[PATCH RFC v2 03/12] mm/pagewalk: Skip dax pages in pagewalk

[Alistair Popple <apopple@xxxxxxxxxx>]

Previously dax pages were skipped by the pagewalk code as pud_special() or
vm_normal_page{_pmd}() would be false for DAX pages. Now that dax pages are
refcounted normally that is no longer the case, so add explicit checks to
skip them.

Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>
---
 include/linux/memremap.h | 11 +++++++++++
 mm/pagewalk.c            | 12 ++++++++++--
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/include/linux/memremap.h b/include/linux/memremap.h
index 4aa1519..54e8b57 100644
--- a/include/linux/memremap.h
+++ b/include/linux/memremap.h
@@ -198,6 +198,17 @@ static inline bool folio_is_fsdax(const struct folio *folio)
 	return is_fsdax_page(&folio->page);
 }
 
+static inline bool is_devdax_page(const struct page *page)
+{
+	return is_zone_device_page(page) &&
+		page_pgmap(page)->type == MEMORY_DEVICE_GENERIC;
+}
+
+static inline bool folio_is_devdax(const struct folio *folio)
+{
+	return is_devdax_page(&folio->page);
+}
+
 #ifdef CONFIG_ZONE_DEVICE
 void zone_device_page_init(struct page *page);
 void *memremap_pages(struct dev_pagemap *pgmap, int nid);
diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index e478777..0dfb9c2 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -884,6 +884,12 @@ struct folio *folio_walk_start(struct folio_walk *fw,
 		 * support PUD mappings in VM_PFNMAP|VM_MIXEDMAP VMAs.
 		 */
 		page = pud_page(pud);
+
+		if (is_devdax_page(page)) {
+			spin_unlock(ptl);
+			goto not_found;
+		}
+
 		goto found;
 	}
 
@@ -911,7 +917,8 @@ struct folio *folio_walk_start(struct folio_walk *fw,
 			goto pte_table;
 		} else if (pmd_present(pmd)) {
 			page = vm_normal_page_pmd(vma, addr, pmd);
-			if (page) {
+			if (page && !is_devdax_page(page) &&
+			    !is_fsdax_page(page)) {
 				goto found;
 			} else if ((flags & FW_ZEROPAGE) &&
 				    is_huge_zero_pmd(pmd)) {
@@ -945,7 +952,8 @@ struct folio *folio_walk_start(struct folio_walk *fw,
 
 	if (pte_present(pte)) {
 		page = vm_normal_page(vma, addr, pte);
-		if (page)
+		if (page && !is_devdax_page(page) &&
+		    !is_fsdax_page(page))
 			goto found;
 		if ((flags & FW_ZEROPAGE) &&
 		    is_zero_pfn(pte_pfn(pte))) {
-- 
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