[PATCH] mm/migrate: preserve exact soft-dirty state

[Muhammad Usama Anjum]

From: Paul Gofman <pgofman@xxxxxxxxxxxxxxx>

pte_mkdirty() sets both _PAGE_DIRTY and _PAGE_SOFT_DIRTY bits. The
_PAGE_SOFT_DIRTY can get set even if it wasn't set on original page
before migration. This makes non-soft-dirty pages soft-dirty just
because of migration/compaction. Clear the _PAGE_SOFT_DIRTY flag if
it wasn't set on original page.

By definition of soft-dirty feature, there can be spurious soft-dirty
pages because of kernel's internal activity such as VMA merging or
migration/compaction. This patch is eliminating the spurious soft-dirty
pages because of migration/compaction.

Cc: Michał Mirosław <emmir@xxxxxxxxxx>
Cc: Andrei Vagin <avagin@xxxxxxxxx>
Signed-off-by: Paul Gofman <pgofman@xxxxxxxxxxxxxxx>
Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
---
 mm/migrate.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index cc9f2bcd73b4..05d6ca437321 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -211,14 +211,17 @@ static bool remove_migration_pte(struct folio *folio,
 		folio_get(folio);
 		pte = mk_pte(new, READ_ONCE(vma->vm_page_prot));
 		old_pte = ptep_get(pvmw.pte);
-		if (pte_swp_soft_dirty(old_pte))
-			pte = pte_mksoft_dirty(pte);
 
 		entry = pte_to_swp_entry(old_pte);
 		if (!is_migration_entry_young(entry))
 			pte = pte_mkold(pte);
 		if (folio_test_dirty(folio) && is_migration_entry_dirty(entry))
 			pte = pte_mkdirty(pte);
+		if (pte_swp_soft_dirty(old_pte))
+			pte = pte_mksoft_dirty(pte);
+		else
+			pte = pte_clear_soft_dirty(pte);
+
 		if (is_writable_migration_entry(entry))
 			pte = pte_mkwrite(pte, vma);
 		else if (pte_swp_uffd_wp(old_pte))
-- 
2.42.0