linux-kernel - Re: [PATCH v3 4/4] ext4: Do not fallback to buffered-io for DIO atomic write

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Message-ID: <20241031215111.GF21832@frogsfrogsfrogs>
Date: Thu, 31 Oct 2024 14:51:11 -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 v3 4/4] ext4: Do not fallback to buffered-io for DIO
 atomic write

On Wed, Oct 30, 2024 at 09:27:41PM +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. We should not 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].
> 
> [1]: https://lore.kernel.org/linux-xfs/cover.1729825985.git.ritesh.list@gmail.com/T/#m9dbecc11bed713ed0d7a486432c56b105b555f04
> 
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@...il.com>
> ---
>  fs/ext4/file.c  |  7 +++++++
>  fs/ext4/inode.c | 14 +++++++++-----
>  2 files changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index 8116bd78910b..61787a37e9d4 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 fcdee27b9aa2..26b3c84d7f64 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3449,12 +3449,16 @@ static int ext4_iomap_end(struct inode *inode, loff_t offset, loff_t length,
>  {
>  	/*
>  	 * 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 atomic writes we will simply fail the I/O request if we coudn't
> +	 * write anything. 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.
>  	 */
> -	if (flags & (IOMAP_WRITE | IOMAP_DIRECT) && written == 0)
> +	if (!(flags & IOMAP_ATOMIC) && (flags & (IOMAP_WRITE | IOMAP_DIRECT))

Huh.  The WRITE|DIRECT check doesn't look right to me, because the
expression returns true for any write or any directio.  I think that's
currently "ok" because ext4_iomap_end is only called for directio
writes, but this bugs me anyway.  For a directio write fallback, that
comparison really should be:

	(flags & (WRITE|DIRECT)) == (WRITE|DIRECT)

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;
}

	if (ext4_want_directio_fallback(flags, written))
		return -ENOTBLK;

> +			&& written == 0)

Nit: put the '&&' operator on the previous line when there's a multiline
expression.

--D

>  		return -ENOTBLK;
>  
>  	return 0;
> -- 
> 2.46.0
> 
>