On Wed, 22 Sept 2021 at 15:21, Chengguang Xu <cgxu519@xxxxxxx> wrote: > > 在 2021/9/22 16:24, Huang Jianan 写道: > > > > > > 在 2021/9/22 16:06, Chengguang Xu 写道: > >> 在 2021/9/22 15:23, Huang Jianan 写道: > >>> From: Huang Jianan <huangjianan@xxxxxxxx> > >>> > >>> At present, overlayfs provides overlayfs inode to users. Overlayfs > >>> inode provides ovl_aops with noop_direct_IO to avoid open failure > >>> with O_DIRECT. But some compressed filesystems, such as erofs and > >>> squashfs, don't support direct_IO. > >>> > >>> Users who use f_mapping->a_ops->direct_IO to check O_DIRECT support, > >>> will read file through this way. This will cause overlayfs to access > >>> a non-existent direct_IO function and cause panic due to null pointer: > >> > >> I just looked around the code more closely, in open_with_fake_path(), > >> > >> do_dentry_open() has already checked O_DIRECT open flag and > >> a_ops->direct_IO of underlying real address_space. > >> > >> Am I missing something? > >> > >> > > > > It seems that loop_update_dio will set lo->use_dio after open file > > without set O_DIRECT. > > loop_update_dio will check f_mapping->a_ops->direct_IO but it deal > > with ovl_aops with > > noop _direct_IO. > > > > So I think we still need a new aops? > > > It means we should only set ->direct_IO for overlayfs inodes whose > underlying fs has DIRECT IO ability. First let's fix the oops: ovl_read_iter()/ovl_write_iter() must check real file's ->direct_IO if IOCB_DIRECT is set in iocb->ki_flags and return -EINVAL if not. To fix the loop -> overlay -> squashfs case your suggestion of having separate aops depending on the real inode's ->direct_IO sounds good. Thanks, Miklos
