Re: [PATCH] null_blk: set dma alignment to logical block size

[Keith Busch <kbusch@kernel.org>]

On Wed, Oct 29, 2025 at 02:39:56PM +0100, Hans Holmberg wrote:
> This driver assumes that bio vectors are memory aligned to the logical
> block size, so set the queue limit to reflect that.
> 
> Unless we set up the limit based on the logical block size, we will go
> out of page bounds in copy_to_nullb / copy_from_nullb.
> 
> Signed-off-by: Hans Holmberg <hans.holmberg@wdc.com>
> ---
> 
> A fixes tag would be in order, but I have not figured out exactly when
> this became a problem.

Sorry! This is from the relaxed memory address alignment changes I've
been making to reduce the need for bounce buffers.

> @@ -1949,6 +1949,7 @@ static int null_add_dev(struct nullb_device *dev)
>  		.logical_block_size	= dev->blocksize,
>  		.physical_block_size	= dev->blocksize,
>  		.max_hw_sectors		= dev->max_sectors,
> +		.dma_alignment		= dev->blocksize - 1,
>  	};

It looks like null_blk only needs (SECTOR_SIZE - 1). All the data copies
work in sector_t units and SECTOR_SIZE granularity, so does dma
alignment really need to match the blocksize if it's, for example, 4k?
And if not, since null_blk didn't set dma_alignment before, it should
have been defaulting to 511 already.