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

Darrick J. Wong
2025-10-27T16:10:28Z

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
>
>