On Mon, Jul 27, 2015 at 4:40 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
>> + /*
>> + * 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.
>
>> + 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,
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