[PATCH 04/11] mm/sparse-vmemmap: prepare DAX vmemmap population for section orders

[Muchun Song <songmuchun@bytedance.com>]

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
geometry in the section order, 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
head/first-tail allocations on the normal vmemmap allocation path, while
preserving the existing page reference for reused DAX tail mappings.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 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 e453ce4675a0..54ae8c284324 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
 
 #include "internal.h"
 #include "mm_init.h"
@@ -208,13 +208,17 @@ 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)
+		struct vmem_altmap *altmap, unsigned long flags)
 {
 	struct zone *zone;
 	struct page *page;
 	const unsigned int order = pfn_to_section_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);
@@ -236,7 +240,7 @@ 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;
@@ -251,7 +255,7 @@ 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);
@@ -511,6 +515,7 @@ 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;
 
 	if (reuse_compound_section(start_pfn, pgmap)) {
 		pte = compound_section_tail_page(start);
@@ -522,8 +527,7 @@ static int __meminit vmemmap_populate_compound_pages(unsigned long start_pfn,
 		 * with just tail struct pages.
 		 */
 		return vmemmap_populate_range(start, end, node, NULL,
-					      pte_pfn(ptep_get(pte)),
-					      VMEMMAP_POPULATE_PAGEREF);
+					      pte_pfn(ptep_get(pte)), flags);
 	}
 
 	size = min(end - start, pgmap_vmemmap_nr(pgmap) * sizeof(struct page));
@@ -531,13 +535,13 @@ static int __meminit vmemmap_populate_compound_pages(unsigned long start_pfn,
 		unsigned long next, last = addr + size;
 
 		/* Populate the head page vmemmap page */
-		pte = vmemmap_populate_address(addr, node, NULL, -1, 0);
+		pte = vmemmap_populate_address(addr, node, NULL, -1, flags);
 		if (!pte)
 			return -ENOMEM;
 
 		/* Populate the tail pages vmemmap page */
 		next = addr + PAGE_SIZE;
-		pte = vmemmap_populate_address(next, node, NULL, -1, 0);
+		pte = vmemmap_populate_address(next, node, NULL, -1, flags);
 		if (!pte)
 			return -ENOMEM;
 
@@ -547,8 +551,7 @@ static int __meminit vmemmap_populate_compound_pages(unsigned long start_pfn,
 		 */
 		next += PAGE_SIZE;
 		rc = vmemmap_populate_range(next, last, node, NULL,
-					    pte_pfn(ptep_get(pte)),
-					    VMEMMAP_POPULATE_PAGEREF);
+					    pte_pfn(ptep_get(pte)), flags);
 		if (rc)
 			return -ENOMEM;
 	}
-- 
2.54.0