Re: [PATCH v7 4/6] block: loop: prepare for supporing direct IO

On Mon, Jul 27, 2015 at 05:53:33AM -0400, Ming Lei wrote:

On Mon, Jul 27, 2015 at 5:45 AM, Christoph Hellwig [off-list ref] wrote:

quoted

On Mon, Jul 27, 2015 at 05:41:57AM -0400, Ming Lei wrote:

quoted

quoted

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.

So check that, too. Any >= 4k block size filesystem should be doing
mostly 4k aligned and sized IO...

Cheers,

Dave.
-- 
Dave Chinner
david-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org