[PATCH, RFC] xfs: add heuristic to flush on rename

[Eric Sandeen <sandeen@redhat.com>]

Add a heuristic to flush data to a file which looks like it's
going through a tmpfile/rename dance, but not fsynced.

I had a report of a system with many 0-length files after
package updates; as it turns out, the user had basically
done 'yum update' and punched the power button when it was
done.

Granted, the admin should not do this.  Granted, the package
manager should ensure persistence of files it updated.

Ext4, however, added a heuristic like this for just this case;
someone who writes file.tmp, then renames over file, but
never issues an fsync.

Now, this does smack of O_PONIES, but I would hope that it's
fairly benign.  If someone already synced the tmpfile, it's
a no-op.

And it's not THAT far off our "flush on close if the file was
truncated" heuristic.

Comments?  Flames?  Testing anyone would like to see?

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
---

diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index ef1ca01..5c95ef5 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -371,6 +371,19 @@ xfs_vn_rename(
 	xfs_dentry_to_name(&oname, odentry, 0);
 	xfs_dentry_to_name(&nname, ndentry, odentry->d_inode->i_mode);
 
+	/*
+	 * If we are renaming a just-written file over an existing
+	 * file, be pedantic and flush it out if it looks like somebody
+	 * is doing a tmpfile dance, and didn't fsync.  Best effort;
+	 * ignore errors.
+	 */
+	if (new_inode) {
+		xfs_inode_t *ip = XFS_I(odentry->d_inode);
+
+		if (VN_DIRTY(VFS_I(ip)) && ip->i_delayed_blks > 0)
+			filemap_flush(new_inode->i_mapping);
+	}
+
 	return -xfs_rename(XFS_I(odir), &oname, XFS_I(odentry->d_inode),
 			   XFS_I(ndir), &nname, new_inode ?
 						XFS_I(new_inode) : NULL);

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