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

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Message-ID: <20241025161632.GL2386201@frogsfrogsfrogs>
Date: Fri, 25 Oct 2024 09:16:32 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: "Ritesh Harjani (IBM)" <ritesh.list@...il.com>
Cc: linux-ext4@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@...radead.org>,
	John Garry <john.g.garry@...cle.com>,
	Ojaswin Mujoo <ojaswin@...ux.ibm.com>,
	Dave Chinner <david@...morbit.com>, linux-kernel@...r.kernel.org,
	linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 4/6] ext4: Warn if we ever fallback to buffered-io for
 DIO atomic writes

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@...il.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.

I think it can if the pagecache invalidation fails, so you really do
need the safety net.  I suspect that the xfs version of this series
needs it too, though it may have fallen out?

Reviewed-by: Darrick J. Wong <djwong@...nel.org>

--D

> +		 */
> +		if (WARN_ON_ONCE(iocb->ki_flags & IOCB_ATOMIC))
> +			ret = -EIO;
> +	}
> +
>  	if (extend) {
>  		/*
>  		 * We always perform extending DIO write synchronously so by
> -- 
> 2.46.0
> 
>