[PATCH 2/4] mm, rmap: allocate anon_vma_chain before starting to link anon_vma_chain

[Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>]

If we allocate anon_vma_chain before starting to link, we can reduce
the lock hold time. This patch implement it.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

diff --git a/mm/rmap.c b/mm/rmap.c
index c2f51cb..1603f64 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -240,18 +240,21 @@ 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;
+	LIST_HEAD(avc_list);
+
+	list_for_each_entry(pavc, &src->anon_vma_chain, same_vma) {
+		avc = anon_vma_chain_alloc(GFP_KERNEL);
+		if (unlikely(!avc))
+			goto enomem_failure;
+
+		list_add_tail(&avc->same_vma, &avc_list);
+	}
 
 	list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
 		struct anon_vma *anon_vma;
 
-		avc = anon_vma_chain_alloc(GFP_NOWAIT | __GFP_NOWARN);
-		if (unlikely(!avc)) {
-			unlock_anon_vma_root(root);
-			root = NULL;
-			avc = anon_vma_chain_alloc(GFP_KERNEL);
-			if (!avc)
-				goto enomem_failure;
-		}
+		avc = list_entry((&avc_list)->next, typeof(*avc), same_vma);
+		list_del(&avc->same_vma);
 		anon_vma = pavc->anon_vma;
 		root = lock_anon_vma_root(root, anon_vma);
 		anon_vma_chain_link(dst, avc, anon_vma);
@@ -259,8 +262,11 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
 	unlock_anon_vma_root(root);
 	return 0;
 
- enomem_failure:
-	unlink_anon_vmas(dst);
+enomem_failure:
+	list_for_each_entry_safe(avc, pavc, &avc_list, same_vma) {
+		list_del(&avc->same_vma);
+		anon_vma_chain_free(avc);
+	}
 	return -ENOMEM;
 }
 
-- 
1.7.9.5

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