Re: [PATCHv2 3/3] block: relax direct io memory alignment

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On Wed, May 18, 2022 at 10:11:31AM -0700, Keith Busch wrote:
> diff --git a/block/fops.c b/block/fops.c
> index b9b83030e0df..d8537c29602f 100644
> --- a/block/fops.c
> +++ b/block/fops.c
> @@ -54,8 +54,9 @@ static ssize_t __blkdev_direct_IO_simple(struct kiocb *iocb,
>  	struct bio bio;
>  	ssize_t ret;
>  
> -	if ((pos | iov_iter_alignment(iter)) &
> -	    (bdev_logical_block_size(bdev) - 1))
> +	if ((pos | iov_iter_count(iter)) & (bdev_logical_block_size(bdev) - 1))
> +		return -EINVAL;
> +	if (iov_iter_alignment(iter) & bdev_dma_alignment(bdev))
>  		return -EINVAL;

The block layer makes a lot of assumptions that bios can be split at any bvec
boundary.  With this patch, bios whose length isn't a multiple of the logical
block size can be generated by splitting, which isn't valid.

Also some devices aren't compatible with logical blocks spanning bdevs at all.
dm-crypt errors out in this case, for example.

What am I missing?

- Eric



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