Re: [PATCH v3 4/4] block: loop: support DIO & AIO

[Christoph Hellwig <hch@infradead.org>]

> @@ -441,6 +500,12 @@ static void do_loop_switch(struct loop_device *lo, struct switch_request *p)
>  		mapping->host->i_bdev->bd_block_size : PAGE_SIZE;
>  	lo->old_gfp_mask = mapping_gfp_mask(mapping);
>  	mapping_set_gfp_mask(mapping, lo->old_gfp_mask & ~(__GFP_IO|__GFP_FS));
> +
> +	lo->support_dio = mapping->a_ops && mapping->a_ops->direct_IO;
> +	if (lo->support_dio)
> +		lo->use_aio = true;
> +	else
> +		lo->use_aio = false;

We need an explicit userspace op-in for this.  For one direct I/O can't
handle sub-sector size access and people use the loop device as a
workaround for that.  Second this doesn't give anyone seeing negative
results from aio a way to disable it easily.

It think the best way is to require a setup time flag, but enable it to
on in losetup versions that know about it.

> @@ -761,6 +826,13 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
>  	if (!(lo_flags & LO_FLAGS_READ_ONLY) && file->f_op->fsync)
>  		blk_queue_flush(lo->lo_queue, REQ_FLUSH);
>  
> +	/* use aio if it is possible */
> +	lo->support_dio = mapping->a_ops && mapping->a_ops->direct_IO;
> +	if (lo->support_dio)
> +		lo->use_aio = true;
> +	else
> +		lo->use_aio = false;

Please always factor out checks like this insted of duplicating them.