[PATCH v3 3/9] mm/rmap: skip unfaulted VMAs on anon_vma clone, unlink

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

For both anon_vma_clone() and unlink_anon_vmas(), if the source VMA or the
VMA to be linked are unfaulted (e.g.  !vma->anon_vma), then the functions
do nothing.  Simply exit early in these cases.

In the unlink_anon_vmas() case we can also remove a conditional that
checks whether vma->anon_vma is set.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
---
 mm/rmap.c | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index a45b011e9846..8e846ccf7835 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -314,6 +314,9 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
 
 	check_anon_vma_clone(dst, src);
 
+	if (!src->anon_vma)
+		return 0;
+
 	list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
 		struct anon_vma *anon_vma;
 
@@ -479,7 +482,10 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
 	mmap_assert_locked(vma->vm_mm);
 
 	/* Unfaulted is a no-op. */
-	VM_WARN_ON_ONCE(!vma->anon_vma && !list_empty(&vma->anon_vma_chain));
+	if (!vma->anon_vma) {
+		VM_WARN_ON_ONCE(!list_empty(&vma->anon_vma_chain));
+		return;
+	}
 
 	/*
 	 * Unlink each anon_vma chained to the VMA.  This list is ordered
@@ -503,15 +509,13 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
 		list_del(&avc->same_vma);
 		anon_vma_chain_free(avc);
 	}
-	if (vma->anon_vma) {
-		vma->anon_vma->num_active_vmas--;
 
-		/*
-		 * vma would still be needed after unlink, and anon_vma will be prepared
-		 * when handle fault.
-		 */
-		vma->anon_vma = NULL;
-	}
+	vma->anon_vma->num_active_vmas--;
+	/*
+	 * vma would still be needed after unlink, and anon_vma will be prepared
+	 * when handle fault.
+	 */
+	vma->anon_vma = NULL;
 	unlock_anon_vma_root(root);
 
 	/*
-- 
2.52.0