[PATCH 3/4] mm, rmap: minimize lock hold when unlink_anon_vmas

[Joonsoo Kim]

Currently, we free the avc objects with holding a lock. To minimize
lock hold time, we just move the avc objects to another list
with holding a lock. Then, iterate them and free objects without holding
a lock. This makes lock hold time minimized.

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

diff --git a/mm/rmap.c b/mm/rmap.c
index 1603f64..9cfb282 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -330,6 +330,7 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
 {
 	struct anon_vma_chain *avc, *next;
 	struct anon_vma *root = NULL;
+	LIST_HEAD(avc_list);
 
 	/*
 	 * Unlink each anon_vma chained to the VMA.  This list is ordered
@@ -348,10 +349,14 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
 		if (RB_EMPTY_ROOT(&anon_vma->rb_root))
 			continue;
 
+		list_move(&avc->same_vma, &avc_list);
+	}
+	unlock_anon_vma_root(root);
+
+	list_for_each_entry_safe(avc, next, &avc_list, same_vma) {
 		list_del(&avc->same_vma);
 		anon_vma_chain_free(avc);
 	}
-	unlock_anon_vma_root(root);
 
 	/*
 	 * Iterate the list once more, it now only contains empty and unlinked
@@ -363,7 +368,6 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
 
 		put_anon_vma(anon_vma);
 
-		list_del(&avc->same_vma);
 		anon_vma_chain_free(avc);
 	}
 }
-- 
1.7.9.5

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