[PATCH v5 2/8] mm: factor zone-device page init helpers out of __init_zone_device_page

["Li Zhe" <lizhe.67@bytedance.com>]

memmap_init_zone_device() currently mixes refcount policy and core
ZONE_DEVICE page setup in a single helper.

Factor the refcount-reset predicate into pagemap_resets_refcount(), move
the common page initialization into __zone_device_page_init(), and wrap
the existing slow path in zone_device_page_init_slow().

This keeps the slow-path behaviour unchanged and gives later patches
reusable helper boundaries.

No functional change intended.

Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
 mm/mm_init.c | 57 ++++++++++++++++++++++++++++++++++------------------
 1 file changed, 38 insertions(+), 19 deletions(-)

diff --git a/mm/mm_init.c b/mm/mm_init.c
index 95808ab5cfdb..4c7fad440c2a 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -1005,11 +1005,38 @@ static void __init memmap_init(void)
 }
 
 #ifdef CONFIG_ZONE_DEVICE
-static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
+/*
+ * Return true when the free path for this pagemap type restores the page
+ * refcount to 1, so memmap_init_zone_device() can keep the count set by
+ * __init_single_page(). Otherwise initialize the refcount to 0 and leave
+ * it to the allocator or pgmap callbacks to raise it when the page is
+ * handed out again.
+ */
+static inline bool pagemap_resets_refcount(const struct dev_pagemap *pgmap)
+{
+	/*
+	 * MEMORY_DEVICE_GENERIC pages regain a refcount of 1 in the free
+	 * path. The remaining ZONE_DEVICE types start from 0 here and raise
+	 * the count again when the allocator or driver hands the page out.
+	 */
+	switch (pgmap->type) {
+	case MEMORY_DEVICE_FS_DAX:
+	case MEMORY_DEVICE_PRIVATE:
+	case MEMORY_DEVICE_COHERENT:
+	case MEMORY_DEVICE_PCI_P2PDMA:
+		return false;
+	case MEMORY_DEVICE_GENERIC:
+		return true;
+	default:
+		WARN_ONCE(1, "Unknown memory type!");
+		return true;
+	}
+}
+
+static void __ref __zone_device_page_init(struct page *page, unsigned long pfn,
 					  unsigned long zone_idx, int nid,
 					  struct dev_pagemap *pgmap)
 {
-
 	__init_single_page(page, pfn, zone_idx, nid);
 
 	/*
@@ -1028,23 +1055,15 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
 	 */
 	page_folio(page)->pgmap = pgmap;
 	page->zone_device_data = NULL;
+}
 
-	/*
-	 * MEMORY_DEVICE_GENERIC pages regain a refcount of 1 in the free
-	 * path. The remaining ZONE_DEVICE types start from 0 here and raise
-	 * the count again when the allocator or driver hands the page out.
-	 */
-	switch (pgmap->type) {
-	case MEMORY_DEVICE_FS_DAX:
-	case MEMORY_DEVICE_PRIVATE:
-	case MEMORY_DEVICE_COHERENT:
-	case MEMORY_DEVICE_PCI_P2PDMA:
+static void __ref zone_device_page_init_slow(struct page *page,
+		unsigned long pfn, unsigned long zone_idx, int nid,
+		struct dev_pagemap *pgmap)
+{
+	__zone_device_page_init(page, pfn, zone_idx, nid, pgmap);
+	if (!pagemap_resets_refcount(pgmap))
 		set_page_count(page, 0);
-		break;
-
-	case MEMORY_DEVICE_GENERIC:
-		break;
-	}
 }
 
 /*
@@ -1090,7 +1109,7 @@ static void __ref memmap_init_compound(struct page *head,
 	for (pfn = head_pfn + 1; pfn < end_pfn; pfn++) {
 		struct page *page = pfn_to_page(pfn);
 
-		__init_zone_device_page(page, pfn, zone_idx, nid, pgmap);
+		zone_device_page_init_slow(page, pfn, zone_idx, nid, pgmap);
 		prep_compound_tail(page, head, order);
 		set_page_count(page, 0);
 	}
@@ -1126,7 +1145,7 @@ void __ref memmap_init_zone_device(struct zone *zone,
 	for (pfn = start_pfn; pfn < end_pfn; pfn += pfns_per_compound) {
 		struct page *page = pfn_to_page(pfn);
 
-		__init_zone_device_page(page, pfn, zone_idx, nid, pgmap);
+		zone_device_page_init_slow(page, pfn, zone_idx, nid, pgmap);
 
 		if (IS_ALIGNED(pfn, PAGES_PER_SECTION))
 			cond_resched();
-- 
2.20.1