[PATCH] mm/migrate: Fix wrongly apply write bit after mkdirty on sparc64

[Peter Xu]

Nick Bowler reported another sparc64 breakage after the young/dirty
persistent work for page migration (per "Link:" below).  That's after a
similar report [2].

It turns out page migration was overlooked, and it wasn't failing before
because page migration was not enabled in the initial report test environment.

David proposed another way [2] to fix this from sparc64 side, but that
patch didn't land somehow.  Neither did I check whether there's any other
arch that has similar issues.

Let's fix it for now as simple as moving the write bit handling to be after
dirty, like what we did before.

Note: this is based on mm-unstable, because the breakage was since 6.1 and
we're at a very late stage of 6.2 (-rc8), so I assume for this specific
case we should target this at 6.3.

[1] https://lore.kernel.org/all/20221021160603.GA23307@xxxxxxxxxxxx/
[2] https://lore.kernel.org/all/20221212130213.136267-1-david@xxxxxxxxxx/

Cc: regressions@xxxxxxxxxxxxx
Fixes: 2e3468778dbe ("mm: remember young/dirty bit for page migrations")
Link: https://lore.kernel.org/all/CADyTPExpEqaJiMGoV+Z6xVgL50ZoMJg49B10LcZ=8eg19u34BA@xxxxxxxxxxxxxx/
Reported-by: Nick Bowler <nbowler@xxxxxxxxxx>
Tested-by: Nick Bowler <nbowler@xxxxxxxxxx>
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
 mm/huge_memory.c | 6 ++++--
 mm/migrate.c     | 2 ++
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 1343a7d88299..4fc43859e59a 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3274,8 +3274,6 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new)
 	pmde = mk_huge_pmd(new, READ_ONCE(vma->vm_page_prot));
 	if (pmd_swp_soft_dirty(*pvmw->pmd))
 		pmde = pmd_mksoft_dirty(pmde);
-	if (is_writable_migration_entry(entry))
-		pmde = maybe_pmd_mkwrite(pmde, vma);
 	if (pmd_swp_uffd_wp(*pvmw->pmd))
 		pmde = pmd_mkuffd_wp(pmde);
 	if (!is_migration_entry_young(entry))
@@ -3283,6 +3281,10 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new)
 	/* NOTE: this may contain setting soft-dirty on some archs */
 	if (PageDirty(new) && is_migration_entry_dirty(entry))
 		pmde = pmd_mkdirty(pmde);
+	if (is_writable_migration_entry(entry))
+		pmde = maybe_pmd_mkwrite(pmde, vma);
+	else
+		pmde = pmd_wrprotect(pmde);
 
 	if (PageAnon(new)) {
 		rmap_t rmap_flags = RMAP_COMPOUND;
diff --git a/mm/migrate.c b/mm/migrate.c
index ef68a1aff35c..40c63e77e91f 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -225,6 +225,8 @@ static bool remove_migration_pte(struct folio *folio,
 			pte = maybe_mkwrite(pte, vma);
 		else if (pte_swp_uffd_wp(*pvmw.pte))
 			pte = pte_mkuffd_wp(pte);
+		else
+			pte = pte_wrprotect(pte);
 
 		if (folio_test_anon(folio) && !is_readable_migration_entry(entry))
 			rmap_flags |= RMAP_EXCLUSIVE;
-- 
2.39.1