[Yaffs] yaffs2 with onenand on kerenel 35 issue

Hi
We have an issue when mounting yaffs image on onenand with kernel 35, after mount, only "lost+found" in the mounted directory. We tried to add new files in the mounted directory, and checked the new file is indeed written to onenand flash. But after umount and mount again, the new file is lost and again "lost + found" is the only thing .
We suspect whether it's onenand driver's problem or yaffs's problem. so we tried to replace all yaffs files in drivers/fs/yaffs2 in kernel 35 with kernel 32 yaffs files, and this time the mount has no problem at all. Mount/umount both will succeed. So maybe it's something that yaffs changed from .32 to .35, but we didn't noticed.
Could you help with problem?

Thanks
Xiaoguang