Re: [PATCH] NFS support for btrfs - v2

[David Woodhouse <dwmw2@infradead.org>]

On Mon, 2008-08-18 at 15:47 -0400, Chris Mason wrote:
> Lets pretend I had put in commments something like the code below.
> The important part is that directories have only one link, so they
> have only one backref.

OK. Now can I rip that code out anyway? The VFS will never call
btrfs_lookup() for ".." -- not since the 2.2 kernel :)

I'm still a little confused about precisely what btrfs_search_slot()
returns when it doesn't find a match -- that's probably where the
documentation would be more useful.

What I have in btrfs_get_parent() is correct, I think -- but could
probably be simplified to your version, searching for (u64)-1 and then
not needing to call btrfs_next_leaf().

Something like this?

(And am I fixing a use-after-free bug by moving that btrfs_free_path()
down by a line, at the end?)

diff --git a/export.c b/export.c
index 797b4cb..830e87a 100644
--- a/export.c
+++ b/export.c
@@ -156,27 +156,27 @@ static struct dentry *btrfs_get_parent(struct dentry *child)
 
 	key.objectid = dir->i_ino;
 	btrfs_set_key_type(&key, BTRFS_INODE_REF_KEY);
-	key.offset = 0;
+	key.offset = (u64)-1;
 	ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
-	BUG_ON(ret == 0);
-	ret = 0;
 
 	leaf = path->nodes[0];
 	slot = path->slots[0];
-	nritems = btrfs_header_nritems(leaf);
-	if (slot >= nritems) {
-		ret = btrfs_next_leaf(root, path);
-		if (ret) {
-			btrfs_free_path(path);
-			goto out;
-		}
-		leaf = path->nodes[0];
-		slot = path->slots[0];
+	if (ret < 0 && slot == 0) {
+		btrfs_free_path(path);
+		goto out;
 	}
+	/*
+	  This will be in the slot _before_ the one that btrfs_search_slot()
+	   returns. And for some reason which dwmw2 doesn't quite understand,
+	   that'll definitely be in the same leaf that btrfs_search_slot()
+	   returned -- even if btrfs_search_slot() had to look in the _next_
+	   leaf to find the first key which is greater than what we asked for 
+	*/
+	slot--;
 
+	btrfs_item_key_to_cpu(leaf, &key, slot);
 	btrfs_free_path(path);
 
-	btrfs_item_key_to_cpu(leaf, &key, slot);
 	if (key.objectid != dir->i_ino || key.type != BTRFS_INODE_REF_KEY)
 		goto out;
 


-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@intel.com                              Intel Corporation