Re: [PATCHv2 4/6] loop: set dma_alignment from the backing file for direct I/O

[Christoph Hellwig <hch@lst.de>]

On Mon, Jun 22, 2026 at 10:42:39AM -0700, Keith Busch wrote:
>  	/*
> -	 * Use the minimal dio alignment of the file system if provided.
> +	 * Use the dio alignment of the file system if provided.  dio_offset_align
> +	 * is the minimum dio size and offset; dio_mem_align is the buffer memory
> +	 * alignment, kept as a mask to become the loop device's dma_alignment in
> +	 * direct I/O mode where the buffer is handed to the backing file unchanged.

A bunch of overly long lines here.

> +	 * In direct I/O the user pages are handed to the backing file as-is, so
> +	 * the backing's DMA alignment requirement applies to them.  Advertise it
> +	 * so misaligned I/O is rejected at this device's entry instead of being
> +	 * dispatched to the backend.  Buffered I/O copies through the page cache
> +	 * and imposes no such requirement.
> +	 */

More line spillover here.

> +	if (lo->lo_flags & LO_FLAGS_DIRECT_IO)
> +		lim->dma_alignment = lo->lo_dio_mem_align;
> +	else
> +		lim->dma_alignment = SECTOR_SIZE - 1;

Despite the comment above this does enforce a SECTOR_SIZE dma
alignment for buffered I/O.  Shouldn't this be our lowest supported
value (or dword alignment to match real devices)?

> +	lim = queue_limits_start_update(lo->lo_queue);
> +	if (lo->lo_flags & LO_FLAGS_DIRECT_IO)
> +		lim.dma_alignment = lo->lo_dio_mem_align;
> +	else
> +		lim.dma_alignment = SECTOR_SIZE - 1;

Should this and the above copy of this assignment be factored into a
helper?