linux-ext4 - Re: [RFC 1/7] iomap: Don't fall back to buffered write if the write is atomic

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Date: Fri, 1 Dec 2023 08:10:38 +1100
From: Dave Chinner <david@...morbit.com>
To: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
Cc: linux-ext4@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
	Ritesh Harjani <ritesh.list@...il.com>,
	linux-kernel@...r.kernel.org,
	"Darrick J . Wong" <djwong@...nel.org>, linux-block@...r.kernel.org,
	linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	John Garry <john.g.garry@...cle.com>, dchinner@...hat.com
Subject: Re: [RFC 1/7] iomap: Don't fall back to buffered write if the write
 is atomic

On Thu, Nov 30, 2023 at 07:23:09PM +0530, Ojaswin Mujoo wrote:
> Currently, iomap only supports atomic writes for direct IOs and there is
> no guarantees that a buffered IO will be atomic. Hence, if the user has
> explicitly requested the direct write to be atomic and there's a
> failure, return -EIO instead of falling back to buffered IO.
> 
> Signed-off-by: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
> ---
>  fs/iomap/direct-io.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 6ef25e26f1a1..3e7cd9bc8f4d 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -662,7 +662,13 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
>  			if (ret != -EAGAIN) {
>  				trace_iomap_dio_invalidate_fail(inode, iomi.pos,
>  								iomi.len);
> -				ret = -ENOTBLK;
> +				/*
> +				 * if this write was supposed to be atomic,
> +				 * return the err rather than trying to fall
> +				 * back to buffered IO.
> +				 */
> +				if (!atomic_write)
> +					ret = -ENOTBLK;

This belongs in the caller when it receives an -ENOTBLK from
iomap_dio_rw(). The iomap code is saying "this IO cannot be done
with direct IO" by returning this value, and then the caller can
make the determination of whether to run a buffered IO or not.

For example, a filesystem might still be able to perform an atomic
IO via a COW-based buffered IO slow path. Sure, ext4 can't do this,
but the above patch would prevent filesystems that could from being
able to implement such a fallback....

-Dave.
-- 
Dave Chinner
david@...morbit.com