[PATCH v4 13/20] mm/migrate: calculate large folio page index using PFN

[Lorenzo Stoakes (ARM)]

Rather than having to figure out the page index to use using
linear_page_index(), calculate it using PFN.

This is a more natural fit as the linear page index is immaterial to
determining the folio page index.

Derive the page index from the offset between migration entry PFN and folio
PFN - pvmw.pfn (set via DEFINE_FOLIO_VMA_WALK() which uses folio_pfn() to
obtain it).

Additionally remove a not so useful comment and clean the code layout up.

No functional change intended.

Suggested-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
---
 mm/migrate.c | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index d08eff028483..50d0b547bca3 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -356,16 +356,11 @@ static bool remove_migration_pte(struct folio *folio,
 
 	while (page_vma_mapped_walk(&pvmw)) {
 		rmap_t rmap_flags = RMAP_NONE;
-		pte_t old_pte;
-		pte_t pte;
+		unsigned long idx = 0;
 		softleaf_t entry;
 		struct page *new;
-		unsigned long idx = 0;
-
-		/* pgoff is invalid for ksm pages, but they are never large */
-		if (folio_test_large(folio) && !folio_test_hugetlb(folio))
-			idx = linear_page_index(vma, pvmw.address) - pvmw.pgoff;
-		new = folio_page(folio, idx);
+		pte_t old_pte;
+		pte_t pte;
 
 #ifdef CONFIG_ARCH_HAS_PMD_SOFTLEAVES
 		/* PMD-mapped THP migration entry */
@@ -381,14 +376,18 @@ static bool remove_migration_pte(struct folio *folio,
 						pvmw.pte);
 		else
 			old_pte = ptep_get(pvmw.pte);
+
+		entry = softleaf_from_pte(old_pte);
+		if (folio_test_large(folio) && !folio_test_hugetlb(folio))
+			idx = softleaf_to_pfn(entry) - pvmw.pfn;
+
 		if (rmap_walk_arg->map_unused_to_zeropage &&
 		    try_to_map_unused_to_zeropage(&pvmw, folio, old_pte, idx))
 			continue;
 
 		folio_get(folio);
+		new = folio_page(folio, idx);
 		pte = mk_pte(new, READ_ONCE(vma->vm_page_prot));
-
-		entry = softleaf_from_pte(old_pte);
 		if (!softleaf_is_migration_young(entry))
 			pte = pte_mkold(pte);
 		if (folio_test_dirty(folio) && softleaf_is_migration_dirty(entry))

-- 
2.55.0