Re: [PATCH 1/6] mm/sparse-vmemmap: drop VMEMMAP_POPULATE_DAX

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

On 9/13/26 4:37 PM, Muchun Song wrote:
> VMEMMAP_POPULATE_DAX currently distinguishes DAX vmemmap population in two
> places: it keeps allocations on the normal path and takes a reference when
> a backing page is supplied for reuse.
> 
> After Device DAX switched to the common per-zone shared tail page, both
> conditions can be determined locally. DAX supplies ptpfn for every shared
> tail mapping and requests an allocation only for compound head mappings,
> whose PFNs are not optimizable. Therefore, vmemmap_optimizable_pfn() alone
> selects the correct allocation path.
> 
> When ptpfn is supplied, the caller is reusing an existing backing page.
> Once the slab allocator is available, take a reference for each reused

Does the availability of slab mean the buddy allocator is already being
used? Could there be a window where the buddy allocator is functional
but slab hasn't become available yet?

> mapping to balance the release performed by vmemmap_free(). Early mappings
> are backed by memblock/reserved memory and do not need page reference
> accounting.
> 
> Remove VMEMMAP_POPULATE_DAX and the flags argument from the vmemmap
> population helpers.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>   mm/sparse-vmemmap.c | 36 +++++++++++++-----------------------
>   1 file changed, 13 insertions(+), 23 deletions(-)
> 
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index 96506f594924..878d29a4e862 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -32,12 +32,6 @@
>   #include <asm/dma.h>
>   #include <asm/tlbflush.h>
>   
> -/*
> - * Flags for vmemmap_populate_range and friends.
> - */
> -/* Vmemmap population for ZONE_DEVICE compound pages */
> -#define VMEMMAP_POPULATE_DAX		0x0001
> -
>   #include "internal.h"
>   #include "mm_init.h"
>   #include "sparse.h"
> @@ -208,7 +202,7 @@ struct page __ref *vmemmap_shared_tail_page(unsigned int order, struct zone *zon
>   }
>   
>   static __meminit void *vmemmap_alloc_pte(unsigned long pfn, int node,
> -		struct vmem_altmap *altmap, unsigned long flags)
> +		struct vmem_altmap *altmap)
>   {
>   	struct zone *zone;
>   	struct page *page;
> @@ -218,7 +212,7 @@ static __meminit void *vmemmap_alloc_pte(unsigned long pfn, int node,
>   	 * 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)
> +	if (!vmemmap_optimizable_pfn(pfn))
>   		return vmemmap_alloc_block_buf(PAGE_SIZE, node, altmap);
>   
>   	zone = pfn_to_zone(pfn, node);
> @@ -230,8 +224,7 @@ static __meminit void *vmemmap_alloc_pte(unsigned long pfn, int node,
>   }
>   
>   static pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, int node,
> -				       struct vmem_altmap *altmap,
> -				       unsigned long ptpfn, unsigned long flags)
> +		struct vmem_altmap *altmap, unsigned long ptpfn)
>   {
>   	pte_t *pte = pte_offset_kernel(pmd, addr);
>   	unsigned long pfn = page_to_pfn((struct page *)addr);
> @@ -240,7 +233,7 @@ 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, flags);
> +			void *p = vmemmap_alloc_pte(pfn, node, altmap);
>   
>   			if (!p)
>   				return NULL;
> @@ -255,7 +248,7 @@ 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_DAX)
> +			if (slab_is_available())
>   				get_page(pfn_to_page(ptpfn));
>   		}
>   		entry = pfn_pte(ptpfn, PAGE_KERNEL);
> @@ -318,8 +311,7 @@ static pgd_t * __meminit vmemmap_pgd_populate(unsigned long addr, int node)
>   
>   static pte_t * __meminit vmemmap_populate_address(unsigned long addr, int node,
>   					      struct vmem_altmap *altmap,
> -					      unsigned long ptpfn,
> -					      unsigned long flags)
> +					      unsigned long ptpfn)
>   {
>   	pgd_t *pgd;
>   	p4d_t *p4d;
> @@ -339,7 +331,7 @@ static pte_t * __meminit vmemmap_populate_address(unsigned long addr, int node,
>   	pmd = vmemmap_pmd_populate(pud, addr, node);
>   	if (!pmd)
>   		return NULL;
> -	pte = vmemmap_pte_populate(pmd, addr, node, altmap, ptpfn, flags);
> +	pte = vmemmap_pte_populate(pmd, addr, node, altmap, ptpfn);
>   	if (!pte)
>   		return NULL;
>   	vmemmap_verify(pte, node, addr, addr + PAGE_SIZE);
> @@ -350,15 +342,14 @@ static pte_t * __meminit vmemmap_populate_address(unsigned long addr, int node,
>   static int __meminit vmemmap_populate_range(unsigned long start,
>   					    unsigned long end, int node,
>   					    struct vmem_altmap *altmap,
> -					    unsigned long ptpfn,
> -					    unsigned long flags)
> +					    unsigned long ptpfn)
>   {
>   	unsigned long addr = start;
>   	pte_t *pte;
>   
>   	for (; addr < end; addr += PAGE_SIZE) {
>   		pte = vmemmap_populate_address(addr, node, altmap,
> -					       ptpfn, flags);
> +					       ptpfn);
>   		if (!pte)
>   			return -ENOMEM;
>   	}
> @@ -369,7 +360,7 @@ static int __meminit vmemmap_populate_range(unsigned long start,
>   int __meminit vmemmap_populate_basepages(unsigned long start, unsigned long end,
>   					 int node, struct vmem_altmap *altmap)
>   {
> -	return vmemmap_populate_range(start, end, node, altmap, -1, 0);
> +	return vmemmap_populate_range(start, end, node, altmap, -1);
>   }
>   
>   /*
> @@ -498,7 +489,6 @@ 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;
>   	struct page *page;
>   	unsigned int order = pfn_to_section_compound_order(start_pfn);
>   
> @@ -508,14 +498,14 @@ static int __meminit vmemmap_populate_compound_pages(unsigned long start_pfn,
>   
>   	if (reuse_compound_section(start_pfn, pgmap))
>   		return vmemmap_populate_range(start, end, node, NULL,
> -					      page_to_pfn(page), flags);
> +					      page_to_pfn(page));
>   
>   	size = min(end - start, (1UL << order) * sizeof(struct page));
>   	for (addr = start; addr < end; addr += size) {
>   		unsigned long next, last = addr + size;
>   
>   		/* Populate the head page vmemmap page */
> -		pte = vmemmap_populate_address(addr, node, NULL, -1, flags);
> +		pte = vmemmap_populate_address(addr, node, NULL, -1);
>   		if (!pte)
>   			return -ENOMEM;
>   
> @@ -525,7 +515,7 @@ static int __meminit vmemmap_populate_compound_pages(unsigned long start_pfn,
>   		 */
>   		next = addr + PAGE_SIZE;
>   		rc = vmemmap_populate_range(next, last, node, NULL,
> -					    page_to_pfn(page), flags);
> +					    page_to_pfn(page));
>   		if (rc)
>   			return -ENOMEM;
>   	}