[PATCH 2/8] mm/rmap: skip unfaulted VMAs on anon_vma clone, unlink

[Lorenzo Stoakes]

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@xxxxxxxxxx>
---
 mm/rmap.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 0e34c0a69fbc..9332d1cbc643 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -309,6 +309,9 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
 	struct anon_vma_chain *avc, *pavc;
 	struct anon_vma *root = NULL;
 
+	if (!src->anon_vma)
+		return 0;
+
 	check_anon_vma_clone(dst, src);
 
 	list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
@@ -441,7 +444,8 @@ 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)
+		return;
 
 	/*
 	 * Unlink each anon_vma chained to the VMA.  This list is ordered
@@ -465,15 +469,13 @@ 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