Re: [PATCH] iomap: add IOMAP_DIO_ALIGNED flag for btrfs

[Christoph Hellwig <hch@infradead.org>]

On Mon, Oct 13, 2025 at 04:38:40PM +1030, Qu Wenruo wrote:
> For now only btrfs will utilize this flag, as btrfs needs to calculate
> checksum for direct read.

Maybe reword this as: 

The initial user of this flag is btrfs, whichs needs to calculate
the checksum for direct read and thus requires the biovec to be
file system block size aligned?

> index 802d4dbe5b38..15aff186642d 100644
> --- a/fs/btrfs/direct-io.c
> +++ b/fs/btrfs/direct-io.c

Please split the patch to use the flag in btrfs from the one adding
the the flag to iomap.

> +	const unsigned int alignment = (dio->flags & IOMAP_DIO_ALIGNED) ?
> +		max(fs_block_size, bdev_logical_block_size(iomap->bdev)) :
> +		bdev_logical_block_size(iomap->bdev);

Please unwind this into an if/else to be easily readable.  Also a
comment on why you still need the max when the flag is set would be
useful.

> +		ret = bio_iov_iter_get_pages(bio, dio->submit.iter, alignment - 1);

Please avoid overly long lines in the iomap code.

> +/*
> + * Ensure each bio is aligned to fs block size.
> + *
> + * For filesystems which need to calculate/verify data checksum for each data bio.

Another overly long line here.

> + */
> +#define IOMAP_DIO_ALIGNED		(1 << 3)

Maybe call the flag IOMAP_DIO_FSBLOCK_ALIGNED to make it clear
what alignment is implied by the flag.