Re: [PATCH 4/6] ext4: Warn if we ever fallback to buffered-io for DIO atomic writes

[Dave Chinner <david@fromorbit.com>]

On Fri, Oct 25, 2024 at 09:15:53AM +0530, Ritesh Harjani (IBM) wrote:
> iomap will not return -ENOTBLK in case of dio atomic writes. But let's
> also add a WARN_ON_ONCE and return -EIO as a safety net.
> 
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> ---
>  fs/ext4/file.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index f9516121a036..af6ebd0ac0d6 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -576,8 +576,16 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  		iomap_ops = &ext4_iomap_overwrite_ops;
>  	ret = iomap_dio_rw(iocb, from, iomap_ops, &ext4_dio_write_ops,
>  			   dio_flags, NULL, 0);
> -	if (ret == -ENOTBLK)
> +	if (ret == -ENOTBLK) {
>  		ret = 0;
> +		/*
> +		 * iomap will never return -ENOTBLK if write fails for atomic
> +		 * write. But let's just add a safety net.
> +		 */
> +		if (WARN_ON_ONCE(iocb->ki_flags & IOCB_ATOMIC))
> +			ret = -EIO;
> +	}

Why can't the iomap code return EIO in this case for IOCB_ATOMIC?
That way we don't have to put this logic into every filesystem.

When/if we start supporting atomic writes for buffered IO, then it's
worth pushing this out to filesystems, but right now it doesn't seem
necessary...

-Dave.
-- 
Dave Chinner
david@fromorbit.com