Re: [PATCH v4 4/4] ext4: Do not fallback to buffered-io for DIO atomic write

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

On Fri, Nov 01, 2024 at 12:20:54PM +0530, Ritesh Harjani (IBM) wrote:
> atomic writes is currently only supported for single fsblock and only
> for direct-io. We should not return -ENOTBLK for atomic writes since we
> want the atomic write request to either complete fully or fail
> otherwise. Hence, we should never fallback to buffered-io in case of
> DIO atomic write requests.
> Let's also catch if this ever happens by adding some WARN_ON_ONCE before
> buffered-io handling for direct-io atomic writes. More details of the
> discussion [1].
> 
> While at it let's add an inline helper ext4_want_directio_fallback() which
> simplifies the logic checks and inherently fixes condition on when to return
> -ENOTBLK which otherwise was always returning true for any write or directio in
> ext4_iomap_end(). It was ok since ext4 only supports direct-io via iomap.
> 
> [1]: https://lore.kernel.org/linux-xfs/cover.1729825985.git.ritesh.list@gmail.com/T/#m9dbecc11bed713ed0d7a486432c56b105b555f04
> Suggested-by: Darrick J. Wong <djwong@kernel.org> # inline helper

Looks good to me now,
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> ---
>  fs/ext4/file.c  |  7 +++++++
>  fs/ext4/inode.c | 27 ++++++++++++++++++++++-----
>  2 files changed, 29 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index 96d936f5584b..a7de03e47db0 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -599,6 +599,13 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  		ssize_t err;
>  		loff_t endbyte;
> 
> +		/*
> +		 * There is no support for atomic writes on buffered-io yet,
> +		 * we should never fallback to buffered-io for DIO atomic
> +		 * writes.
> +		 */
> +		WARN_ON_ONCE(iocb->ki_flags & IOCB_ATOMIC);
> +
>  		offset = iocb->ki_pos;
>  		err = ext4_buffered_write_iter(iocb, from);
>  		if (err < 0)
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 3e827cfa762e..5b9eeb74ce47 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3444,17 +3444,34 @@ static int ext4_iomap_overwrite_begin(struct inode *inode, loff_t offset,
>  	return ret;
>  }
> 
> +static inline bool ext4_want_directio_fallback(unsigned flags, ssize_t written)
> +{
> +	/* must be a directio to fall back to buffered */
> +	if ((flags & (IOMAP_WRITE | IOMAP_DIRECT)) !=
> +		    (IOMAP_WRITE | IOMAP_DIRECT))
> +		return false;
> +
> +	/* atomic writes are all-or-nothing */
> +	if (flags & IOMAP_ATOMIC)
> +		return false;
> +
> +	/* can only try again if we wrote nothing */
> +	return written == 0;
> +}
> +
>  static int ext4_iomap_end(struct inode *inode, loff_t offset, loff_t length,
>  			  ssize_t written, unsigned flags, struct iomap *iomap)
>  {
>  	/*
>  	 * Check to see whether an error occurred while writing out the data to
> -	 * the allocated blocks. If so, return the magic error code so that we
> -	 * fallback to buffered I/O and attempt to complete the remainder of
> -	 * the I/O. Any blocks that may have been allocated in preparation for
> -	 * the direct I/O will be reused during buffered I/O.
> +	 * the allocated blocks. If so, return the magic error code for
> +	 * non-atomic write so that we fallback to buffered I/O and attempt to
> +	 * complete the remainder of the I/O.
> +	 * For non-atomic writes, any blocks that may have been
> +	 * allocated in preparation for the direct I/O will be reused during
> +	 * buffered I/O. For atomic write, we never fallback to buffered-io.
>  	 */
> -	if (flags & (IOMAP_WRITE | IOMAP_DIRECT) && written == 0)
> +	if (ext4_want_directio_fallback(flags, written))
>  		return -ENOTBLK;
> 
>  	return 0;
> --
> 2.46.0
> 
>