Implement unioned directories, whiteouts, and fallthrus in pathname
lookup routines. do_lookup() and lookup_hash() call lookup_union()
after looking up the dentry from the top-level file system.
lookup_union() is centered around __lookup_hash(), which does cached
and/or real lookups and revalidates each dentry in the union stack.
The added cost to a non-union mount pathname lookup in a
CONFIG_UNION_MOUNT kernel is either one or two mount flag tests per
pathname component, in needs_union_lookup().
XXX - implement negative union cache entries
---
fs/namei.c | 199 ++++++++++++++++++++++++++++++++++++++++++++++++-
fs/union.c | 67 +++++++++++++++++
include/linux/union.h | 9 ++
3 files changed, 274 insertions(+), 1 deletions(-)
diff --git a/fs/namei.c b/fs/namei.c
index 7e2c31f..a72187b 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -32,6 +32,7 @@
#include <linux/fcntl.h>
#include <linux/device_cgroup.h>
#include <linux/fs_struct.h>
+#include <linux/union.h>
#include <asm/uaccess.h>
#include "internal.h"
@@ -722,6 +723,189 @@ static __always_inline void follow_dotdot(struct nameidata *nd)
follow_mount(&nd->path);
}
+static struct dentry *__lookup_hash(struct qstr *name, struct dentry *base,
+ struct nameidata *nd);
+
+/*
+ * __lookup_union - Given a path from the topmost layer, lookup and
+ * revalidate each dentry in its union stack, building it if necessary
+ *
+ * @nd - nameidata for the parent of @topmost
+ * @name - pathname from this element on
+ * @topmost - path of the topmost matching dentry
+ *
+ * Given the nameidata and the path of the topmost dentry for this
+ * pathname, lookup, revalidate, and build the associated union stack.
+ * @topmost must be either a negative dentry or a directory.
+ *
+ * This function is called both to build a new union stack and to
+ * revalidate a pre-existing union stack. So we must cope with
+ * already existing union cache entries.
+ *
+ * This function may stomp ...
