Subject: [PATCH v1 1/6] mm/mprotect: allow clean exclusive anon pages to be writable

From: David Hildenbrand <[email-masked]>
Date: Wed, 2 Nov 2022 20:12:04 +0100

From: Nadav Amit <[email-masked]>

Anonymous pages might have the dirty bit clear, but this should not
prevent mprotect from making them writable if they are exclusive.
Therefore, skip the test whether the page is dirty in this case.

Note that there are already other ways to get a writable PTE mapping an
anonymous page that is clean: for example, via MADV_FREE. In an ideal
world, we'd have a different indication from the FS whether writenotify
is still required.

Signed-off-by: Nadav Amit <[email-masked]>
[ return directly; update description ]
Signed-off-by: David Hildenbrand <[email-masked]>
---
 mm/mprotect.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/mm/mprotect.c b/mm/mprotect.c
index 8d770855b591..86a28c0e190f 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -46,7 +46,7 @@ static inline bool can_change_pte_writable(struct vm_area_struct *vma,
 
 	VM_BUG_ON(!(vma->vm_flags & VM_WRITE) || pte_write(pte));
 
-	if (pte_protnone(pte) || !pte_dirty(pte))
+	if (pte_protnone(pte))
 		return false;
 
 	/* Do we need write faults for softdirty tracking? */
@@ -65,11 +65,10 @@ static inline bool can_change_pte_writable(struct vm_area_struct *vma,
 		 * the PT lock.
 		 */
 		page = vm_normal_page(vma, addr, pte);
-		if (!page || !PageAnon(page) || !PageAnonExclusive(page))
-			return false;
+		return page && PageAnon(page) && PageAnonExclusive(page);
 	}
 
-	return true;
+	return pte_dirty(pte);
 }
 
 static unsigned long change_pte_range(struct mmu_gather *tlb,
-- 
2.38.1