On Mon, Jul 27, 2015 at 5:45 AM, Christoph Hellwig <h...@infradead.org> wrote: > On Mon, Jul 27, 2015 at 05:41:57AM -0400, Ming Lei wrote: >> > Why the hardcoded value? I suspect this should be more like: >> > >> > if (dio && inode->i_sb->s_bdev && >> > (lo->lo_offset & (bdev_io_min(inode->i_sb->s_bdev) - 1)) != 0) >> > dio = false; >> >> The above can't work if the backing device has a bigger sector size >> (such as 4K), that is why loop's direct-io requires 512 min_io_size of >> backing device. > > Why doesn't it work? If the backing device sector size is 4k > and lo_offset is 0 or a multiple of 4k it should allow direct I/O, > and my code sniplet will allow that.
Because size has to be 4k aligned too. And it can't work in the example posted by Dave Chinner: > I have a 4k sector backing device and a 512 byte sector filesystem > image. I can't do 512 byte direct IO to the filesystem image, so I > can't run tools that handle fs images in files using direct Io on > that file. Create a loop device with the filesystem image, and now I > can do 512 byte direct IO to the filesystem image, because all that > direct IO to the filesystem image is now buffered by the loop > device. > > If the loop device does direct io in this situation, the backing > filesystem rejects direct IO from the loop device because it is not > sector (4k) sized/aligned. User now swears, shouts and curses you > from afar. Thanks, -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
