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. > Ok, I got the swap code working with ->read_iter/->write_iter without too much trouble. I wanted to double check before I submit if there's any gotchas involved with adding the O_DIRECT flag to a file pointer after it has been opened -- swapon opens the swapfile before we know if we're using the SWP_FILE infrastructure, and we need to add O_DIRECT so ->{read,write}_iter use direct I/O, but we can't add O_DIRECT to the original open without excluding filesystems that support the old bmap path but not direct I/O. -- Omar -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>