
> +		if (!p)
> +			return NULL;
> +
> +		entry = pfn_pte(PHYS_PFN(__pa(p)), PAGE_KERNEL);
>   		set_pte_at(&init_mm, addr, pte, entry);
>   	} else if (WARN_ON_ONCE(vmemmap_optimizable_pfn(pfn)))
>   		return NULL;