Re: [PATCH v8 4/7] fs: iomap: Atomic write support

[Christoph Hellwig <hch@lst.de>]

On Tue, Oct 15, 2024 at 09:01:39AM +0000, John Garry wrote:
> Support direct I/O atomic writes by producing a single bio with REQ_ATOMIC
> flag set.
> 
> Initially FSes (XFS) should only support writing a single FS block
> atomically.
> 
> As with any atomic write, we should produce a single bio which covers the
> complete write length.
> 
> Signed-off-by: John Garry <john.g.garry@oracle.com>
> ---
>  .../filesystems/iomap/operations.rst          | 11 ++++++
>  fs/iomap/direct-io.c                          | 38 +++++++++++++++++--
>  fs/iomap/trace.h                              |  3 +-
>  include/linux/iomap.h                         |  1 +
>  4 files changed, 48 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/filesystems/iomap/operations.rst b/Documentation/filesystems/iomap/operations.rst
> index 8e6c721d2330..fb95e99ca1a0 100644
> --- a/Documentation/filesystems/iomap/operations.rst
> +++ b/Documentation/filesystems/iomap/operations.rst
> @@ -513,6 +513,17 @@ IOMAP_WRITE`` with any combination of the following enhancements:
>     if the mapping is unwritten and the filesystem cannot handle zeroing
>     the unaligned regions without exposing stale contents.
>  
> + * ``IOMAP_ATOMIC``: This write is being issued with torn-write
> +   protection. Only a single bio can be created for the write, and the
> +   write must not be split into multiple I/O requests, i.e. flag
> +   REQ_ATOMIC must be set.
> +   The file range to write must be aligned to satisfy the requirements
> +   of both the filesystem and the underlying block device's atomic
> +   commit capabilities.
> +   If filesystem metadata updates are required (e.g. unwritten extent
> +   conversion or copy on write), all updates for the entire file range
> +   must be committed atomically as well.
> +
>  Callers commonly hold ``i_rwsem`` in shared or exclusive mode before
>  calling this function.
>  
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index f637aa0706a3..c968a0e2a60b 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -271,7 +271,7 @@ static int iomap_dio_zero(const struct iomap_iter *iter, struct iomap_dio *dio,
>   * clearing the WRITE_THROUGH flag in the dio request.
>   */
>  static inline blk_opf_t iomap_dio_bio_opflags(struct iomap_dio *dio,
> -		const struct iomap *iomap, bool use_fua)
> +		const struct iomap *iomap, bool use_fua, bool atomic)
>  {
>  	blk_opf_t opflags = REQ_SYNC | REQ_IDLE;
>  
> @@ -283,6 +283,8 @@ static inline blk_opf_t iomap_dio_bio_opflags(struct iomap_dio *dio,
>  		opflags |= REQ_FUA;
>  	else
>  		dio->flags &= ~IOMAP_DIO_WRITE_THROUGH;
> +	if (atomic)
> +		opflags |= REQ_ATOMIC;
>  
>  	return opflags;
>  }
> @@ -293,7 +295,8 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>  	const struct iomap *iomap = &iter->iomap;
>  	struct inode *inode = iter->inode;
>  	unsigned int fs_block_size = i_blocksize(inode), pad;
> -	loff_t length = iomap_length(iter);
> +	const loff_t length = iomap_length(iter);
> +	bool atomic = iter->flags & IOMAP_ATOMIC;
>  	loff_t pos = iter->pos;
>  	blk_opf_t bio_opf;
>  	struct bio *bio;
> @@ -303,6 +306,9 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>  	size_t copied = 0;
>  	size_t orig_count;
>  
> +	if (atomic && (length != fs_block_size))

Nit: no need for the inner braces here.

> +		if (atomic && n != length) {
> +			/*
> +			 * This bio should have covered the complete length,
> +			 * which it doesn't, so error. We may need to zero out
> +			 * the tail (complete FS block), similar to when
> +			 * bio_iov_iter_get_pages() returns an error, above.
> +			 */
> +			ret = -EINVAL;

Do we want a WARN_ON_ONCE here because this is a condition that should be
impossible to hit?

Otherwise looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>