Re: [PATCH v3 11/16] iomap: add read_folio_range() handler for buffered writes

Darrick J. Wong
2025-07-02T17:51:57Z

On Mon, Jun 23, 2025 at 07:21:30PM -0700, Joanne Koong wrote:
> Add a read_folio_range() handler for buffered writes that filesystems
> may pass in if they wish to provide a custom handler for synchronously
> reading in the contents of a folio.
>
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> [hch: renamed to read_folio_range, pass less arguments]
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> .../filesystems/iomap/operations.rst | 6 +++++
> fs/iomap/buffered-io.c | 25 +++++++++++--------
> include/linux/iomap.h | 10 ++++++++
> 3 files changed, 31 insertions(+), 10 deletions(-)
>
> diff --git a/Documentation/filesystems/iomap/operations.rst b/Documentation/filesystems/iomap/operations.rst
> index 1f5732835567..813e26dbd21e 100644
> --- a/Documentation/filesystems/iomap/operations.rst
> +++ b/Documentation/filesystems/iomap/operations.rst
> @@ -68,6 +68,8 @@ The following address space operations can be wrapped easily:
> void (*put_folio)(struct inode *inode, loff_t pos, unsigned copied,
> struct folio *folio);
> bool (*iomap_valid)(struct inode *inode, const struct iomap *iomap);
> + int (*read_folio_range)(const struct iomap_iter *iter,
> + struct folio *folio, loff_t pos, size_t len);
> };
>
> iomap calls these functions:
> @@ -123,6 +125,10 @@ iomap calls these functions:
> ``->iomap_valid``, then the iomap should considered stale and the
> validation failed.
>
> + - ``read_folio_range``: Called to synchronously read in the range that will
> + be written to. If this function is not provided, iomap will default to
> + submitting a bio read request.

Excellent!
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

> +
> These ``struct kiocb`` flags are significant for buffered I/O with iomap:
>
> * ``IOCB_NOWAIT``: Turns on ``IOMAP_NOWAIT``.
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index f6e410c9ea7b..897a3ccea2df 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -667,22 +667,23 @@ iomap_write_failed(struct inode *inode, loff_t pos, unsigned len)
> pos + len - 1);
> }
>
> -static int iomap_read_folio_sync(loff_t block_start, struct folio *folio,
> - size_t poff, size_t plen, const struct iomap *iomap)
> +static int iomap_read_folio_range(const struct iomap_iter *iter,
> + struct folio *folio, loff_t pos, size_t len)
> {
> + const struct iomap *srcmap = iomap_iter_srcmap(iter);
> struct bio_vec bvec;
> struct bio bio;
>
> - bio_init(&bio, iomap->bdev, &bvec, 1, REQ_OP_READ);
> - bio.bi_iter.bi_sector = iomap_sector(iomap, block_start);
> - bio_add_folio_nofail(&bio, folio, plen, poff);
> + bio_init(&bio, srcmap->bdev, &bvec, 1, REQ_OP_READ);
> + bio.bi_iter.bi_sector = iomap_sector(srcmap, pos);
> + bio_add_folio_nofail(&bio, folio, len, offset_in_folio(folio, pos));
> return submit_bio_wait(&bio);
> }
>
> -static int __iomap_write_begin(const struct iomap_iter *iter, size_t len,
> +static int __iomap_write_begin(const struct iomap_iter *iter,
> + const struct iomap_write_ops *write_ops, size_t len,
> struct folio *folio)
> {
> - const struct iomap *srcmap = iomap_iter_srcmap(iter);
> struct iomap_folio_state *ifs;
> loff_t pos = iter->pos;
> loff_t block_size = i_blocksize(iter->inode);
> @@ -731,8 +732,12 @@ static int __iomap_write_begin(const struct iomap_iter *iter, size_t len,
> if (iter->flags & IOMAP_NOWAIT)
> return -EAGAIN;
>
> - status = iomap_read_folio_sync(block_start, folio,
> - poff, plen, srcmap);
> + if (write_ops && write_ops->read_folio_range)
> + status = write_ops->read_folio_range(iter,
> + folio, block_start, plen);
> + else
> + status = iomap_read_folio_range(iter,
> + folio, block_start, plen);
> if (status)
> return status;
> }
> @@ -848,7 +853,7 @@ static int iomap_write_begin(struct iomap_iter *iter,
> else if (srcmap->flags & IOMAP_F_BUFFER_HEAD)
> status = __block_write_begin_int(folio, pos, len, NULL, srcmap);
> else
> - status = __iomap_write_begin(iter, len, folio);
> + status = __iomap_write_begin(iter, write_ops, len, folio);
>
> if (unlikely(status))
> goto out_unlock;
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index 8d20a926b645..5ec651606c51 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -166,6 +166,16 @@ struct iomap_write_ops {
> * locked by the iomap code.
> */
> bool (*iomap_valid)(struct inode *inode, const struct iomap *iomap);
> +
> + /*
> + * Optional if the filesystem wishes to provide a custom handler for
> + * reading in the contents of a folio, otherwise iomap will default to
> + * submitting a bio read request.
> + *
> + * The read must be done synchronously.
> + */
> + int (*read_folio_range)(const struct iomap_iter *iter,
> + struct folio *folio, loff_t pos, size_t len);
> };
>
> /*
> --
> 2.47.1
>
>