Re: [PATCH 1/4] iomap: add IOMAP_DIO_FSBLOCK_ALIGNED flag

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Oct 23, 2025 at 03:55:42PM +0200, Christoph Hellwig wrote:
> From: Qu Wenruo <wqu@suse.com>
> 
> Btrfs requires all of its bios to be fs block aligned, normally it's
> totally fine but with the incoming block size larger than page size
> (bs > ps) support, the requirement is no longer met for direct IOs.
> 
> Because iomap_dio_bio_iter() calls bio_iov_iter_get_pages(), only
> requiring alignment to be bdev_logical_block_size().
> 
> In the real world that value is either 512 or 4K, on 4K page sized
> systems it means bio_iov_iter_get_pages() can break the bio at any page
> boundary, breaking btrfs' requirement for bs > ps cases.
> 
> To address this problem, introduce a new public iomap dio flag,
> IOMAP_DIO_FSBLOCK_ALIGNED.
> 
> When calling __iomap_dio_rw() with that new flag, iomap_dio::flags will
> inherit that new flag, and iomap_dio_bio_iter() will take fs block size
> into the calculation of the alignment, and pass the alignment to
> bio_iov_iter_get_pages(), respecting the fs block size requirement.
> 
> The initial user of this flag will be btrfs, which needs to calculate the
> checksum for direct read and thus requires the biovec to be fs block
> aligned for the incoming bs > ps support.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> Reviewed-by: Pankaj Raghav <p.raghav@samsung.com>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/iomap/direct-io.c  | 13 ++++++++++++-
>  include/linux/iomap.h |  8 ++++++++
>  2 files changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 5d5d63efbd57..ce9cbd2bace0 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -336,10 +336,18 @@ static int iomap_dio_bio_iter(struct iomap_iter *iter, struct iomap_dio *dio)
>  	int nr_pages, ret = 0;
>  	u64 copied = 0;
>  	size_t orig_count;
> +	unsigned int alignment = bdev_logical_block_size(iomap->bdev);
>  
>  	if ((pos | length) & (bdev_logical_block_size(iomap->bdev) - 1))
>  		return -EINVAL;
>  
> +	/*
> +	 * Align to the larger one of bdev and fs block size, to meet the
> +	 * alignment requirement of both layers.
> +	 */
> +	if (dio->flags & IOMAP_DIO_FSBLOCK_ALIGNED)
> +		alignment = max(alignment, fs_block_size);
> +
>  	if (dio->flags & IOMAP_DIO_WRITE) {
>  		bio_opf |= REQ_OP_WRITE;
>  
> @@ -434,7 +442,7 @@ static int iomap_dio_bio_iter(struct iomap_iter *iter, struct iomap_dio *dio)
>  		bio->bi_end_io = iomap_dio_bio_end_io;
>  
>  		ret = bio_iov_iter_get_pages(bio, dio->submit.iter,
> -				bdev_logical_block_size(iomap->bdev) - 1);
> +					     alignment - 1);
>  		if (unlikely(ret)) {
>  			/*
>  			 * We have to stop part way through an IO. We must fall
> @@ -639,6 +647,9 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
>  	if (iocb->ki_flags & IOCB_NOWAIT)
>  		iomi.flags |= IOMAP_NOWAIT;
>  
> +	if (dio_flags & IOMAP_DIO_FSBLOCK_ALIGNED)
> +		dio->flags |= IOMAP_DIO_FSBLOCK_ALIGNED;
> +
>  	if (iov_iter_rw(iter) == READ) {
>  		/* reads can always complete inline */
>  		dio->flags |= IOMAP_DIO_INLINE_COMP;
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index 73dceabc21c8..4da13fe24ce8 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -518,6 +518,14 @@ struct iomap_dio_ops {
>   */
>  #define IOMAP_DIO_PARTIAL		(1 << 2)
>  
> +/*
> + * Ensure each bio is aligned to fs block size.
> + *
> + * For filesystems which need to calculate/verify the checksum of each fs
> + * block. Otherwise they may not be able to handle unaligned bios.
> + */
> +#define IOMAP_DIO_FSBLOCK_ALIGNED	(1 << 3)

A new flag requires an update to IOMAP_F_FLAGS_STRINGS in trace.h for
tracepoint decoding.

The rest of the changes look ok to me, modulo hch's subsequent fixups.

--D

>  ssize_t iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
>  		const struct iomap_ops *ops, const struct iomap_dio_ops *dops,
>  		unsigned int dio_flags, void *private, size_t done_before);
> -- 
> 2.47.3
> 
>