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

From: Eric Biggers <ebiggers@kernel.org>
To: Keith Busch <kbusch@fb.com>
Cc: linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
	axboe@kernel.dk, Kernel Team <Kernel-team@fb.com>,
	hch@lst.de, bvanassche@acm.org, damien.lemoal@opensource.wdc.com,
	Keith Busch <kbusch@kernel.org>
Subject: Re: [PATCHv2 3/3] block: relax direct io memory alignment
Date: Wed, 18 May 2022 17:14:49 -0700	[thread overview]
Message-ID: <YoWL+T8JiIO5Ln3h@sol.localdomain> (raw)
In-Reply-To: <20220518171131.3525293-4-kbusch@fb.com>

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