On Mon, Dec 15, 2014 at 08:56:15AM -0800, Christoph Hellwig wrote: > On Mon, Dec 15, 2014 at 05:27:05PM +0100, Jan Kara wrote: > > On Sun 14-12-14 21:26:56, Omar Sandoval wrote: > > > The generic write code locks i_mutex for a direct_IO. Swap-over-NFS > > > doesn't grab the mutex because nfs_direct_IO doesn't expect i_mutex to > > > be held, but most direct_IO implementations do. > > I think you are speaking about direct IO writes only, aren't you? For DIO > > reads we don't hold i_mutex AFAICS. And also for DIO writes we don't > > necessarily hold i_mutex - see for example XFS which doesn't take i_mutex > > for direct IO writes. It uses it's internal rwlock for this (see > > xfs_file_dio_aio_write()). So I think this is just wrong. > > The problem is that the use of ->direct_IO by the swap code is a gross > layering violation. ->direct_IO is a callback for the filesystem, and > the swap code need to call ->read_iter instead of ->readpage and > ->write_tier instead of ->direct_IO, and leave the locking to the > filesystem. The thing is, ->read_iter() and ->write_iter() might decide to fall back to buffered IO path. XFS is unusual in that respect - there O_DIRECT ends up with short write in such case. Other filesystems, OTOH... -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
