On Jul 10, 2007  22:40 -0700, Andrew Morton wrote:

On Sun, 01 Jul 2007 03:38:18 -0400 Mingming Cao [off-list ref] wrote:

quoted

A EXT4_FEATURE_RO_COMPAT_DIR_NLINK flag has been added and it is set if
the subdir count for any directory crosses 65000.

Would I be correct in assuming that a later fsck will clear
EXT4_FEATURE_RO_COMPAT_DIR_NLINK if there are no longer any >65000 subdir
directories?

Correct.

If so, that is worth a mention in the changelog, perhaps?

Please remind us what is the behaviour of an RO_COMPAT flag?  It means that
old ext4, ext3 and ext2 can only mount this fs read-only, yes?

Also correct.  The COMPAT flag behaviour is described in detail in
Documentation/filesystems/ext[234].txt

quoted

+static inline void ext4_inc_count(handle_t *handle, struct inode *inode)
+{
+	inc_nlink(inode);
+	if (is_dx(inode) && inode->i_nlink > 1) {
+		/* limit is 16-bit i_links_count */
+		if (inode->i_nlink >= EXT4_LINK_MAX || inode->i_nlink == 2) {
+			inode->i_nlink = 1;
+			EXT4_SET_RO_COMPAT_FEATURE(inode->i_sb,
+					      EXT4_FEATURE_RO_COMPAT_DIR_NLINK);
+		}
+	}
+}

Why do we set EXT4_FEATURE_RO_COMPAT_DIR_NLINK if i_nlink==2?

Because that means it was previously 1 (inc_nlink() was already called).

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.