mm: workingset: move shadow entry tracking to radix tree exceptional tracking

Currently, we track the shadow entries in the page cache in the upper
bits of the radix_tree_node->count, behind the back of the radix tree
implementation.  Because the radix tree code has no awareness of them,
we rely on random subtleties throughout the implementation (such as the
node->count != 1 check in the shrinking code, which is meant to exclude
multi-entry nodes but also happens to skip nodes with only one shadow
entry, as that's accounted in the upper bits).  This is error prone and
has, in fact, caused the bug fixed in d3798ae8c6f3 ("mm: filemap: don't
plant shadow entries without radix tree node").

To remove these subtleties, this patch moves shadow entry tracking from
the upper bits of node->count to the existing counter for exceptional
entries.  node->count goes back to being a simple counter of valid
entries in the tree node and can be shrunk to a single byte.

This vastly simplifies the page cache code.  All accounting happens
natively inside the radix tree implementation, and maintaining the LRU
linkage of shadow nodes is consolidated into a single function in the
workingset code that is called for leaf nodes affected by a change in
the page cache tree.

This also removes the last user of the __radix_delete_node() return
value.  Eliminate it.

Link: http://lkml.kernel.org/r/20161117193211.GE23430@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox <mawilcox@linuxonhyperv.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 19 deletions

diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index df4ff18dd63c..9dbfaac05e6c 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -541,12 +541,10 @@ out:
  *	radix_tree_shrink    -    shrink radix tree to minimum height
  *	@root		radix tree root
  */
-static inline bool radix_tree_shrink(struct radix_tree_root *root,
+static inline void radix_tree_shrink(struct radix_tree_root *root,
 				     radix_tree_update_node_t update_node,
 				     void *private)
 {
-	bool shrunk = false;
-
 	for (;;) {
 		struct radix_tree_node *node = root->rnode;
 		struct radix_tree_node *child;
@@ -606,26 +604,20 @@ static inline bool radix_tree_shrink(struct radix_tree_root *root,
 		}
 
 		radix_tree_node_free(node);
-		shrunk = true;
 	}
-
-	return shrunk;
 }
 
-static bool delete_node(struct radix_tree_root *root,
+static void delete_node(struct radix_tree_root *root,
 			struct radix_tree_node *node,
 			radix_tree_update_node_t update_node, void *private)
 {
-	bool deleted = false;
-
 	do {
 		struct radix_tree_node *parent;
 
 		if (node->count) {
 			if (node == entry_to_node(root->rnode))
-				deleted |= radix_tree_shrink(root, update_node,
-							     private);
-			return deleted;
+				radix_tree_shrink(root, update_node, private);
+			return;
 		}
 
 		parent = node->parent;
@@ -638,12 +630,9 @@ static bool delete_node(struct radix_tree_root *root,
 		}
 
 		radix_tree_node_free(node);
-		deleted = true;
 
 		node = parent;
 	} while (node);
-
-	return deleted;
 }
 
 /**
@@ -1595,13 +1584,11 @@ unsigned long radix_tree_locate_item(struct radix_tree_root *root, void *item)
  *	After clearing the slot at @index in @node from radix tree
  *	rooted at @root, call this function to attempt freeing the
  *	node and shrinking the tree.
- *
- *	Returns %true if @node was freed, %false otherwise.
  */
-bool __radix_tree_delete_node(struct radix_tree_root *root,
+void __radix_tree_delete_node(struct radix_tree_root *root,
 			      struct radix_tree_node *node)
 {
-	return delete_node(root, node, NULL, NULL);
+	delete_node(root, node, NULL, NULL);
 }
 
 static inline void delete_sibling_entries(struct radix_tree_node *node,