Re: [PATCH v3 05/12] iomap: Support SW-based atomic writes

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

On Thu, Feb 27, 2025 at 06:08:06PM +0000, John Garry wrote:
> Currently atomic write support requires dedicated HW support. This imposes
> a restriction on the filesystem that disk blocks need to be aligned and
> contiguously mapped to FS blocks to issue atomic writes.
> 
> XFS has no method to guarantee FS block alignment for regular,
> non-RT files. As such, atomic writes are currently limited to 1x FS block
> there.
> 
> To deal with the scenario that we are issuing an atomic write over
> misaligned or discontiguous data blocks - and raise the atomic write size
> limit - support a SW-based software emulated atomic write mode. For XFS,
> this SW-based atomic writes would use CoW support to issue emulated untorn
> writes.
> 
> It is the responsibility of the FS to detect discontiguous atomic writes
> and switch to IOMAP_DIO_ATOMIC_SW mode and retry the write. Indeed,
> SW-based atomic writes could be used always when the mounted bdev does
> not support HW offload, but this strategy is not initially expected to be
> used.
> 
> Signed-off-by: John Garry <john.g.garry@oracle.com>

Looks good now, thank you.
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D

> ---
>  Documentation/filesystems/iomap/operations.rst | 16 ++++++++++++++--
>  fs/iomap/direct-io.c                           |  4 +++-
>  include/linux/iomap.h                          |  6 ++++++
>  3 files changed, 23 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/filesystems/iomap/operations.rst b/Documentation/filesystems/iomap/operations.rst
> index 82bfe0e8c08e..b9757fe46641 100644
> --- a/Documentation/filesystems/iomap/operations.rst
> +++ b/Documentation/filesystems/iomap/operations.rst
> @@ -525,8 +525,20 @@ IOMAP_WRITE`` with any combination of the following enhancements:
>     conversion or copy on write), all updates for the entire file range
>     must be committed atomically as well.
>     Only one space mapping is allowed per untorn write.
> -   Untorn writes must be aligned to, and must not be longer than, a
> -   single file block.
> +   Untorn writes may be longer than a single file block. In all cases,
> +   the mapping start disk block must have at least the same alignment as
> +   the write offset.
> +
> + * ``IOMAP_ATOMIC_SW``: This write is being issued with torn-write
> +   protection via a software mechanism provided by the filesystem.
> +   All the disk block alignment and single bio restrictions which apply
> +   to IOMAP_ATOMIC_HW do not apply here.
> +   SW-based untorn writes would typically be used as a fallback when
> +   HW-based untorn writes may not be issued, e.g. the range of the write
> +   covers multiple extents, meaning that it is not possible to issue
> +   a single bio.
> +   All filesystem metadata 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 f87c4277e738..575bb69db00e 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -644,7 +644,9 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
>  			iomi.flags |= IOMAP_OVERWRITE_ONLY;
>  		}
>  
> -		if (iocb->ki_flags & IOCB_ATOMIC)
> +		if (dio_flags & IOMAP_DIO_ATOMIC_SW)
> +			iomi.flags |= IOMAP_ATOMIC_SW;
> +		else if (iocb->ki_flags & IOCB_ATOMIC)
>  			iomi.flags |= IOMAP_ATOMIC_HW;
>  
>  		/* for data sync or sync, we need sync completion processing */
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index e7aa05503763..4fa716241c46 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -183,6 +183,7 @@ struct iomap_folio_ops {
>  #define IOMAP_DAX		0
>  #endif /* CONFIG_FS_DAX */
>  #define IOMAP_ATOMIC_HW		(1 << 9) /* HW-based torn-write protection */
> +#define IOMAP_ATOMIC_SW		(1 << 10)/* SW-based torn-write protection */
>  
>  struct iomap_ops {
>  	/*
> @@ -434,6 +435,11 @@ struct iomap_dio_ops {
>   */
>  #define IOMAP_DIO_PARTIAL		(1 << 2)
>  
> +/*
> + * Use software-based torn-write protection.
> + */
> +#define IOMAP_DIO_ATOMIC_SW		(1 << 3)
> +
>  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);
> -- 
> 2.31.1
>