[PATCH v2 05/17] 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.

>From a CPU perspective, these entries are clean. So only set the PTE dirty
if the folios is 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 539c0f7c6d545..ba33ba3b7ea17 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -723,18 +723,21 @@ 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