Re: [PATCH v6 05/15] ubifs: Rename whiteout atomically

[Zhihao Cheng]

Hi, Richard
> How do you make sure the the whiteout is never written to disk (by writeback) before ubifs_jnl_rename() linked
> it? That's the reason why other filesystems use the tmpfile mechanism for whiteouts too.

The whiteout inode is clean after creation from create_whiteout(), and 
it can't be marked dirty until ubifs_jnl_rename() finished. So, I think 
there is no chance for whiteout being written on disk. Then, 
'ubifs_assert(c, !whiteout_ui->dirty)' never fails in ubifs_jnl_rename() 
during my local stress tests. You may add some delay executions after 
whiteout creation to make sure that whiteout won't be written back 
before ubifs_jnl_rename().

BTW, I considered plan B(Binding whiteout to a negative dentry), but we 
cannot get parent dir's entry in do_rename(), so I abandoned it.
--- a/fs/ubifs/dir.c
+++ b/fs/ubifs/dir.c
@@ -1334,7 +1335,8 @@ static int do_rename(struct inode *old_dir, struct 
dentry *old_dentry,
                        goto out_release;
                }

-               err = do_tmpfile(old_dir, old_dentry, S_IFCHR | 
WHITEOUT_MODE, &whiteout);
+               // tmp_dentry = d_alloc(old_dir_dentry, &slash_name); 
// refer to vfs_tmpfile
+               err = do_tmpfile(old_dir, tmp_dentry, S_IFCHR | 
WHITEOUT_MODE, &whiteout);
                if (err) {
                        kfree(dev);
                        goto out_release;