[PATCH 3/9] mm/rmap: factor lock function out of rmap_walk_anon()

[Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>]

When we traverse anon_vma, we need to take a read-side anon_lock.
But there is subtle difference in the situation so that we can't use
same method to take a lock in each cases. Therefore, we need to make
rmap_walk_anon() taking difference lock function.

This patch is the first step, factoring lock function for anon_lock out
of rmap_walk_anon(). It will be used in case of removing migration entry
and in default of rmap_walk_anon().

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

diff --git a/mm/rmap.c b/mm/rmap.c
index e6d532c..916f2ed 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1683,6 +1683,24 @@ void __put_anon_vma(struct anon_vma *anon_vma)
 }
 
 #ifdef CONFIG_MIGRATION
+static struct anon_vma *rmap_walk_anon_lock(struct page *page)
+{
+	struct anon_vma *anon_vma;
+
+	/*
+	 * Note: remove_migration_ptes() cannot use page_lock_anon_vma_read()
+	 * because that depends on page_mapped(); but not all its usages
+	 * are holding mmap_sem. Users without mmap_sem are required to
+	 * take a reference count to prevent the anon_vma disappearing
+	 */
+	anon_vma = page_anon_vma(page);
+	if (!anon_vma)
+		return NULL;
+
+	anon_vma_lock_read(anon_vma);
+	return anon_vma;
+}
+
 /*
  * rmap_walk() and its helpers rmap_walk_anon() and rmap_walk_file():
  * Called by migrate.c to remove migration ptes, but might be used more later.
@@ -1695,16 +1713,10 @@ static int rmap_walk_anon(struct page *page, int (*rmap_one)(struct page *,
 	struct anon_vma_chain *avc;
 	int ret = SWAP_AGAIN;
 
-	/*
-	 * Note: remove_migration_ptes() cannot use page_lock_anon_vma_read()
-	 * because that depends on page_mapped(); but not all its usages
-	 * are holding mmap_sem. Users without mmap_sem are required to
-	 * take a reference count to prevent the anon_vma disappearing
-	 */
-	anon_vma = page_anon_vma(page);
+	anon_vma = rmap_walk_anon_lock(page);
 	if (!anon_vma)
 		return ret;
-	anon_vma_lock_read(anon_vma);
+
 	anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root, pgoff, pgoff) {
 		struct vm_area_struct *vma = avc->vma;
 		unsigned long address = vma_address(page, vma);
-- 
1.7.9.5

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