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

On Mon, Jul 27, 2015 at 4:40 AM, Christoph Hellwig [off-list ref] wrote:

quoted

+     /*
+      * loop block's logical block size is 512, now
+      * we support direct I/O only if the backing
+      * block devices' minimize I/O size is 512 and
+      * the offset is aligned with 512.
+      */
+     if (dio) {
+             if (inode->i_sb->s_bdev &&
+                     bdev_io_min(inode->i_sb->s_bdev) == 512 &&
+                     !(lo->lo_offset & 511))

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.

quoted

+     blk_mq_freeze_queue(lo->lo_queue);
+     lo->use_dio = use_dio;
+     if (use_dio)
+             lo->lo_flags |= LO_FLAGS_DIRECT_IO;
+     else
+             lo->lo_flags &= ~LO_FLAGS_DIRECT_IO;
+     blk_mq_unfreeze_queue(lo->lo_queue);

Locking?

__loop_update_dio() is only called inside ioctl path, so
mutex of lo->lo_ctl_mutex has been held already.

Thanks,