[PATCH] get stack footprint of pathname resolution back to relative sanity

Somebody had put struct nameidata in stack frame of link_path_walk().
Unfortunately, there are certain realities to deal with:
	* It's in the middle of recursion.  Depth is equal to the nesting
depth of symlinks, i.e. up to 8.
	* struct namiedata is, even if one discards the intent junk,
at least 12 pointers + 5 ints.
	* moreover, adding a stack frame is not free in that situation.
	* there are fs methods called on top of that, and they also have
stack footprint.
	* kernel stack is not infinite.

The thing is, even if one chooses to deal with -ESTALE that way (and it's
one hell of an overkill), the only thing that needs to be preserved is
vfsmount + dentry, not the entire struct nameidata.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -106,7 +106,7 @@
  * any extra contention...
  */
 
-static int link_path_walk(const char *name, struct nameidata *nd);
+static int __link_path_walk(const char *name, struct nameidata *nd);
 
 /* In order to reduce some races, while at the same time doing additional
  * checking and hopefully speeding things up, we copy filenames to the
@@ -563,6 +563,37 @@ walk_init_root(const char *name, struct nameidata *nd)
 	return 1;
 }
 
+/*
+ * Wrapper to retry pathname resolution whenever the underlying
+ * file system returns an ESTALE.
+ *
+ * Retry the whole path once, forcing real lookup requests
+ * instead of relying on the dcache.
+ */
+static __always_inline int link_path_walk(const char *name, struct nameidata *nd)
+{
+	struct path save = nd->path;
+	int result;
+
+	/* make sure the stuff we saved doesn't go away */
+	dget(save.dentry);
+	mntget(save.mnt);
+
+	result = __link_path_walk(name, nd);
+	if (result == -ESTALE) {
+		/* nd->path had been dropped */
+		nd->path = save;
+		dget(nd->path.dentry);
+		mntget(nd->path.mnt);
+		nd->flags |= LOOKUP_REVAL;
+		result = __link_path_walk(name, nd);
+	}
+
+	path_put(&save);
+
+	return result;
+}
+
 static __always_inline int __vfs_follow_link(struct nameidata *nd, const char *link)
 {
 	int res = 0;
@@ -1020,36 +1051,6 @@ return_err:
 	return err;
 }
 
-/*
- * Wrapper to retry pathname resolution whenever the underlying
- * file system returns an ESTALE.
- *
- * Retry the whole path once, forcing real lookup requests
- * instead of relying on the dcache.
- */
-static int link_path_walk(const char *name, struct nameidata *nd)
-{
-	struct nameidata save = *nd;
-	int result;
-
-	/* make sure the stuff we saved doesn't go away */
-	dget(save.path.dentry);
-	mntget(save.path.mnt);
-
-	result = __link_path_walk(name, nd);
-	if (result == -ESTALE) {
-		*nd = save;
-		dget(nd->path.dentry);
-		mntget(nd->path.mnt);
-		nd->flags |= LOOKUP_REVAL;
-		result = __link_path_walk(name, nd);
-	}
-
-	path_put(&save.path);
-
-	return result;
-}
-
 static int path_walk(const char *name, struct nameidata *nd)
 {
 	current->total_link_count = 0;