[PATCH v1 05/12] mm/memory: detect writability in restore_exclusive_pte() through can_change_pte_writable()

[David Hildenbrand]

Let's do it just like mprotect write-upgrade or during NUMA-hinting
faults on PROT_NONE PTEs: detect if the PTE can be writable by using
can_change_pte_writable().

Set the PTE only dirty if the folio is dirty: we might not
necessarily have a write access, and setting the PTE writable doesn't
require setting the PTE dirty.

With this change in place, there is no need to have separate
readable and writable device-exclusive entry types, and we'll merge
them next separately.

Note that, during fork(), we first convert the device-exclusive entries
back to ordinary PTEs, and we only ever allow conversion of writable
PTEs to device-exclusive -- only mprotect can currently change them to
readable-device-exclusive. Consequently, we always expect
PageAnonExclusive(page)==true and can_change_pte_writable()==true,
unless we are dealing with soft-dirty tracking or uffd-wp. But reusing
can_change_pte_writable() for now is cleaner.

Signed-off-by: David Hildenbrand <david at redhat.com>
---
 mm/memory.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 03efeeef895a..db38d6ae4e74 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -725,18 +725,21 @@ static void restore_exclusive_pte(struct vm_area_struct *vma,
 	struct folio *folio = page_folio(page);
 	pte_t orig_pte;
 	pte_t pte;
-	swp_entry_t entry;
 
 	orig_pte = ptep_get(ptep);
 	pte = pte_mkold(mk_pte(page, READ_ONCE(vma->vm_page_prot)));
 	if (pte_swp_soft_dirty(orig_pte))
 		pte = pte_mksoft_dirty(pte);
 
-	entry = pte_to_swp_entry(orig_pte);
 	if (pte_swp_uffd_wp(orig_pte))
 		pte = pte_mkuffd_wp(pte);
-	else if (is_writable_device_exclusive_entry(entry))
-		pte = maybe_mkwrite(pte_mkdirty(pte), vma);
+
+	if ((vma->vm_flags & VM_WRITE) &&
+	    can_change_pte_writable(vma, address, pte)) {
+		if (folio_test_dirty(folio))
+			pte = pte_mkdirty(pte);
+		pte = pte_mkwrite(pte, vma);
+	}
 
 	VM_BUG_ON_FOLIO(pte_write(pte) && (!folio_test_anon(folio) &&
 					   PageAnonExclusive(page)), folio);
-- 
2.48.1