Re: [PATCHv3 6/6] block: relax direct io memory alignment

[Christoph Hellwig <hch@lst.de>]

On Mon, May 23, 2022 at 02:01:19PM -0700, Keith Busch wrote:
>   Removed iomap support for now

Do you plan to add a separate patch for it?  It would be a shame to
miss it especially as you said you already tested xfs.

> +	struct request_queue *q = bdev_get_queue(bio->bi_bdev);
>  	struct bio_vec *bv = bio->bi_io_vec + bio->bi_vcnt;
>  	struct page **pages = (struct page **)bv;
>  	ssize_t size, left;
> @@ -1219,7 +1220,19 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>  	BUILD_BUG_ON(PAGE_PTRS_PER_BVEC < 2);
>  	pages += entries_left * (PAGE_PTRS_PER_BVEC - 1);
>  
> +	/*
> +	 * Each segment in the iov is required to be a block size multiple.
> +	 * However, we may not be able to get the entire segment if it spans
> +	 * more pages than bi_max_vecs allows, so we have to ALIGN_DOWN the
> +	 * result to ensure the bio's total size is correct. The remainder of
> +	 * the iov data will be picked up in the next bio iteration.
> +	 *
> +	 * If the result is ever 0, that indicates the iov fails the segment
> +	 * size requirement and is an error.
> +	 */
>  	size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
> +	if (size > 0)
> +		size = ALIGN_DOWN(size, queue_logical_block_size(q));

I think we can simply use bdev_logical_block_size here and remove the need
for the q local variable.

Given that bio_iov_iter_get_pages is used by more than the block device
direct I/O code maybe split this up further?

> @@ -42,6 +42,16 @@ static unsigned int dio_bio_write_op(struct kiocb *iocb)
>  	return op;
>  }
>  
> +static int blkdev_dio_aligned(struct block_device *bdev, loff_t pos,
> +			      struct iov_iter *iter)
> +{
> +	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;
> +	return 0;
> +}

I'd also split adding this helper into another prep patch to see
the actual change in behavior more easily.