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Message-ID: <20241101111336.j34umexmaww4uyae@quack3>
Date: Fri, 1 Nov 2024 12:13:36 +0100
From: Jan Kara <jack@...e.cz>
To: "Ritesh Harjani (IBM)" <ritesh.list@...il.com>
Cc: linux-ext4@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
Jan Kara <jack@...e.cz>, "Darrick J . Wong" <djwong@...nel.org>,
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 v4 4/4] ext4: Do not fallback to buffered-io for DIO
atomic write
On Fri 01-11-24 12:20:54, 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@...nel.org> # inline helper
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@...il.com>
Looks good. Feel free to add:
Reviewed-by: Jan Kara <jack@...e.cz>
Honza
> ---
> 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
>
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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